1. science
2. ecology
3. pollution

Workshop Volume on
Radon Contamination in
Groundwater and Application of
Isotopes in Groundwater Studies
Central Ground Water Board
Ministry Of Water Resources
Government of India
South Western Region, Bengaluru
Editors
Dr. K.Md.Najeeb
N. Vinayachandran
March 2010
MESSAGE
Ground Water quality deterioration due to various natural and anthropogenic causes is emerging as a
concern in our country, along with increasing extraction of ground water for various uses. Ground Water
Quality monitoring in India has received some what limited attention. Increasing dependence on ground
water as a reliable source of water supply for domestic, industrial and irrigation uses has led to more
importance being accorded to the quality aspects of ground water in different aquifer systems.
Radioactive contamination of water and air, reported from some areas of the country has come to the
notice of scientists in the recent years. This includes contamination of ground water by radon, which is
extremely hazardous to human health, especially in aquifers in granitic and gneissic terrain in India.
There is an urgent need for detailed scientific studies to identify and demarcate areas affected by such
contaminants so that necessary preventive/remedial measures could be under taken for safe water supply
to the affected populace.
I am glad that Central Ground Water Board, South Western Region Bangalore is organizing a one day
workshop on "Radon Contamination in Ground Water and Application of Isotopes in Ground
Water Studies" to discuss various issues related to contamination of ground water by radon as well as
the scope of isotope techniques in ground water studies on 26.3.2010 at Bhujal Bhawan, Bangalore. I
Hope this workshop would dwell upon various aspects of radon contamination in ground water and also
on issues connected with the application of stable and radioactive isotopes in groundwater investigations.
I appreciate the efforts of Shri. T.M.Hunse, Regional Director, CGWB, SWR Bangalore and his team of
officers and staff in organizing this workshop and wish it all success
Faridabad
March 2010
(B.M.JHA)
1
Preface
Radiation is a natural part of the environment in which we live and water
dissolves the radioactive isotopes in varying degrees. The high concentrationof
these radioactive isotopes in the environment is a threat to our health.
However, natural isotopes are useful tools in hydrogeological investigations
and has got wide acceptance over the years.
The largest fraction of the natural radiation we receive comes from the
radioactive gas Radon, which disintegrates by emitting alpha particles.
Although it cannot be detected by a person’s senses, radon and its radioactive
by-products are a health concern because they can cause lung cancer when
inhaled over many years. Radon is present everywhere in the rock, soil, water
and air because of the ubiquitous nature of its parent radioactive element
uranium in all geological terrains. The radio activity of groundwater is mainly
contributed by radon as it is easily dissolvable in water in relation to other
members of the uranium decay series. Groundwater dissolves radon from the
soil or aquifer and releases certain quantity to air when it comes in contact
with it. Drinking of radon contaminated groundwater for a longer period may
cause stomach cancer.
The workshop was aimed at creating awareness on health hazards due to radon
and other radioactive isotopes in groundwater and also to popularise the
application of isotope techniques in groundwater investigations. All the
technical papers are relevant to the theme of the workshop and the efforts of
the authors are gratefully acknowledged. The tremendous effort by the officers
of Central Ground Water Board, Bengaluru in organizing the workshop and
bringing out the volume is appreciated.
The financial assistance extended by Ministry of Water Resources and the
support and encouragement received from the Chairman, Central Ground
Water Board for organizing the workshop are placed on record.
Bengaluru
March2010
T.M.Hunse
Regional Director
2
Editors note
This workshop volume incorporates the technical presentations of eminent
scientists, technocrats and academicians on various topics under the theme of
the workshop ‘Radon Contamination in Groundwater and Application of
Isotopes in Groundwater Studies’. The presentations pertain to a wide
spectrum of knowledge on the radioactive gas, radon and cover the entire
gamut of isotope applications as a tool in the field of groundwater studies. It
provides a range of information on radon and reviews the principles and
examples in the application of isotope in hydrogeological investigations.
Although these techniques are currently used more widely than a few decades
back and there are publications on the subject, it is hoped that this workshop
volume will also contribute to a wider knowledge and use of isotope
techniques in groundwater studies.
Editors
3
CONTENTS
Radon measurement techniques in environmental and geophysical studies
K.P.Eappen
5
Radon - Occurrence, estimation and hydrological applications
Somashekar. R.K, Deljo Davis, Shivanna. K, M. Jiban Singh and Prakash. K.L
14
Radon in groundwater
N.Vinayachandran, K.Md.Najeeb and T.M.Hunse
36
Radiogenic contamination of groundwater in India
P K Mehrotra
41
Radon: health hazards and remedial measures
V. Meenakshisundaram
46
Evaluation of submarine groundwater discharge in coastal regions
Noble Jacob and K. Shivanna
56
Application of Radon measurements in Uranium Exploration
G.B.Rout
74
Groundwater dating using 14C, 4He and 4He/222Rn methods – A case study of
the North Gujarat Cambay region, India
R.D. Deshpande and S.K. Gupta
81
Radioactive isotope tracers for groundwater recharge studies
A Shahul Hameed
100
Estimation of groundwater recharge using isotope and geochemical tracers
D.V. Reddy and P. Nagabhushanam
107
Use of natural isotopes in delineation of multi - aquifer system and
intermixing of waters – A case study
S.Suresh, K.Shivanna, S.Tirumalesh and J.F.Lawrence
124
Isotopic Technique for Evaluating Artificial Recharge to groundwater - A
case study from Maharashtra
B. K. Purandara, Bhishm Kumar, M. S. Rao and S. K. Verma
134
Natural recharge and irigation return flow evaluation using Tritium tracer
technique in a granite watershed, Midjil mandal, Mahboobnagar district,
Andhra Pradesh, India
Pandith Madhnure, P.N. Rao, Rangarajan. R, Shankar. G.B.K, Rajeshwar.K and
A.D.Rao
142
4
Radon measurement techniques in environmental and
geophysical studies
K.P.Eappen
Ex-BARC, Environmental Assessment Division
A-13, Ranjani, Anushaktinagar, Mumbai – 400 094.
E-mail: [email protected]
Abstract
Health effects of radon, most notably lung cancer, have been investigated for several decades.
Initially, investigations focused on underground miners exposed to high concentrations of
radon in their occupational environment. However, in the early 1980s, several surveys of
radon concentrations in homes and other buildings were carried out, and the results of these
surveys, together with risk estimates based on the studies of mine workers, provided indirect
evidence that radon may be an important cause of lung cancer in the general population.
Recently, efforts to directly investigate the association between indoor radon and lung cancer
have provided convincing evidence of increased lung cancer risk causally associated with
radon, even at levels commonly found in buildings. Risk assessment for radon both in mines
and in residential settings have provided clear insights into the health risks due to radon.
Radon is now recognized as the second most important cause of lung cancer after smoking in
the general population. The understanding of radon sources and radon transport mechanisms
has evolved over several decades. The paper discusses the various methods available for radon
measurements at indoors and from different matrices causing radon release to the
environment.
Introduction
Radon belongs to the noble gas series in the periodic table. There are three natural isotopes of
radon namely, radon (222Rn), thoron (220Rn) and actinon (219Rn) resulting from the radioactive
decay of the uranium, thorium and the actinium series. 222Rn is formed from the decay of 226
Ra, the immediate parent from the 238U series, while its isotope 220Rn decays from 224Ra, a
member of the 232Th series. Actinon results from the decay of 223Ra from 235U series and is
normally neglected because its presence is negligible in atmosphere. Radon being a gaseous
element in the natural radioactive series gets diffused into the atmosphere from the earth’s
crust.
Radon emanates from rocks and soils and tends to concentrate in enclosed spaces like
underground mines or houses. Soil gas infiltration is recognized as the most important source
of residential radon. Other sources, including building materials and water extracted from
wells, are of less importance in most circumstances. The presence of radon in the free
atmosphere was first noted by Elster and Geitael (Elster, 1901) and Gish, (1951) around 1901.
Radon is a major contributor to the ionizing radiation dose received by the general population.
Recent studies on indoor radon and lung cancer in Europe, North America and Asia provide
strong evidence that radon causes a substantial number of lung cancers in the general
population. Current estimates of the proportion of lung cancers attributable to radon range
from 3 to 14%, depending on the average radon concentration in the country concerned and
5
the calculation methods. The latest report on World Health Organization (WHO) on the
biological effects of alpha radiation, BEAR VI, estimates the radiation dose due to the
exposure of radon to be 0.025 mSv y-1 per Bq m-3. This is equivalent to a probability of lung
cancer of approximately 2 x 10-6 per Bq m-3 per year per person. The analyses indicate that the
lung cancer risk increases proportionally with increasing radon exposure. As many people are
exposed to low and moderate radon concentrations, the majority of lung cancers related to
radon are caused by these exposure levels rather than by higher concentrations. Thus it is
believed that radon is the second cause of lung cancer after smoking. Most of the radoninduced lung cancer cases occur among smokers due to a strong combined effect of smoking
and radon.
A national reference level for radon represents the maximum accepted radon concentration in
a residential dwelling and is an important component of a national programme. For homes
with radon concentrations above these levels remedial actions may be recommended or
required. When setting a reference level, various national factors such as the distribution of
radon, the number of existing homes with high radon concentrations, the arithmetic mean of
indoor radon level and the prevalence of smoking should be taken into consideration. In view
of the latest scientific data, WHO proposes a reference level of 100 Bqm-3 to minimize health
hazards due to indoor radon exposure. However, if this level cannot be reached under the
prevailing country-specific conditions, the chosen reference level should not exceed 300 Bqm3
which represents approximately 10 mSv per year according to recent calculations by the
International Commission on Radiation Protection.
When one talk about radon, the immediate interpretation comes to the mind is its radiation
effect “lung cancer”. One should also know that radon can be used for many scientific
applications. The best known application of radon is its use in earth quake prediction. Being a
noble gas the tectonic movements enhances the level of radon in soil hence, can be used as a
precursor to an earth quake event. Experiments on mixing and transport of radon in air are
made use in atmospheric studies. Sudden fall of radon levels in sea cost areas is an indication
for the onslaught of rain in the region. Radon has many roles in ground water studies like (i)
dating, (ii) ground water movements (iii) aquifer characteristics etc.
It is pertinent that reliable and sensitive techniques are made available for radon studies.
Radon measurements are relatively simple to perform and are essential to assess radon
concentration in homes. They need to be based on standardized protocols to ensure accurate
and consistent measurements. When one talk about inhalation dose, it is important that the
progeny concentrations are also measured since the major portion of lung dose is contributed
by the progeny nuclides. The paper explains in brief the various methods available for radon
measurements in environmental matrices.
Radon measurements
Radon measurements are often discussed in terms of either a short-term or long-term test
(Quindos et al. 1991). A short-term test for radon, using a Lucas scintillation cell, activated
charcoal detector or another type of detector such as an electret ion chamber, can provide a
first indication of the mean long-term radon concentration in a home. However, diurnal and
seasonal radon variations should be taken into account when performing short-term radon
measurements. Since high radon concentrations commonly occur during periods when homes
are “closed up” (i.e. windows closed), a short-term measurement performed during this period,
or season, can overestimate the yearly mean radon concentration. Alternatively, a short-term
radon measurement performed during a period when the house has increased ventilation (e.g.
windows open) can substantially underestimate the mean annual radon concentration.
Therefore, in order to assess the annual average radon concentration within a home, devices
that provide a long-term integrated radon measurement are preferred.
6
Most important active methods for radon measurements in air employ collection of the gas in
scintillation flask / ionization chamber or suction of the gas through a two filter sampler. On
the other hand, progeny levels are measured by collecting them on filter papers and counting
for radioactivity at suitable intervals. Major difference in the methods of measurements is that
in radon gas estimation using scintillation cell, the progeny in the sampled air is removed by
filtration before the gas is collected in the cell. The alpha activity in the sampling cell is
computed from the build up and decay of radon gas with respect to post sampling delay. In
progeny measurement, filter paper sample is collected and the decay profile is followed in
computing individual progeny concentrations.
Table 1. Radon gas measurement devices and their characteristics
Detector Type
Passive/Active
Uncertaintya
Sampling
Cost
(Abbreviation)
[%]
Period
Alpha-track
Detector(ATD)
Passive
10-25
1-12 Months
low
Activated Charcoal
Detector (ACD)
Passive
10-30
2-7days
low
Electret ion
Chamber(EIC)
Passive
8-15
5days-1year
medium
Electronic Inegrating
Device(EID)
Active
~25
2days-year(s)
medium
Continous Radon
Monitor(CRM)
Active
~10
1hour- year(s)
high
a
Uncertainty expressed for optimal exposure durations and for exposures ~ 200 Bq/m3 .
The most popular radon measuring devices (Table 1) used by countries surveyed within the
WHO International Radon Project (WHO 2007) were alpha-track detectors (ATDs), electret
ion chambers (EICs), and activated charcoal detectors (ACDs). Active devices in use by many
countries included electronic integrating devices (EIDs) and continuous radon monitors
(CRMs). Passive devices do not require electrical power or a pump to work in the sampling
setting, whereas active devices require electricity and include the ability to chart the
concentration and fluctuations of radon gas during the measurement period. For homes, ATDs
are a popular choice to obtain a long-term radon measurement and are often deployed for a
one-year period, while EICs are often used for short (e.g. several days) to intermediate (e.g.
weeks to months) measurement periods. EICs also have the ability to integrate the radon
concentration over time (e.g. 8-hour home occupancy period), using an open-and close feature
of the detector. The use of CRMs has become more prevalent as the price of these detectors
has slowly declined. CRMs can automatically provide time resolved information. Grab
samples are air samples collected, using various devices like scintillation cells, over time
intervals as short as minutes and then taken to the laboratory for analysis. These types of
measurements do not capture the fluctuations in radon or radon decay product concentration
over time. Grab samples are not included within the guidelines as they are not recommended
for assessment of radon exposure or for making decisions regarding the need for mitigation.
Additional details on measurement devices can be found in George (1996) and in reports from
OECD (1985), NCRP (1988), SSK (2002) and USEPA (1992, 1993).
7
Radon gas detectors
Alpha-track detectors
An ATD is a small piece of specially
produced plastic substrate enclosed
within a filter-covered diffusion
chamber that excludes the entry of
radon decay products as shown in
Figure 1. The plastic is generally a
polyallyl diglycol carbonate (PADC or
CR- 39), cellulose nitrate (LR-115), or
polycarbonate (Makrofol) material.
When alpha particles are generated by
radon or radon decay products in
proximity to the detecting material, they
can strike the detecting material,
producing microscopic areas of damage
called latent alpha tracks. Chemical or electro-chemical etching of the plastic detector material
enlarges the size of the alpha tracks, making them observable by light microscopy so that they
can be counted either manually or by an automated counting device. The number of tracks per
unit surface area, after subtracting background counts, is directly proportional to the integrated
radon concentration in Bqhm-3. A conversion factor obtained by controlled exposures at a
calibration facility allows conversion from track density to radon concentration. Alpha-track
detectors are generally deployed for an exposure period ranging from 1 month to 1 year.
Alpha-track detectors are insensitive to humidity, temperature, and background beta and
gamma radiation, but measurements performed at very high altitudes (e.g. above 2 000 m)
may require slight adjustments due to differences in air density that can affect the distance
alpha particles can travel (Vasudevan et al. 1994). Cross-sensitivity to thoron can be avoided
by using a diffusion chamber with a large diffusion resistance to gas entering the chamber. A
minimum detectable concentration (MDC) of 30 Bqm-3, calculated by methods discussed
elsewhere (Strom and MacLellan 2001), for a 1 month exposure is generally achievable for
ATDs. Even lower MDCs can be obtained as prescribed by Durrani and Ilic (1997) and Field
et al. (1998).
Activated charcoal adsorption (ACD) detectors
ACDs are passive devices deployed for 1-7 days to measure indoor radon. The principle of
detection is radon adsorption on the active sites of the activated carbon. After sampling, the
detector is sealed and the radon decay products equilibrate with the collected radon. After a 3hour waiting period, the collectors can be directly gamma counted, or analytically prepared for
liquid scintillation counting techniques. In the gamma counting method, the charcoal canisters
or bags contain 25-90 g of activated carbon. In the alpha counting method, 20 ml liquid
scintillation vials containing 2-3 g of activated carbon are used. The canisters can be openfaced or equipped with a diffusion barrier to extend the measurement period to 7 days.
Because the response of ACD devices is affected by humidity, they must be calibrated under
various levels of humidity. The devices should also be calibrated over the range of exposure
durations and temperatures likely to be encountered in the field. If different types of carbon
are mixed, the calibration may not remain constant. Because charcoal allows continuous
adsorption and desorption of radon, the method only provides a good estimate of the average
radon concentration over the exposure time if changes in radon concentration are small. The
use of a diffusion barrier reduces the effects of drafts and high humidity. Since radon decays
8
with a half-life of 3.8 days, detectors must be returned for analysis as soon as possible after
the exposure period. Further details are given by George (1984) and USEPA (1987).
Electret ion chambers(EIC)
EICs are passive devices that function as integrating detectors for measuring the average
radon gas concentration during the measurement period. The electret serves both as the source
of an electric field and as a sensor in the ion chamber. Radon gas, but not decay products,
enters the chamber by passive diffusion through a filtered inlet. Radiation emitted by radon
and its decay products formed inside the chamber ionizes the air within the chamber volume.
The negative ions are collected by the positive electret located at the bottom of the chamber.
The discharge of the electret over a known time interval is a measure of time-integrated
ionization during the interval. This in turn is related to the radon concentration. The electret
discharge in volts is measured using a non-contact battery-operated electret reader. This value,
in conjunction with a duration and calibration factor, yields the radon concentration in desired
units. Typical short-term EICs are designed to measure radon for 2 to 15 days at a
concentration of 150 Bqm-3. The long-term EICs measure radon over 3 to 12 months at a
concentration of 150 Bqm-3. EICs have been described previously (Kotrappa et al. 1990).
These devices have been used in various countries and have displayed excellent accuracy and
precision if standard operating procedures (routine correction for background gamma
radiation, assuring the electrets are free of dust, etc.) are followed (Sun et al. 2006).
Electronic integrating devices (EID)
Most EIDs use a solid-state silicon detector within a diffusion chamber for counting the alpha
particles emitted by the radon decay products. Due to the small dimensions of the diffusion
chamber, long integration times (> 2 days) are often necessary for a statistically stable reading
at moderate radon concentrations. Higher sensitivities can be achieved by applying high
voltage to collect the charged radon decay products electrostatically by direct contact to the
detector. High air humidity may affect the measurement. An MDC of 20 Bqm-3 is typical for a
7-day exposure period. For several popular EIDs, the ability to routinely calibrate these
detectors is lacking.
Continuous radon monitors (CRM)
There are several types of commercially available CRMs using various types of sensors
including scintillation cells, current or pulse ionization chambers, and solid state silicon
detectors. CRMs either collect air for analysis using a small pump or by allowing air to diffuse
into a sensor chamber. All CRMs have electrical circuitry that provide a summary report, and
often a time-resolved recording, which allows the calculation of the integrated radon
concentration for specified periods. The different types have their specific advantages. For
example, when using solid-state silicon detectors, alpha spectrometry is possible (Tokonami et
al. 1996, Iimoto et al. 1998a), allowing discrimination between radon and thoron. Some
devices eliminate the cross-sensitivity to air humidity by drying the incoming air. Generally,
the MDC of these devices is about 5 Bqm-3 calculated using standard methods. CRMs require
routine calibrations to assure proper functioning and reliable results. Figure 2 shows an
example of an electronic radon measure device using scintillation cell. An electrostatic
collection chamber based radon monitor has also been developed in BARC, Mumbai for
monitoring the real time variations of radon concentration in the environment (Fig.3). The
unit consists of a 5 L Al chamber with perforated walls covered with foam to allow radon to
9
Fig.2. CRM using scintillation cell
diffuse into the chamber. The radon decay products formed in the control volume are
collected on a 15 m Al metal foil maintained at a negative potential of -1200 Volts with
respect to the body of the chamber. A ZnS coated transparent film is placed between the Al
metal foil and a photomultiplier tube that is attached to the pulse counting system. The light
scintillations from the alpha sensitive ZnS film due to the radioactive decay of these collected
radon decay products are counted using photomultiplier assembly. A microprocessor based
software converts the registered counts to radon concentration using an algorithm based on
growth curve analysis of decay product activity. The sensitivity factor of the unit is 5
cph/(Bqm-3) of radon having the lower detection limit of 3 Bqm-3. The unit is capable of
providing variations in radon concentrations with counting intervals ranging from 20 minutes
to 2 hours.
Fig.3. CRM based on
electrostatic collection
Radon in water measurement
devices
The presence of radon in groundwater is
predominantly due to the decay of
radium (226Ra) found in rock and soils and does not mainly originate from the radium
dissolved in water. Radon can also be generated within water distribution systems with high
radium concentrations from radium adsorbed iron pipe scales (Field et al. 1995, Fisher et al.
1998a). Radon exposure from waterborne radon sources may occur either from ingestion or
from inhalation of radon released from water. The cancer risk resulting from the release of
waterborne radon (showering, dish washing, etc.) is generally considered much greater than
the risk from drinking water containing radon (NRC 1998). A commonly used estimate for the
transfer coefficient of radon between water and air for homes in North America is 1.0 x 10-4
(Nazaroff et al. 1987). In most parts of the world, radon released into the indoor air from
10
waterborne sources is much less than the radon emanating from ground sources beneath the
home. Several well-established methods exist for the collection (Field and Kross 1996) and
measurement (Vitz 1991) of radon in water. Techniques for measuring radon in water include
direct gamma counting (Galli et al. 1999), electret ion chambers (Kotrappa and Jester 1993),
and gas transfer by membranes (Surbeck 1996, Freyer et al. 2003). Liquid scintillation
counting and the de-emanation radon measurement techniques are the most prevalent methods
for measuring radon concentrations in water (Prichard et al. 1991, Prichard and Gesell 1977,
Lucas 1957, 1964) and will be discussed in detail.
Liquid scintillation counting
Liquid scintillation counting (LSC) is the most sensitive and widely used method to measure
radon in water. The popularity of liquid scintillation for radon analysis is due to several factors
including the excellent accuracy and precision of the method, the low level of detection, the
limited need for sample preparation, the ability to rapidly measure a large number of samples,
and the ability of the counter to change samples while unattended. Because of the high
solubility of radon in organic solvents, properly collected water samples (Field and Kross
1996) can be added directly to the scintillation cocktail (e.g. toluene, xylene, or mineral oil) to
form a two-phase aqueous/organic system. The radon will be partitioned between the
water/scintillation cocktail and the air space in the vial and will become available for
measurement by LSC methods. The LSC technique quantifies the activity of radon and decay
products from the rate of photons emitted from the scintillation fluid (Prichard and Gesell
1977, Prichard et al. 1991). Limitations of the LSC technique include the initial start-up cost
to purchase the counter and the need to perform the analyses in a laboratory.
De-emanation counting
Measurement of radon in water by de-emanation involves extracting the dissolved radon from
water into a radon-free gas that is subsequently transferred to a radon measuring device, such
as a scintillation cell. For water to be analysed, a water sample is transferred to a bubbler. By
bubbling the water sample with a radon free gas (e.g. nitrogen), whose volume is five-to
tenfold greater than the volume of the liquid, de-emanation of water at normal temperatures
can be achieved. In this example, an evacuated scintillation cell is refilled by the gas enriched
with the extracted radon. Fig. 4 shows the typical Lucas cell. The cell is counted after a delay
of about 3 hours to establish radioactive equilibrium between radon and its decay products.
Depending on the counting time, a detection limit below 1 BqL-1 can be achieved. Besides
EIC, two other techniques for measuring radon in water are direct gamma counting and gas
transfer by membranes (Galli et al. 1999, Surbeck 1996, Freyer et al. 2003).
Fig.4. Lucas scintillation cell
11
For the measurement of relatively low concentrations, it is desirable to use a large collection
volume and then concentrate the sample into a smaller volume for counting. Pre-concentration
can be accomplished by passing the air through an activated charcoal trap submerged in liquid
nitrogen. At this temperature, radon is retained in the charcoal. This charcoal is heated in a
chamber resulting in the release of the trapped radon. This is then collected from the closed
chamber using a scintillation cell or an ionisation chamber and the concentration is estimated
using the method described above.
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Prichard HM, Venso EA, Dodson CL (1991). Liquid scintillation analysis of 222Rn in water by
alpha/beta discrimination. Radioactivity and Radiochemistry, 3:28-36.
Quindós LS, Fernández PL, Soto YL (1991). Short versus long-term indoor radon
measurements. Health Phys, 61:539-542.
Raghavayya, M. (1977), A Study on the distribution of radon in uranium mines, Rep. No.
BARC I/452, Bhabha Atomic Research Centre, Bombay
Srivastava, G.K., 1994. A study of the potential internal radiation hazards and their control in
uranium mining and milling. PhD thesis submitted to University of MUmbay.
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Veröffentlichungen der SSK Bd. 47, Urban & Fischer, München.
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Sun K et al. (2006). Field comparison of commercially available short-term radon detectors.
Health Phys, 91:221-226.
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Tokonami S et al. (1996) Calculation procedure of potential alpha energy concentration with
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Nucleonics, 13, 68.
.
13
Radon - Occurrence, estimation and hydrological applications
Somashekar. R.K, Deljo Davis, Shivanna. K,* M. Jiban Singh and Prakash. K.L
Department of Environmental Science, Bangalore University, Bangalore – 56
*
Isotope Hydrology Section, IAD, Bhabha Atomic Research Centre, Mumbai
[email protected]
Abstract
This paper presents the latest research work on natural radioactivity of radon. The work
evaluates the occurrence, estimation and analysis of radon gas in air and water followed at
national and international levels. The observed concentration in groundwater and surface
water supply from Bangalore city is provided along with the hydrological application and the
associated health risk. The study emphasizes on further need of a detailed research in this field
especially, the application of radon in hydrology.
Introduction
Background radiation levels are a combination of terrestrial (radium, thorium, radon, etc.) and
cosmic radiation (photons, muons, etc.) Natural radioactivity is common in the rocks and soil
that make up our planet. Over 60 radionuclides (radioactive elements) can be found in nature.
The present day content of radioactivity in the sea is estimated to be nearly 10,000
exabecquerel (Ebq = 1018 Bq). Terrestrial radiation comes from radioactive elements that
were present at the time the earth was formed. The surface of the Earth and the terrestrial crust
happens to be an enormous reservoir of primordial radioactivity. Essentially, all substances
contain radioactive elements of natural origin to some extent or the other.
Each radioactive element, or radionuclide, has a characteristic half-life. Half-life is a measure
of the time taken for one half of the atoms of a particular radionuclide to disintegrate (or
decay) into another nuclear form. Half-lives vary from millionths of a second to billions of
years. The half-life of Uranium-238 is about 4.5 billion years, Uranium-235 about 700 million
years, and Uranium-234 about 25 thousand years. In fact, very long half-life (and thus low
radioactivity) is the reason why Uranium still exists. The chain of successive decays
constitutes a radioactive family or series. In the Uranium and Thorium series (4n+2 / 4n+3)
after consecutive transformation state Radium and Radon isotopes are built up. Radium is
element 88 in the group IIA periodic table. It can exist in the form of different isotopes, 226Ra
(α emitter, half life =1600 years and 228Ra (β emitter half life 5.8 years) are the most abundant.
From the health point of view the two are also most radiotoxic; when ingested over a long
period of time they deliver the highest radiation doses to bone, where radium tends to
accumulate.
Radon (222Rn) and Thoron (220Rn ) from the Uranium and Thorium decay chains are noble
gases produced by the decay of their immediate respective parent nuclides, Radon decay
products are divided into two groups; the short lived radon daughters 218Po ( A; 3.05m), 214Pb
14
(B; 26.8m); 214Bi(C; 19.7m), and 214Po(C;164µs) with effective half lives ~30min; and the
long lived radon decay product is 210Pb (T1/2 = 22y). Most important radionuclide in this chain
is the lead isotope 212Pb with half life of 10.6H. These daughter products of Radon being the
isotopes of heavy metals get attached to the existing aerosol particles in the atmosphere. Their
elimination from the atmosphere occurs either by radioactive decay or by other removal
processes and surface deposition that is washout by rain.
238
U concentration in most rocks and soils is low (a few parts per million or less), the
corresponding 226Ra and 222Rn concentrations are usually small as well. Larger concentrations
of 222Rn are typically associated with granitic rocks that contain elevated concentrations of
238
U (typically ten or more parts per million). Hall et al., (1987) measured 222Rn
concentrations in waters from the granitic regions in excess of 3.7 × 104 Bq L-1. Larger than
average 222Rn have also been measured in other 238U naturally enriched sites such as
phosphate bearing rocks. The observably enhanced concentrations of 222Rn in waters in granite
regime are due to the moving out of Radon gas from the parent rock and dissolving in
surrounding water under geologic pressure which tend to get released into atmosphere in
normal atmospheric conditions.
It is found at low levels in virtually all rocks, soil, and water. Significant concentrations of
Uranium occur in some substances such as phosphate rock deposits and minerals such as
lignite, and monazite sands in uranium-rich ores (it is recovered commercially from these
sources). Uranium has been mined in Canada, the southwest United States, Australia, and
parts of Europe, the former Soviet Union, Namibia, South Africa, Niger and elsewhere.
Radon released by radioactive decay of Radium-226 (T 1/2 = 1620 years), Radium-226 is
produced through the radioactive decay of Uranium-238 (T 1/2 = 4.51 billion yrs) that
comprises 4 ppm of the earth’s crust, Radon-222 is a non-reactive noble gas and itself is not a
health hazard, Radon is an odorless, tasteless, invisible gas that mixes with air, chemically
inert and essentially non-reactive, heaviest noble gas with highest melting and boiling point,
highly soluble in non-polar solvents, moderately soluble in cold water, able to diffuse through
rock and soil, decays by alpha particle emission (T1/2 = 3.8 days), decay products are solids
and are called daughters or progeny. The ratio of progeny to radon gas ranges from 0.2 - 0.8
(average 0.4), radon progeny are short lived (0.2 milli seconds to 26.8 minutes), seven decay
steps from Radon-222 produces stable Lead-206, chemically active and charged radon
progeny can attach to air particles. The radon daughters Polonium-218, 214 and 210 are alpha
particle emitters, alpha particles, when inspired, can potentially cause physical cellular
damage.
Two thirds of the total effective radiation dose to the average humans from all natural sources
comes from radon and its progeny. Radon in homes is more concentrated and far more
dangerous than outdoors. The National Academy of Sciences estimates that the outdoor radon
causes only 800 out of the total of 21,000 lung cancer deaths in the US each year. But radon
decay products, radioactive solid particles, much smaller than household dust, float in the air
and get trapped in our lungs, trachea, and bronchi. At 4 pCi/L each liter of air contains 70,000
radon atoms. But less than 1% of the inhaled atoms get trapped and we thus accumulate in our
airways about 600,000 radioactive particles every hour. They shoot out alpha particles that can
pose severe health risk to the people.
The present paper estimates the status and advances in the Radon studies and Estimation
procedures, Hydrological applications, possible health risk besides its occurrence as a case
study from Bangalore city.
15
Research in radon
National status
Extensive studies have been conducted on the hydrological behavior and radiological
significance of Radon and Radium world over. Radium, radon and thoron are important
radionuclides in the uranium and thorium decay series. (Birchard and Libby, 1980;
Hermansson et al. 1991) and Sengupta et al.(2001) documented radon variation in air to be as
low as 2.3 Bqm-2 h-1 in the vicinity of a shallow, low grade Uranium deposit at Narwapahar to
a high of 9.9 Bqm-2 h-1 close to Jaduguda in Jharkhand India. Several reports on the presence
of dissolved radon in groundwater from northern India are also available (Vivek et al. 2002).
The study conducted by the Sridhar Babu. et al., 2008 in pockets of Kolar district of
Karnataka, the groundwater uranium concentration spanned between 0.3 and 1442.9 µg/L.
According to the World Health Organization 30 ppb is the permissible limit for uranium in
drinking water. Computing to this prescribed level, 21.8% samples exceeded the permissible
limits. The Uranium concentration in the drinking water samples of Chikmagalur, Karnataka,
has been reported to vary from 0.2 to 27.9 µg/L (ppb).
Radon concentrations in soil, air and groundwater in Bhilangana Valley, Garhwal Himalaya,
by using an LR-115 plastic track detector and radon emanometer, showed measured radon
concentrations to vary from 1 KBq/m3 to 57 KBq/m3 in soil , 5 Bq/l to 887 Bq/l in water and
95 Bq/m3 to 208 Bq/m3 in air. The recorded values are quite high due to associated uranium
mineralization in the area. Radon concentration was also found to depend on the tectonic
structure and geology of the area (Choubey, 1997).
The gamma radiation level due to the radiological isotopes were checked in the dwelling using
a gamma survey meter (Khokhar; 2007) and reported annual average indoor radon
concentration in air inside the dwellings varied from 9.91 to 87.84 Bq m-3 with overall mean
value of 26.48 Bq/ m3. It was also observed that the radon concentration is relatively higher in
the houses where the floor is bare but relatively lower in the tiled or cemented floor.
Relatively higher concentrations of radon (25–92 Bq/l) were reported by Choubey et al.,
(2003) for groundwater from Quaternary alluvial gravels associated with uranium-rich
sediments in the Doon Valley of the outer Himalayas.
The radon concentration recorded in natural Springs of Uttarkhand, West Bengal, Sikkim, and
Bhutan, the lowest and highest concentration recorded is 0.1 and 441.2 Bq/l (Swastik Burtu
village near Gangtok, Sikkim) respectively. Due to high radon concentration in natural
springs, the residents in the city and villages around Gangtok are likely to be exposed to
radiation hazards following Consumption of potable spring water (Virk, 2002).
It is well established that some areas of Himachal Pradesh situated in the environs of
Himalayan Mountains are relatively rich in uranium-bearing minerals. Some earlier studies
have indicated high levels of radon in the dwellings of these zones. An indoor Radon/Thoron
survey has been carried out in some villages of four districts in the state of H.P. This survey
has been conducted using twin chamber dosimeter cups. The indoor radon levels have been
found to be varying from 17.4 Bq/m3 to 140.3 Bq/m3, whereas indoor Thoron levels varied
from 5.2 Bq/m3 to 131.9 Bq/m3. The yearly average dose rate for the local population varied
16
from 0.03 µSv/h to 0.83 µSv/h. The annual exposure dose to inhabitants in these dwellings lies
below the upper limit of 10 mSv (WHO).
Generally, radon concentration have been found to be higher in thermal springs at Kasol with
highest value of 792.0±9.0 Bq/l. Radon concentration in the soil-gas has been found to be vary
from 1.024±0.27 KBq/m3 to 75.4±2.62 KBq/m3 (Vivek et al.., 2000).
Virk et al., (2000) also monitored Radon at Palampur and Dallhousie stations in Kangra and
Champa valleys using emmanometry for discrete measurement of time series radon data since
1992 under Himalayan seismicity programs of Government of India. The radon data was
correlated with micro earthquakes recorded by Indian Meteorological department (IMD)
network during 1992- 1997. Radon concentration in Kangran and Champa Valley of N-W
Himalayan also suggest that the temperature, rainfall and humidity have positively correlated,
whereas wind velocity exhibited negative correlation with radon. Virk et al., (2001) found
concentrations of radon in groundwater from tube wells in Bathinda and Gurdaspur districts of
Punjab, India in the range 3.0–8.8 Bq/l.
International Status
Lico and Rowee (1991) studied the areal distribution and characteristics of 222Rn in the
groundwater from the Aquifers of Carson Valley, Nevada USA, to determine the sources of
222
Rn. Groundwater 222Rn concentrations ranged from less than 4 to 400 Bq/l with a median
concentration of 18Bq/l. The largest 222Rn concentrations were present in the groundwater
confined to alluvial deposits on the western side of Carson valley. These large concentrations
were attributed to large amount of 238U present in granitic rocks in the Carson Range (2.9 to
10.1 mg Kg-1). The second factor could be direct flow into the alluvial aquifer from subsurface
fractures in the underlying granitic bedrock, rich in dissolved 222Rn. A third factor was the
gradients that existed along the western side of the valley. At the edge of the valley, the
bedrock was closer to the surface. A thinner alluvial at this point caused less dilution of the
222
Rn coming from the granite. Water quality indicates that the granitic rocks of the Carson
Range are the major source of the 222Rn in groundwater from the western side of the valley as
evidenced by increased inverse relationship of specific conductance and dissolved 222Rn
concentrations.
Water samples collected in 1995 from 57 monitoring wells (48 shallow and 9 deep) in the
fluvial aquifers of the White River Basin were analyzed for radon. Radon concentrations in
the shallow wells ranged from 140 to 1,600 pCi/L (picocuries per liter); the median
concentration was 420 pCi/L. In comparison, analyses of the samples from the nine deep wells
indicated that radon concentrations decrease with depth within the fluvial aquifers; the median
concentration was 210 pCi/L. (Joseph and Rhett; 1998).
In the Chickies quartzite of southern Pennsylvania, USA, (Cecil et al., (1991) measured 222Rn
concentrations ranging from 3.2 to 907 Bq/l and 226Ra concentrations ranged from less than
0.004 to 4.44 Bq/l in water samples from 103 wells.
Szabo and Zapeza (1991) studied geologic and geochemical factors that controlled
radionuclide concentrations in groundwater in the Newark basin, New Jersey, USA. Analysis
of 30 groundwater samples showed 222Rn concentrations ranging from less than 3 to almost
600 Bq/l, with median concentration of 51.4 Bq/l. Eighty seven percent had concentration
greater than 400 Bq/l and were located in three different geologic formations in which
17
elevated gross alpha particle radioactive series in the water existed. At concentrations less than
400Bq/l dissolved 222Rn did not correlate with dissolved gross alpha particles or with
concentrations of 238U.
The concentrations of both 238U and 226Ra varied with changes in groundwater chemistry, but
those of 222Rn did not. Thus, the dissolved concentrations of 222Rn was not controlled by the
geochemical composition of the groundwater, but by the 238U and the 226Ra content of the rock
and the physical characteristics of the aquifers (Szabo and Rhett, 1998). . This is contrary to
the report by Torgersen et al., (1992) who opined that the large variability of 222Rn
concentrations in groundwater is independent of dissolved 226Ra concentrations, supporting
the idea that controls on 222Rn concentrations in a given aquifer are dependent on local aquifer
characteristics.
Again in a study of the Kirkwood- Cohansy system in southern New Jersey by Kozin Ki et al.
(1995) showed that the aquifer materials were the source of the dissolved 222Rn. The majority
of the wells (53) contained 222Rn concentrations between 7 and 70 Bq/l with median of 10
Bq/l. Concentration of 222Rn were approximately two orders of magnitude greater than the
concentrations of 226Ra in the groundwater samples, indicating that the source of the dissolved
222
Rn was not dissolved 226Ra within this aquifer system and the data correlated to the depth
and the chemical composition of the water samples. In contrast, the concentration of dissolved
222
Rn correlated to the radioactivity of the aquifer sediment adjacent to the well screen, as
determined by gamma ray logging.
Recently proposed water quality regulations for 222Rn in ground water is a maximum
contaminant level (MCL) of 11.1 kBq m-3; 300 pCiL-1 (EPA, 1991)
Sampling and analysis for 222rn
222
Rn in Air: Radon-222 gas is measured by its activity or rate of decay in units called "curies"
, One curie equals 3.7 x 107 radioactive disintegrations per second, One pico curie (pCi)
equals 3.7 x 10-2 radioactive disintegrations per sec.
The radon concentration measurement techniques fall in either of grab sampling, continuous
and active sampling, and integrative sampling. Grab sampling: consist of essentially
instaneous measurements of the radon or radon progeny concentration in air over time
intervals that are short (on the order of minutes) compared to the time scale of fluctuations in
concentration. The air is collected in a container and brought back to the laboratory for
analysis. Typical containers include plastic containers, metal cans and glass containers. The
volume of the containers are usually between 5 liters and 20 liters. Continuous sampling
involves the automatic taking of measurements at closely spaced time intervals over a long
period of time. The result is a series of measurements which can give information on the
pattern with which the concentration varied throughout the measurement interval. Integrating
Sampling involves integrating devices that collect information on the total number of radiation
events which occur throughout some fairly long period of time, usually on the order of several
days to months. The result from integrating devices is an estimate of the approximate average
concentration in the environment.
Radon analysis in air involves both direct and indirect approaches. Several protocols are
employed in the measurement of Indoor Radon Concentrations. Like Continuous Radon
Monitors (CR), Alpha Track Detectors (AT or ATD), Activated Charcoal Adsorption Devices
(AC), Charcoal Liquid Scintillation (LS) Devices, Grab Radon Sampling (GB, GC, GS),
18
Pump/Collapsible Bag Devices (PB), Three-Day Integrating Evacuated Scintillation Cells
(SC), Unfiltered Track Detectors (UT), Indoor Radon Decay Product Measurement Device
Protocols like Radon Progeny Integrating Sampling Units (RPISU or RP), etc.
Water sample and analysis
Concentrations in water typically is reported in disintegration per minute (dpm), picocurie per
liter pCiL-1) or Becquerel per liter (Bql-1).One BqL-1 is equal to 60dpm l-1 or 27.03pCiL-1.
Water for 222Rn analyses can be collected with the bubbler, liquid scintillation, or field
screening methods. These three sampling and associated analytical technique are summarized
here.
The liquid scintillation method uses direct liquid scintillation counting. The sampling vial,
with 10ml of water sample and 5ml of a mineral oil based liquid scintillation solution, is
shaken in the laboratory to mix fluids. This is followed by a wait of at least 3hours to allow
measurement the short lived daughters of Radon (214Po, 218Po) to achieve secular equilibrium
measurement of the radioactivity. The detection limit is about 0.04 BqL-1 for 10 ml of sample.
Analyses of water sample in Bubbler method are by direct de emanations and alpha
scintillation counting. The de emanation bubbler containing the samples is attached to de
emanation system in the laboratory. The de-emanation system consists of following parts 1.
Bubbler 2. A glass drying tube packed with anhydrous magnesium per chlorate to remove
moisture and sodium hydroxide on asbestos plus soda lime to remove carbon dioxide. 3. A
manometer to indicate the pressure inside the system during the transfer of 222Rn from the
bubbler to the alpha scintillation cell. 4. An alpha scintillation cell, 5. Vacuum pump. The
222
Rn is purged from the water sample with helium gas into the alpha scintillation cell. The
radioactivity of 222Rn is determined by the alpha scintillation counter. The detection limit for
60ml sample is 0.01 Bql-1.
The infield analytical instrument is an alpha scintillation meter that contains a phosphor
coated cell. These cells should be airtight in order to quantitatively accept a small gas sample
from the sample syringe. Such a system has been described by counter for 1 minute. Each
analytical system is calibrated and gas extraction efficiency is compared with laboratory
standard. For first order approximations of 222Rn concentrations, the temperature of the water
can be ignored but it should be considered more accurate calculations. The detection limit is
about 0.115 Bql-1 for a 100ml gas sample that has been withdrawn from the water sample.
A Lucas cell technique, using a portable device contain 750 ml of water sample is taken in
radon-tight reagent bottle of 1 litre capacity connected in a close circuit with a ZnS coated
detection chamber through a hand operated rubber pump and a glass bulb containing CaCl2 to
absorb the moisture. Air is then circulated in close circuit for a period of ten minutes till the
radon forms a uniform mixture with the air and the resulting alpha activity is recorded. The
calibration factor, 1 count/min. = 0.11 Bq/l is used to convert the recorded alpha counts in
Bq/l.
19
Fig1. A Lucas cell and associated counting device
Radon Monitor (RAD- 7)
The in situ field monitor system is similar to the system proposed by Brunett et al., for the
radon in air. The radon in the air is continuously pumped through a desiccant. The purpose of
desiccant is to remove moisture. In this case the water sample collected in the 250 ml bottle
establishes a closed air – water loop to aid the radon to strip in water. The diagrammatic
presentation of radon detector is given in fig 1 and 1a and 2.
The RAD monitor uses a high electric field above a silicon semi conductor that detects the
ground potential to attract the positively charged polonium daughters,218Po+(t1/2=3.1 min;
alpha energy =6.00 MeV) and 214Po+(t1/2= 164 µs; alpha energy =7.67 MeV), which are
counted as a measure of 222Rn concentration in air. The ions are collected in energy specific
windows which eliminate interference and maintain very low backgrounds. 222Rn activities are
expressed in Bq/m3 (disintegration per second per m3) with 2 σ uncertainties. A specially
fabricated aerating system is used to strip the radon in water. The system uses the bubbling air
in closed loop. This radon gas after passing through the desiccant which will free from
moisture is collect through the energy specific windows and counted for the radon
concentration. The time elapsed for the sample collection and analysis will corrected with the
following equation
C = C0e-λt
(1)
Where C= measured concentration, C0= initial concentration (to be calculate) after the decay
correction, t= time elapsed since collection (days). This instrument is helpful in analysis
radon in air and water.
20
Desiccant
Sample bottle
Radon in air monitor
Aerator
Fig. 1. Radon water monitoring system
Fig. 1a. The aerator of the monitoring system
21
Fig. 2. Diagrammatic view of Radon monitor in water
The in situ field monitor system is similar to the system proposed by Brunett et al., for the
radon in air. The radon in the air is continuously pumped through a desiccant. The purpose of
desiccant is to remove moisture. In this case the water sample collected in the 250 ml bottle
establishes a closed air – water loop to aid the radon to strip in water. The diagrammatic
presentation of radon detector is given in fig 1 and 1a and 2. The ions are collected in energy
specific windows which eliminate interference and maintain very low backgrounds. 222Rn
activities are expressed in Bq/m3 (disintegration per second per m3) with 2 σ uncertainties. A
specially fabricated aerating system is used to air the water sample that free the radon in
water. This radon gas is collected through the energy specific windows and counted for the
radon concentration. The time elapsed for the sample collection and analysis is corrected with
the following equation
C = C0e-λt
(1)
22
Where C= measured concentration, C0= initial concentration (to be calculate) after the decay
correction, t= time elapsed since collection (days). This instrument is helpful in the analysis
radon in air and water.
Health risk
An Internationally prescribed radioactivity exposure limit is one msv/year. Radiological
effects owing to ingestion of dissolved radon in drinking water are defined in terms of
effective radiation dose received by the population during habitual consumption of water. The
annual effective dose to an individual consumer due to intake of radon from drinking water is
evaluated using the relationship (Alam et al, 1999)
DW = CwCRwDcw
(2)
Where Dw is the annual effective dose (Svy–1) due to ingestion of radionuclide from the
consumption of water, Cw is the concentration of 222Rn in the ingested drinking water (Bq l–1),
CRw is the annual intake of drinking water (l y–1) and Dcw is the ingested dose conversion
factor for 222Rn (SvBq–1). For calculation of effective dose, a dose conversion factor of 5 x 10–
9
Sv Bq–1 suggested by the United Nations Scientific Committee on the Effects of Atomic
Radiation (UNSCEAR) is used. Annual effective dose due to intake of 222Rn from drinking
water is calculated considering that an adult (Age > 18 y), on average, takes 730 L water
annually (Cevik et al, 2006). Following ingestion of 222Rn dissolved in drinking water, mean
effective doses per liter (nSv l–1) and annual effective doses (µSv y–1) are calculated.
In assessing the dose from radon ingestion, it is important that water processing technology
that can remove radon be considered before consumption is taken into account. Moreover, the
use of radon-containing groundwater supplies not treated for radon removal (usually by
aeration) for general domestic purposes will increase the levels of radon in the indoor air, thus
increasing the dose from indoor inhalation. UNSCER (2000) reported the “average doses from
radon in drinking water to be as low as 0.025mSv/year via inhalation and 0.002mSv/year from
ingestion” compared with the inhalation dose of 1.1mSv/year from radon and its decay
products in air.
A study conducted in USA estimates that 12% of lung cancer deaths (Grans, 1985) are linked
to radon (radon-222 and its short-lived decay products) in indoor air. Erlandsson et al. (2001)
estimated approximately 100 fold smaller risk from exposure to radon in drinking-water. The
recent work assessed that the risk of stomach cancer caused by drinking-water containing
dissolved radon is extremely small compared to lung cancer in the cold countries.
Indoor air concentrations vary with underlying or surrounding rock or soil type, occupational
radon levels are highest for miners (uranium, iron and fluorospar) , residential radon levels are
highest in basements and ground floor rooms, building type, construction, level of repair and
ventilation effect levels , average radon level in homes is about 1.25 pCi/L. Radon
remediation recommended at residential levels above 4.0 pCi/L. Indoor radon is a significant
health hazard (3 - 32,00 lung cancer deaths/ year). Exposure through inhalation virtually
occurs in all humans through the respiratory system and ingestion provides for minimal
exposure. The delivered human dose from radon exposure cannot be measured directly and
radon exposure assessment is modeled beginning with measured air concentration
Potential Dose of Radon-222 decay products are the radon concentration in air and duration of
exposure. The potential dose is affected by respiratory rate, lung tidal volume and bronchial
23
morphology as well as aerosol characteristics. A high percent of airborne radon decay
products attach to ambient aerosols. The quantity, size and density of airborne particles affect
deposition in respiratory tract, intake rate of media (air) is approximately 0.5 L/Min or 20
m3/day
Internal (Absorbed) dose of Radon-222 decay products are the potential dose multiple with
absorption factor. Absorption factor is affected by mucous thickness and clearance rate.
Radiation absorbed dose is defined as energy deposited per unit mass of tissue, and is
expressed in "rads" or "greys" (Gy) (1 rad = 100 ergs/gram, 1 Gy = 100 rads). The absorbed
dose is modified by a qualitative factor related to linear energy transfer and the modified
absorbed dose is measured in "rems" (roentgen equivalent man). The average effective dose
equivalent exposure of ionizing radiation from all sources to the US population is estimated at
300 mrem / year, radon decay products account for 200 mrem / year of this total solid radon222 decay products (primarily polonium 218 and 214) ratio of solid radon-222 progeny to
radon-222 gas in air is 0.2 - 0.8, solid radon-222 progeny deliver the actual radiation dose to
lung tissue. The radiation dose of radon-222 decay products is expressed as "working levels"
(WL's). One WL equals a combination of radon decay products which releases 1.3 x 105 MeV
of potential alpha energy (equivalent to the decay of 100 pCi/L of radon-222). The cumulative
exposure is expressed in WLM (working level months) for exposure to one WL for one
working month (170 hours). The risk to the average person of dying of radon-caused lung
cancer due to a lifetime exposure to 4 pCi/L radon level at home is 2.3 percent. Radon
concentrations in soil gas depends on the concentration of the immediate precursor of Rn-222,
Ra-226, in rocks and soils. Rn-222 is the most important source of natural radiation and is
responsible for approximately half of the received dose from all the sources. Most of this dose
is from inhalation of the Rn-222 progeny, especially in closed atmospheres.
Hydrological applications of radon
222
Rn occurs naturally in all groundwater systems the concentrations have been used for range
of applications. These concentrations may vary considerably between aquifers depending on
lithology and geologic structure. Radon concentrations in stream water are usually several
orders of magnitude lower than concentrations in groundwater. Radon is easy to sample and
analyze. The great potential lies in the study of rapid mixing processes occurring on time
scales from hours to days. Over short time scales, increase in radon concentration has been
related to aquifer residence time, and used to measure infiltration rates. Comparisons between
radon concentrations in groundwater with those in surface waters receiving aquifer discharge
have enabled locations groundwater discharge to be identified, and determination of discharge
rate. Recently Radon concentration has been used to infer flow rates through fracture rock
aquifer and as portioning tracer in the contaminated studies.
The short half life of 222Rn limits its applications in most regional flow systems, where
residence times exceed a few weeks. The large variability of 222Rn concentrations in
groundwater, independent of dissolved 226Ra concentrations, further supports the idea that
controls on 222Rn concentrations in a given aquifer are dependent on local aquifer
characteristics (Krishnaswamy et al., 1982).
222
Rn is a radioactive noble gas. It is produced from 226Ra in the radioactive decay. When
rainfall or surface water infiltrate into rock or soil, the concentration of 222Rn in the
groundwater increases due to radioactive decay of 226Ra in soil or rock. After the rainfall or
the surface water infiltrate into rock or soil, about 3 weeks later, the rate of 222Rn supply by
24
radioactive decay of 226Ra will be in a state of balance. At this time, the concentration of 222Rn
in the groundwater reaches a stable state. Therefore, the concentration of 222Rn in groundwater
is much higher than surface water. The present study shows that based on the knowledge of
radon concentration distribution in groundwater and surface water, groundwater discharge
points to the surface water can be confirmed. By calculating quantitatively the mass balance
equation of 222Rn, the fraction of groundwater discharged to surface water can be assessed.
222
Rn in surface flow comes from riverbed sediment or is recharged by groundwater; however,
222
Rn in the riverbed sediment is very scarce and can be omitted. Groundwater is the only
source of 222Rn in the surface flow. 222Rn diffuses into the river, transfers to the atmosphere
due to radioactive decay and gas exchange.
Groundwater dating with the 222Rn
Over short timescales 222Rn can be used to determine the groundwater residences time. Since
222
Rn is chemically inert, chemical reactions do not limit the 222Rn input into solution. The
growth curve of 222Rn in solution can therefore be used to estimate an apparent age of
groundwater. To arrive at an apparent age, the hypothetical steady state 222Rn concentration of
water must first be determined. The computation can be done with the following equation
222
Rn = 12.38 AρU/φ Bq/l-1
(3)
Where A is the fraction of 222Rn that is generated in the rock that migrates to the groundwater
( emanation coefficient), U is the 238U concentration in rock matrix in ppm, φ fractional
porosity of the rock (unit less)and ρ is bulk density of the rock(g cm-3). Values for the U, φ
and ρ can be measured for a given rock, but the value of A is difficult to ascertain. If the
concentration of dissolved 222Rn is measured instead, then A can be calculated to help
understand how 222Rn migrates through the rock. A small value of A indicates that the parent
nuclides are tightly bound in mineral phases whereas a value of close unity indicates large
degree of 222Rn migration. A high emanation coefficient indicates that the rock has an open
texture or that the parent nuclides are present mainly as coatings on the mineral grains or on
facture surfaces. Studies have shown that coarse grained granites may have the emanation
factor near to unity. A laboratory study through the sandbox at a constant flow determined the
ingrowths 222Rn in a flowing system of water describes radioactive decay equation
A1+ Ae [1- exp (-λt)]
(4)
Where At and Ae are radio activities of 222Rn at time t and steady state respectively, and λ is
the decay constant of 222Rn. The radon tracing method could possibly apply to aquifers
composed of sandy material with inhomogeneous production of 222Rn and flow velocities ≥0
.2 m/day.
Radon-222 can also be used in conjunction with other tracer like 4He to determine the
apparent age of groundwater. The 222Rn concentration in groundwater is mostly dependent
upon the 226Ra content of the rock matrix. On the other hand, the 4He concentration is direct
result of maximum thorium concentration in rock as well as the age of the water. The ratio of
4
He to 222Rn can indicate changes in parent nuclide concentrations as well as in apparent age
of the water. The problem in relating the concentration of these inert gases to the apparent age
of the water is that an increase in the 4He: 222Rn ratio might indicate escape of crystal 4He, or
sites where localized diffusion occurs; neither is related to apparent age of the water.
Therefore, Changes in 222Rn as a result of variation in 238U and thorium concentration must be
eliminated, and fluctuations in the rates at which 4He and 222Rn escape into the must be taken
25
to account. The enhancement of 4He concentrations from the crystal diffusion unusually
results in apparent ages that are too young.
Surface water infiltration
222
Rn is a natural tracer in river studies. It is documented that in freshly infiltrated
groundwater, 222Rn increases with increasing flow distances. Thus the measurement of 222Rn
allows the dating of in filtering water to groundwater.
The studies using the 222Rn in alluvial aquifers established that continuous pumping of
groundwater causes the river to recharge the aquifer. The distribution of 226Ra in the aquifer
and 222Rn losses to the unsaturated zone however was constant with time. Ground water
concentrations of dissolved 222Rn increased with distance of the river until a steady state is
achieved, at which the point the rate of 222Rn loss from the solution by reactive decay
remained balanced by the rate of its input. Therefore, the steady state concentration of 222Rn
with respect to conductivity and chloride concentration and the fraction of water coming from
river is a function of distance from the river. Despite the mixing of groundwater, away from
the river, it is possible to trace the infiltration of river water into the aquifer besides
calculating residence time and also estimate the flow velocity of the groundwater.
Groundwater discharge
Radon can also used as a tracer of groundwater discharge to streams and other surface water
bodies. The concentration of 222Rn in stream flow is measured to determine the location and
magnitude of groundwater seepage. A mass balance approach is used to determine the relation
between groundwater and surface water flows for a given reach of stream.
Qs × As+ Qgw × Agw= Qm × Am
(5)
Where Qs is the rate of stream flow, As is the concentration of 222Rn stream flow at an
upstream point, Qgw is the rate of groundwater seepage, Agw is the concentration of 222Rn in
groundwater, Qm is the rate of mixed stream flow and groundwater flow downstream and Am
is the concentration of 222Rn in the mixed water downstream. Qs + Qgw = Qm This equation
incorporate the rate of stream flow and the rate of groundwater seepage. The ratio of
groundwater seepage to the total stream flow at the measured point can be determined using
equation.
Qgw/Qm = Am-As/Agw- As
(6)
The regional maps of radon in surface waters could be used to delineate the major geological
structures. Such information is best contoured using the exclusion isolines constructed in a
way that they enclose all data points that have concentrations equal to or greater than the
concentration of isolines in an area. The high concentration is the indication of 222Rn
concentration in groundwater while lower factor may be due to the others, including the
turbulent degassing of surface water.
26
Radon transport in the fractured rock
Aquifers that are composed of fractured crystalline rock can have dissolved 222Rn
concentration greater than those expected, based on aquifer mineralogy, when the 238U and
226
Ra content of the bedrock concentrated along fracture walls. 238U and 226Ra commonly
migrated to sites along fracture surface that are close to rock water interfaces. The migration
of 238U and 226Ra enhanced the dissolved 222Rn concentrations in groundwater. A study
measured 222Rn emanations rate from fracture surface in Stripe granite to be 1100 atoms S-1m2
. Comparison of this value with 222Rn concentration measured in mine waters and shallow
groundwater allowed estimation of fracture apertures. A fractured granite aquifer aperture
affects the 222Rn concentrations. Radon-222 varied by a factor of seven and transmissivity
varied by four orders of magnitude within a single rock type. The different hydraulic apertures
of water bearing fractures yielded large variations in 222Rn concentrations measured in wells.
The lithologic controls of groundwater and 222Rn concentrations in fractured rock media
experimented with 150 samples of groundwater from the wells established primary controls of
222
Rn in pore fluid among rock type as the 238U concentrations of the horst rock, variations of
222
Rn concentrations within the pore fluid in each individual rock unit is controlled by other
geologic factors such as the facture surface area per unit volume of pore fluid and fracture
width the fraction. The 222Rn concentration of the single well experiment showed cone of
depression is depended and extracted groundwater from the smaller fractures. Another studies
found 222Rn concentration increased with the fracture length. Also found the 222Rn
concentration independent of the pumping rate and of the hydrodynamic dispersion process.
222
Rn in contamination studies
As portioning tracer to determine the amount of non aqueous phase liquid in contaminated
aquifers a laboratory found 222Rn as tracer to detect and quantify the amount of diesel fuel in
quartz sands. 222Rn concentrations measured at different saturations indicated that saturation
increases due to portioning the emanated radon into the organic liquid. The 222Rn
concentration decreased by 40 percent when it passed into an area contaminated with diesel
fuel. When the water was passed through the contaminated area, the concentration of 222Rn
returned to its steady state value. The contamination present was estimated to be 1.5 ± 0.35
per cent which actually represented 1.9 percent of diesel fuel saturation (Hunker et al. 1997).
Applications of natural 222Rn hydrological investigations include delineation of groundwater
recharge, determination of groundwater flow paths, groundwater and surface water
interactions and determining the groundwater apparent ages over short timescales. The short
half life of 222Rn limits its applications in most regional flow systems, where residence times
exceeds a few weeks. In some cases steady state concentration may be difficult to determine
because of local heterogeneity. Thus even short flow path, differentiating between non stay
state and steady state concentrations although necessary are difficult to perform.
Occurrence of 222rn in Bangalore city
Bangalore is one of the fastest growing cities in India with a current estimated population of
more than 8.0 million people, spread over an area of about more than 800 Sq.km. and, is
situated at an altitude of about 895 m. above MSL. Geologically the area is composed of
Younger Granites and Precambrian Gneisses (Fig 3). Water flow is aided through the fractures
in the crystalline aquifers. Samples were collected in the sample survey from the Air, Soil,
Groundwater (shallow wells and bore wells), surface water and corporation drinking water.
27
The radon concentrations were measured using the instrument RAD-7, supplied by Durridge
Company, USA.
Fig. 3. Lithology and Lineaments in the Bangalore city and adjacent areas
Fig. 4. Rn concentration in air vs Time (weekly)
28
Outdoor air
The concentration of radon in outdoor was measured through experiments for a day and for a
week in the first quarter of April. The experiments are conducted near the Department of
Environmental Science, Bangalore University, Jnanabharathi campus, Bangalore. The average
outdoor radon concentration is found to be 12.6 Bq/m3. The average radon concentration for a
week was 5.28 Bq/m3 (0.142 pci/m3) per day which is very low compared to the EPA
permissible level of 3.7Bq/m3-14.8 Bq/m3 and WHO (5 Bq/m3 and 15 Bq/m3), The highest
value were observed in the mornings and late evenings and lowest during evenings between 3
to 7 pm. Diurnal temperature inversion affects the concentration of radon in the mornings and
late nights (Fig.4)
Surface water
Radon in the lake waters of Bangalore city was measured. The highest average concentration
was 1.45Bq/l in Sanky tank while lowest was found in the 0.213Bq/l in Venkipuram Lake (Fig
5). The overall concentration of radon in the surface waters of Bangalore city is found to be
far below the permissible limits. The highest value of Sanky tank indicates the presence of
ground water discharge.
Soil
Radon in soil gives an estimate of its distribution in soil. The values ranged between 7670
Bq/m3 to 9900 Bq/m3 with a average of 8710Bq/m3 (Fig. 6 ). The radon soil equilibrium was
recorded after an hour of the experiment. The wide scale fluctuation can be correlated to the
minor trimmers in the earth crust.
Ground water
Radon was measured in 55 samples of ground water collected all over the central core of
Bangalore city Radon concentration in the wells varied from 55.96 Bq/l to 1000 Bq/l with an
average mean valve of 377.448 Bq/l. The open well had the lower concentration than the bore
wells. The concentration of radon in the deeper bore wells in Bangalore city is higher, may be
due to the presence younger granites and the weaker structures like fractures and liniments.
The contour analysis of radon concentration in the bore well waters indicates the background
distribution as 200Bq/L (Fig 7). Higher concentrations were found in some part of the Chikpet
and Hebbal which might be due to change in geological regime or exposure of younger
granitic rocks associated with high abstraction of ground water. The results of the radon
survey vs. the depth confirmed that (Fig.8) radon activity is a function of depth, with higher
activities at deeper depths, with some exceptions. Thus, the population of Bangalore City at
some locations vulnerable to the health risk because of the higher activities of radon in the
drinking water.
29
Fig 5. Average Radon concentration in lakes
Fig. 6. Radon levels in the Soil
30
Fig. 7. Spatial distribution of Radon in the bore well waters
Fig. 8. Radon in bore well waters Vs. total depth
The effective dose per liter and annual effective dose values varied with increase in radon
concentration. The effective dose was 1846.9(nSv-1) with annual concentration of 1385.2
(µSvy-1). The consumers at locations having greater depth are likely to be exposed to higher
radon.
31
Conclusions
As the climate changes in rapid phase, with the other challenges, the radioactive decay from
the nature also carries utmost importance in the present world. Terrestrial radiation together
with natural radioactivity and associated heath risk therefore assumes great importance in the
coming years. Radon is the prominent element that account for 53% natural radiation over the
world. Nonetheless, the beneficial use of the radon as natural tracer also carries significant
importance in the present day context particularly to understand the regional hydrological
regime.
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35
Radon in groundwater
N.Vinayachandran, K.Md.Najeeb and T.M.Hunse
Central Ground Water Board, South Western Region
Bhujal Bhavan, 27th main, HSR lay out, Bangalore- 560102
[email protected]
Abstract
Radon is a naturally occurring colourless odourless radioactive gas that is soluble in water. In
recent years a lot of data about radon concentration in water has been collected for uranium
prospecting, earthquake prediction, groundwater exploration etc. Radon as a radioactive gas in
the air got serious attention of scientists as excess inhalation of the same may lead to lung
cancer. However, its presence in groundwater is not considered seriously as most of the
dissolved radon escapes when it comes in contact with the atmosphere. Use of radon
contaminated groundwater increase the radon levels in the air, especially in poorly ventilated
houses, which is hazardous to health. Ingestion of such water for quite long period may lead to
stomach cancer. The genesis of radon isotopes and its application in various studies and health
hazards are discussed here.
Introduction
Radioactive isotopes in nature occur both in the atmosphere and in the lithosphere. The most
important radioactive series in the lithosphere are the uranium and thorium series. The first
members of these series and their decay products are leached out of the rocks and dissolved by
groundwater to varying degrees. The gaseous radioactive member of the uranium series, radon
is easily dissolvable in water and is enriched in relation to other members of the series. Hence,
the radioactivity of groundwater is mainly contributed by radon. Radon-222 and Radon-220,
the gaseous daughter products of U-238 and Thorium respectively accounts for more than
50% of the human exposure due to natural radiation.
The higher frequency radiation emitted from nuclei of unstable radioactive atoms is called
ionising radiation. It has enough power to knockout electrons from atoms and convert them to
electrically charged ions, which can damage the large molecules of living cells. Ionising
radiation damages DNA and just one mutant cell can cause cancer. There are several types of
ionising radiation as briefed below.
Alpha particles consist of two protons and two neutrons (like the nucleus of helium) and
because of their relatively large size, alpha particles collide readily with matter and therefore
do not penetrate deep. They are absorbed in a few centimetres of air, where they produce
intense ionisation. Hence, alpha particles can inflict more biological damage than any other
radiation.
36
Beta particles are fast moving electrons (or the anti-matter positrons) ejected from the nuclei
of atoms. Being much smaller than alpha particles, they can penetrate 1 to 2 centimeters of
water or human flesh.
Gamma rays and X-rays are pure energy (electromagnetic energy) transmitted in a wave
without the movement of matter. But, unlike light, they have great penetrating power and
easily pass through the human body. Gamma rays are similar to X-rays but more energetic.
Nuclear explosions emit one more radiation called neutron emissions. Cosmic radiation
consists of a variety of energetic particles, including protons, muons, and neutrinos.
Radon decay chain offers a full menu of ionising radiations such as alpha and beta particles
and gamma rays. Radon emits alpha particles and has a half-life of 3.8 days. The decay chain
of Uranium 238 with its ionising radiation and half-life period is given in table 1.
The concentration of radon gas is not measured directly but rather by the radioactivity it
produces. In metric system it is expressed in Becquerels per cubic meter (Bq/m3). One
Becquerel means one radioactive disintegration per second. It is also expressed in Pico Curies
per litre of air (pCi/L). A Curie is a unit of radioactivity equivalent to 1 gram of radium and
the prefix ‘pico’ means trillionth. 4pCi/L equals to 148 Bq/m3. Radon concentration in water
is generally expressed in Becquerels per liter (Bq/l).
Mineral
Uranium-238
Thorium-234
Proctanium-234
Thorium-230
Radium-226
Radon-222
Polonium-218
Lead-214
Bismuth-214
Table1: The Uranium –238 decay series
Particle/energy
Half life
4.5
billion years
α
24.5 days
β and γ
2,69,000 years
β and γ
83000 years
α and γ
1590 years
α and γ
3.825 days
α
3.05minutes
α
26.8 minutes
β and γ
19.7 minutes
α or β and γ
Polonium-214
Thallium-210
Lead-210
Bismuth-210
Polonium-210
α
β
β and γ
β
α and γ
150 micro seconds
1.32 minutes
22 years
5 days
138 days
Daughter product
Thorium-234
Proctanium-234
Thorium- 230
Radium-226
Radon-222
Polonium-218
Lead-214
Bismuth-214
Thallium-210/
Polonium-214
Thallium-210
Lead-210
Bismuth-210
Polonium-210
Lead-206
The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR)
have suggested a value of radon concentration in water for human consumption between 4 and
40 Bq/l. Radon concentrations in groundwater can range up to three orders of magnitude with
high spatial variations in the same geologic unit. An important factor affecting the
concentration of radon in groundwater is the underlying geologic unit. Its occurrence and
distribution in groundwater vary among different rock types and can vary considerably within
the same geologic formation. Many factors affect the presence and movement of radon in
groundwater, such as mineralogy, uranium content, grain size, permeability, the nature and
extent of fracturing in the host rock, and is tectonic history. Rocks such as schist typically
37
contain higher concentrations of uranium than rocks such as limestone and hence, higher
groundwater radon concentrations are expected in schistose rocks. The range of radon
concentrations probably reflects the variable distribution of uranium or radium in the aquifer
and the variable distribution of aquifer properties. Radon moves from its source in rocks and
soils through voids and fractures and carried by groundwater.
Radon concentration in groundwater
When groundwater moves through radium / radon bearing rocks they are dissolved and
transported with water. The adverse health effects of radon in water are largely due to transfer
of radon to the air. The daughter products of radon gas are radioactive solids. The deposition
of inhaled short lived radon progeny in the respiratory tract may cause lung cancer. The
International commission on Radiological Protection (ICRP) in its publication 65 has
stipulated an action level of 200 Bq/m3 for radon concentration in dwellings. Radon activity
of approximately 400Bq/l in water would result in a concentration of about 150 Bq/m3 in the
air above the water. Dunduli et al.(1984) suggested that even water borne radon levels up to
400 Bq/l do not increase the risk of stomach or intestinal cancer by direct ingestion. A
Maximum Concentration Level (MCL) of radon in public drinking water supplies in the range
of 37-740 Bq/l was suggested by Cross et al.(1985). The US Environmental protection agency
in 1982 recommended remedial action at an activity level of 400 Bq/l in water and in 1991 it
proposed an MCL of 11.1 Bq/l for public water supply. The health risks due to the inhalation
and ingestion of radon and its short lived daughter products have been well documented by
Mills(1990) and Craw ford Brown (1991).
Radon concentration in groundwater may vary with time because of factors such as dilution by
recharge and changes in recharge area due to pumping etc. The seasonal changes may be high
or low depending on the factors responsible for enrichment of radon in groundwater. A study
on radon concentration in tube wells by Sonkawade et al.(1984) found that de-ionisation of
water reduces the radon concentration. Also, the concentration of radon was found to be
inversely correlated with the pH value of water samples.
Radon contamination of groundwater in India
Radon contamination in groundwater is reported from many parts of India. Radon
contamination in groundwater of Dehradun city, India was reported by Ramola et al.((1999).
They have recorded radon concentration in the range of 27 to 154 Bq/l in water from hand
pumps with an average of 67 Bq/l and in the range of 26 to 129 Bq/l with an average of 59
Bq/l in tube wells. These values are much above the safe limits specified in international
recommendations. They attribute the tectonic features (thrust, fault, lineament, fracture etc)
which provide high permeability for movement of radon from depth responsible for high
concentration of radon in groundwater. Similarly, radon concentration in groundwater and soil
in the Uttar kashi region of the Himalayan terrains is found to be tectonically controlled
(Ramola et al. 2008). The radon concentration in the soil and water does not show any
correlation with the uranium and thorium content in the soil in this region, which is indicative
of its structural control.
The study by Choubey et al. (2001) in Doon Valley found a positive correlation of radon in
groundwater with depth of the wells. There was no correlation with water temperature and pH
of water. The spatial variations in radon concentration in groundwater was attributed to neotectonic activity and geohydrological processes that occurs in the area.
38
Choubey and Ramola (1997) reported high radon concentrations in soil, air and groundwater
in Bhilangana valley, Garhwal Himalaya, India that varies from 1KBq/m3 to 57KBq/m3 in
soil, 5KBq/l to 887KBq/l in water and 95KBq/m3 to 208KBq/m3 in air. These high
concentrations are due to associated uranium mineralisation in that area.
A recent study conducted by the Central Ground Water Board in Bengaluru city has indicated
radon concentration in the range of 81 to 1189 Bq/l in in the bore wells and 56 Bq/l in a dug
well. The study also indicated positive correlation between geology and radon concentration
(CGWB-SWR 2009).
Applications of radon
Radon emanation and migration in the lithosphere and atmosphere has been subjected to
numerous studies and efforts have been made to correlate soil gas radon concentration with
factors such as geology, soil porosity, shears and faults ( Fleischer and Mogrocampero,1978;Ramola et.al,1989; Choubey et al, 1997). An increased content of radon is
often observed in soil gases over faults and fractured zones, due to increased flow of water
along these discontinuities. This property has been successfully used in some areas for
groundwater exploration in basement hard rocks (pointet,1989; wright,1992). Radon has been
used as a pointer to buried uranium deposits and as a possible tool for earthquake predictions
(king,1980). Radon as a natural tracer can be used as an indicator of submarine groundwater
discharge along sea coast (Noble et al. 2009).
Conclusions
Various studies conducted in different terrains on the concentration of radon in groundwater
indicates a direct relation between the presence of uranium and thorium in the parent rock and
radon enrichment in groundwater. In tectonically disturbed areas high radon concentration in
groundwater is observed due to contribution of radon from greater depths. Spatial variations in
radon concentrations are generally related to changes in geology, soil type, and structural
controls. High radon concentrations in groundwater and soil are observed above structural
planes like fault, fracture, fold, and lineaments. It is used as a natural tracer in many
hydrogeological investigations and for quantifying submarine discharge along sea coats.
References
Central Ground Water Board -SWR (2009) Occurrence of radon gas in groundwater in
Bangalore, Bangalore urban district, Karnataka. Unp. Report.
Choubey VM, Ramola RC, (1997) Correlation between geology and radon levels in
groundwater, soil,and indoor air in Bhilangana Valley, Garhwal Himalaya, India. Envron Geol
32(4): 258-262.
Choubey VM, Sharma KK, Ramola RC, (1997) Preliminary investigations of geology and
radon occurrence around Jari in Parvati valley, Himachal Pradesh, India. Jour Environ
Radioactivity 34: 139-148.
Choubey V.M, Bartarya S.K, Saini N.K, Ramola R.C (2001) impact of geohydrology and
neotectonic activity on radon concentration in groundwater of intermontane Doon valley,
India. Environmental Geology 40(3): 257-266.
Craw ford-Brown D.J (1991) Risk and uncertainty analysis for radon in dinking water: Final
report. Chapel Hill, American Water Works Association.
39
Cross F.T, Harley N.H, Hofmann W (1985) Health effects and risks from Radon-222 in
drinking water. Health Phys. 48: 649-670.
Dunduli W.P, Bell W.J, Keene B.E, Dostie P.J, (1984). Radon-222 in the gastrointestinal tract
: A proposed modification of ICRP 30 model. Health Phys. 47: 243-252.
Fleischer R.L, Mogro-Campero A (1978) Mapping of integrated radon emanation for
detection of long distance migration of gases within the earth: techniques and principle. J
Geophys Res 83: 3539-3549.
King CY (1980) Episodic radon changes in subsurface soil gas along active fault and possible
relation to earthquakes. J Geophys Res 85(B6): 3065-3078.
Mills W.A (1990) Risk assessment and control management of radon in drinking water, In
Cthern CR, Robers PA (eds): Radon radium and uranium in water. Chalsea, Lewis publishers,
pp 27-37.
Noble Jacob,.Suresh Babu D.S, Shivanna.K (2009) Radon as an indicator of submarine
groundwater discharge in coastal regions. Current Sci. vol.9 pp 1313-1320
Pointet,T. (1989) Hydrogological prospecting of hard rock aquifers in semi-arid climates for
small scale irrigation. Abstracts 28th IGC, Vol.2, p.620, Washington.
Ramola R.C, Sandhu AS, Singh M, Singh S, Virk HS (1989) Geochemical exploration of
uranium using radon measurement techniques. Nucl Geophys 3: 57-69.
RamolaR.C, Choubey V.M, Saini N.K, Bartarya S.K (1999) Occurrence of radon in the
drinking water of dehradun city, India. Indoor Built Environ. 8: 67-70.
Ramola R.C, Choubey V.M, Negi M.S, Yogesh Prasad, Ganesh Prasad (2008). Radon
occurrence in soil-gas and groundwater around an active landslide. Radiation Measurements.
43: 98-101.
Sonkawade R.G, Rewa Ram, Knjilal D.K, Ramola R.C (2004) Radon in tube well drinking
water and indoor air. Indoor Built Environ. 13: 383-385
United Nations Scientific Committee on the Effects of Atomic Radiation: sources and effects
of ionising radiation. New York, United Nations, 1993.
UNSCEAR (1982) Exposures to radon and thoron and their decay products. Report to the
general Assembly.
US EPA (1991) National Primary drinking water regulations: Radionuclids: Proposed rules.
Federal register. 56: 33050-33127.
Wright, E.P. (1992) The hydrogeology of crystalline basement aquifers in Africa, in ‘The
hydrogeology of crystalline basement aquifers in Africa’ (eds. E.P.Wright and W.G.Burgess)
Geol. Soc. Special Publ. No. 66, The Geological Society, London, pp. 1-27.
40
Radiogenic contamination of groundwater in India
P K Mehrotra
Director
Ministry of Water Resources
[email protected]
Introduction
Water quality is a major emerging concern throughout the world. Drinking water sources are
threatened from contamination, with far-reaching consequences for the health of children and
for the economic and social development of communities. Deteriorating water quality also
threatens the targets of MDG of reducing by 50% the proportion of people without sustainable
access to safe water. The theme of the World Water Day – 2010 focuses on water quality.
Ground water is emerging as an essential and vital component of our life support system.
Today, the ground water resources are being utilized for drinking, irrigation and industrial
purposes. However, due to rapid increase in population, urbanization, industrialization and
intensive agriculture activities ground water resources are getting stressed with contamination.
There is growing concern on deterioration of ground water quality due to geogenic and
anthropogenic activities.
Increase in overall salinity of the ground water, presence of high concentrations of fluoride,
nitrate, iron, arsenic, total hardness and few toxic metal ions have been noticed in several
states of India. These constituents determine the suitability of ground water for various uses.
The latest yet, a potential addition to this are radiogenic contamination. Uranium and its
daughter product radon are two naturally occurring element that can lead to health problems if
present in high concentrations in groundwater. Countries that have problems with natural
radioactivity in their water include Sweden, Finland, Norway, USA, Canada, India, Iran and
Brazil among other. Various standards are in vogue for determining quality of ground water
for drinking, agriculture and industrial water uses.
Status of Ground Water Quality in India
The ground water quality of India overall is good and generally suitable for drinking,
irrigation, and industrial use. Ground water quality analysis reveals that the ground water in
shallow aquifers is of calcium bicarbonate type and mixed type. However, other types of water
are also available including sodium chloride type of water. Occurrences of high salinity and
contamination of fluoride, arsenic, iron, nitrate etc has been observed in ground water in some
pockets of the country.
Like surface water pollution, groundwater is also susceptible to contamination from various
natural and man-made sources including radiogenic contaminants. Groundwater is connected
41
to surface water such as rivers, streams and lakes. In fact, there is continuous exchange of
water between surface water and groundwater.
Coal, used in the thermal power plants, contains low levels of uranium, thorium and other
‘naturally occurring radioactive materials’ (NORMs). Their release into the environment leads
to radio-active contamination. These NORMs are present only as trace impurities in the coal,
but they tend to get concentrated in the residues once the coal is burnt in thermal plants. A
substantial amount of these NORMs are released into the environment through the flue gases,
fly-ash and the furnace bottom ash which keep accumulating in the ash-ponds. From ashponds they tend to leach into the groundwater sources causing its contamination.
Living beings have always been exposed throughout their period of existence to naturally
occurring radiation and radioactive materials specifically, naturally occurring radio-nuclides
present in variable amounts in the environment. The distribution of naturally-occurring
uranium, radon, and other radioactive elements, radio-nuclides, depends on the distribution of
rocks from which they originate and the processes which concentrate them. The largest
amount of the natural radiation exposure which we receive comes from a radioactive gas,
radon. Today, radon monitoring has become a global phenomenon due to its health hazard.
More than 55% radiation dose delivered to human kind on this globe from all natural sources
is due to radon alone. Radon is emitted from uranium, a naturally occurring mineral in rocks
and soil; thus, radon is present virtually everywhere on the earth, but particularly over land.
Although it cannot be detected by a person’s senses, radon and its radioactive by-products are
a health concern because they can cause lung cancer when inhaled over long durations.
Contamination of Groundwater by Radon
Radiation is a natural part of the environment in which we live (including water and air). All
living being receive exposure from naturally occurring radioactivity in soil, water, air and
food. The largest portion of the natural radiation exposure which we receive comes from a
single source, the radioactive gas, radon. Radon monitoring has become a global phenomenon
due to its health hazard. More than 55% radiation dose delivered to human kind in the world
from all natural sources is due to radon alone. Radon is emitted from uranium, a naturally
occurring mineral in rocks and soil; within the rock strata as an intermediate decay product of
the U/Th radioactive series thus, radon is present virtually everywhere on the earth, but
particularly over land. It cannot be detected by a person’s senses; radon and its radioactive byproducts are a health concern because they can cause lung cancer when inhaled over long
durations.
Radon gas is partially soluble in water. The solubility co-efficient of radon in water is 0.254 at
20 ºC. Although, radon gas in drinking water itself does not pose a direct health risk, the main
concern is that the levels of radon in indoor air of any housing dwellings can be enhanced
partially by radon derived from water supply. High radon concentration has been reported in
river waters of Garhwal and Siwalik Himalayas and underground waters of the Doon valley,
Himachal Pradesh (Kullu, Kangra, Una and Hamirpur Districts), Punjab (Gurdaspur and
Bathinda Districts) and Bangalore in Karnataka. Extremely high uranium content was reported
in groundwater of Bathinda district in the Punjab state which may be one of the causes of high
radon content. While the thermal springs at Kasol in the Himachal Pradesh show the highest
value of radon concentration, the natural spring of Chinnjra in the Kullu district of Himachal
Pradesh shows the highest value in the cold water. The radon values in groundwater of
Himachal Pradesh, which is part of Siwalik Himalaya, have higher values than in Punjab. The
42
average radon concentration increases as we move from Punjab plains towards Siwalik
Himalaya. Radon concentration is usually much higher in groundwater than in surface water.
The observed values of radon concentration in ground waters of different areas of districts of
Himachal Pradesh and Punjab states are within the international recommended limit and hence
safe for drinking.
Contamination by Uranium
The Uranium concentration in groundwater depends on lithology, geomorphology and other
geological conditions of the region. In groundwater uranium is present both in dissolved and
particulate form. More than radioactivity Uranium has been identified as a nephrotoxin by the
world Health Organisation (WHO).implying it is a naturally produced chemical, which may
cause kidney damage problems.
A‘high’ concentration of uranium has been detected in the groundwater of the Bathinda area.
This presence of uranium makes the availability of clean drinking water to village residents
extremely difficult. Ninety samples of groundwater taken from tube wells and hand pumps
from 22 villages of Bathinda district were tested jointly with experts from BARC and Kota
Research Centre of the Kota Atomic Research Station. Compared to the liberal’ WHO limit of
15 mg/litre, more than 77 per cent of samples failed the limit and groundwater of 17 out of 22
villages tested, remains unfit for human consumption.
A part from Punjab uranium has been reported in the groundwater of Varanasi in UP, Ranchi
in Jharkhand, Chikmangalore and Kolar in Karnataka, Sikar in Rajasthan and West Khasi Hill
district in Meghalaya.
The safe level of uranium in drinking water needs to be carefully fixed. Additionally,
safeguards need to be built into our exiting and proposed thermal power plants to reduce the
risks of uranium contamination of the ground water resources. Apart from the safe drinking
water for human, we also need to consider the safety of the entire food chain involving
livestock as well as the food-crops which would be using the pumped-out groundwater.
Radium Contamination
Radium (Ra) is again a naturally occurring radioactive element that is present in varying
amounts in rocks and soil within the earth’s crust. Small amounts of radium also can be found
in groundwater supplies. Radium can be present in form of several isotopes. Surface water is
usually low in radium but groundwater can contain high levels of radium depending on local
geology. Deep bedrock aquifers used for drinking water sometimes contain levels of Ra-226
and Ra-228 that exceed health-based regulatory standards.
Radium in water may pose a hazard to human health when the water is used for drinking or
cooking purposes. Only a small portion of ingested radium is absorbed from the digestive tract
and distributed throughout the body. The rest is passed unchanged from the body. Some
portion of the absorbed radium is excreted in urine. Absorbed radium behaves similarly to
calcium and is deposited in the tissues of the body, especially bone. Any radiation received
externally through showering, washing, or other uses is not a hazard since alpha particles do
not travel through your skin. Internally deposited radium emits alpha particles that may then
damage surrounding tissues.
43
Based upon our current knowledge, it is assumed that any radiation exposure carries some
degree of risk. A number of treatment methods are available to remove radium from water.
Ion exchange, lime softening, and reverse osmosis are the most common and can remove up to
90 percent of radium present. Ion exchange (i.e. water softeners) can often remove 90 percent
of radium present along with water hardness.
Thorium in Groundwater
Thorium is a radioactive element that occurs naturally in low concentrations (about 10 parts
per million) in the earth’s crust. It is about three times as abundant as uranium and about as
abundant as lead or molybdenum. Thorium can be taken into the body by eating food,
drinking water, or breathing air. Most thorium that is inhaled or ingested in food and water is
excreted within a few days, with only a small fraction being absorbed into the bloodstream.
Gastrointestinal absorption from food or water is the principal source of internally deposited
thorium in the general population. Thorium is generally a health hazard only if it is taken into
the body. External gamma exposure is not a major concern because thorium emits only a small
amount of gamma radiation. The main health concern for environmental exposures is
generally bone cancer. The acidic leaching of uranium tailing piles in certain areas is a source
of thorium-230 in surface water and groundwater. The Jadugoda area of Jharkhand is typical
example of it.
Issues in Preventing Groundwater Contamination and Pollution
The first step towards evolving measures to prevent and cure groundwater quality
deterioration, as stated above, is generating reliable and accurate information and data bank
through water quality monitoring (WQM). This will enable us to understand the actual
source/cause, type and level of contamination. However, there are a few observation stations
in the country that cover all the essential parameters for water quality. These are mostly under
Central Pollution Control Board (CPCB) and CGWB. Hence, there is limited data and the
data obtained are not decisive on the water quality status. Further, WQM involve expensive
and sophisticated equipments. These are difficult to operate and maintain and require
substantial expertise in collecting, analyzing and managing samples and the data collected.
The existing methodology, availability of instruments and trained manpower for WQM is
inadequate to identify the various sources of pollution. In the absence of any stringent norms
on water quality testing, results can change across agencies depending on sampling procedure,
time of testing, and testing instruments and procedure. However, the Water Quality
Assessment Authority (WQAA), has circulated to all the states Uniform Water Quality
Testing Protocol for maintaining the uniformity.
In India, groundwater quality monitoring is primarily the concern of the Central Ground Water
Board and state groundwater agencies, where each of them set up their monitoring network.
CPCB also monitors few hundred stations. But, there are issues concerning adequacy of
scientific data available from them. The network of monitoring stations is not dense enough
and they are tested only once in a year. Water quality analysis excludes critical parameters.
The pollution by fertilizer and pesticide, heavy metals and other toxic effluents including
radiogenic pollutants are normally not tested by them. There is virtually no agency taking up
the matter of pollution by radioactive contaminants.
44
The available scientific data, particularly that on pollution is of civil society institutions. There
is a paucity of such institutions that are capable of carrying out such professionally
challenging, technologically sophisticated, and often politically sensitive tasks.
Challenges Ahead
The available treatment systems work on the principles of physics and chemistry. Thus, the
efficiency of the treatment process/equipment depends heavily on maintaining certain
specified operating conditions. This would call upon qualified technical manpower for system
operations and regular maintenance.
Most of the treatment systems for drinking water have to be tried out at the community level
to be cost effective and affordable. Since, there is no major revenues being generated from
domestic water supply services, any additional investment for provision of safe water for
drinking and cooking purposes would induce unprecedented financial burden. Therefore, there
is an urgent need of research in this area for developing low cost technology.
The unit cost of production can also be brought down considerably by running the plant at
peak capacity. This means there should be sufficient demand for clean water generated
through this process. Optimal plant design, proper selection of membrane, generating
sufficient demand etc. are important for bringing down the cost of production. The level of
professional inputs that go into management of public water supply systems would be far less
than adequate to manage these systems. Over and above, operating costs for agency-run
systems are likely to be high due to high administrative overheads. As a result, new techno
institutional models need to be evolved to manage the system in order to make them self
sustaining. Involving private sector in provision of clean and safe drinking water would be a
major step towards achieving this. Alternatively, this is a fit area for Public Private Partnership
(PPP) with viability gap funding model.
Pollution Control Framework
The task of controlling radiogenic pollution today is not easy. The tremendous amount of
types and sources of water pollution, in addition to its complex nature, calls for conducting
much study and research into pollution problems. There is also a need to systematically
develop database on possible effects of the radioactive pollutants on the human body. The
most effective means of controlling pollution results from cooperation between scientists,
legislators, citizens and industry.
The Views expressed here are of the author and do not represent the view of the
Government/Ministry
45
Radon: health hazards and remedial measures
V. Meenakshisundaram
Radiation Safety Section
Radiological Safety Division, Safety Group
Indira Gandhi Centre for Atomic Research
Kalpakam – 603102
[email protected]
Abstract
Natural radiation is the largest contributor to the collective dose to the world population. The
major contribution of dose from natural radiation in normal background regions arises due to
inhalation of air-borne radon and its progeny. The need for measurement of radon in the
environment, the quantities and units for measurement of radon isotopes, health hazards due to
intake of radon gas especially from building materials and water and remedial measures to
avoid their intake are highlighted. An overview of the increased health hazards due to new
scientific findings and thereby increasing the effective dose/unit of exposure to radon and its
progeny by nearly a factor of two by International Commission on Radiation Protection is
summarized in brief.
Introduction
Exposure to human population from ionizing radiation originates mainly from naturally
occurring radioactive sources (approximately 90%). Naturally occurring radiation and
radioactivity present in the atmosphere and on the earth’s crust can further be classified into
‘virgin’ and ‘modified’ natural sources. ‘Virgin’ sources of radiation are of cosmogenic or
primordial origin and have existed on our earth since primordial times known as Naturally
Occurring Radioactive Material (NORM). ‘Modified’ natural sources are mainly from
activities like mining, usage of fossil fuel, production of fertilizers or usage of natural
materials for building constructions. The latter is known as Technologically Enhanced
Naturally occurring Radioactive Material (TENORM). As mentioned earlier, natural radiation
is the largest contributor to the collective dose to the world population. Relatively constant
exposure to the population at a location is the distinctive characteristics of this radiation. The
major contribution of dose from natural radiation in normal background regions arises due to
inhalation of air-borne radon and its progeny and to a certain extent, due to thoron and its
progeny and also to a lesser extent via the ingestion of radon dissolved in water. The dynamics
of radon, thoron and their progeny in the atmosphere has been the subject of scientific studies
for several decades.
Isotopes of radon
Of the three isotopes of radon viz., Rn-219 (Actinon), Rn-220 (Thoron) and Rn-222 (Radon),
Rn-222 has a long half-life of 3.82 days and decays by alpha emission through a series of
solid, short lived radioisotopes that are collectively called radon daughters or progeny. Two
of the daughters in the uranium chain Po-218 and Po-214 are also alpha emitters. Po-218 has
a half-life of 3.11 minutes and Po-214 has a half-life of just 0.0002 seconds. There are two
46
daughter products in the radon-222 chain viz., Pb-210 and Po-210, with the half-lives of 21.8
years and 140 days respectively. In view of their relatively long half-lives and consequent
insignificant activity buildup they are not considered in the hazard evaluation of radon
daughters since they do not cause almost no damage to tissues.
Radon-220 which is also called as thoron since it occurs in the thorium decay chain has halflife of 55.3 seconds. Because of this shorter half-life, in most of the radon exposure studies,
the contribution due to Rn-220 is usually neglected, though from the health hazards to lungs
point of view, the daughter products of Rn-220 viz., Po-216 and Po-212 have the same effect
as that of Rn-222 daughter products. In fact, the alpha energies of Po-216 (6.78 MeV) is
higher than Po-218 and similarly the alpha energy of Po-212 (8.78 MeV) is higher than Po214 (7.69 MeV). Since in India, the thorium content in the soil along the coastal line is
appreciable, the effect due to thoron is equally important and hence the same has to be
evaluated along with Rn-222.
Radon-219, which is also called as Actinon, has a negligibly shorter half-life of 3.9 seconds
and hence it is not being considered at all.
The decay products of radon and thoron are divided into two groups viz., short-lived radon
daughters 218Po (RaA: 3.05 m), 214Pb (RaB: 26.8 m), 214Bi (RaC: 19.7 m) and 214Po (RaC’: 164
µsec) with half-lives below 30 minutes and long-lived radon decay products 210Pb (RaD: 22.3
y), 210Bi (RaE: 5.01 d) and 210Po (RaF: 138.4 d). There is no long-lived group for the thoron
progeny. Most important radionuclide in this chain is the lead isotope 212Pb with a half-life of
10.6 h. These daughter products, being the isotopes of heavy metals, get attached to the
existing aerosol particles in the atmosphere. Their elimination from the atmosphere occurs
either by radioactive decay or by other removal processes such as plate-out or surface
deposition and washout by rain. The characteristics of 222Rn and their daughter products are
summarized in the following Table.
Radionuclide
Half-life
Rn-222
Po-218
Pb-214
Bi-214
Po-214
Pb-210
Bi-210
Po-210
Pb-206
3.8 d
3.05 m
26.8 m
19.7 m
164 µs
22 y
5d
138 d
Stable
Alpha energy
(MeV)
5.49
6.0
7.69
5.31
Beta energy
(MeV)
0.98, 0.71, 0.65
3.26, 1.51, 1.0
0.016, 0.061
1.16
-
Gamma energy
(MeV)
0.35, 0.30
0.61, 1.77, 1.12
0.799
0.047
-
Need for measurement of radon
Ever since studies on uranium miners established the presence of a positive risk coefficient for
the occurrence of lung cancer in miners exposed to elevated levels of radon and its progeny,
there has been a great upsurge of interest in programs concerned with the measurement of
radon in the environment. This interest was accentuated by the observations of elevated radon
levels in the indoor environment in many countries that led to the realization of residential
radon as being a possible public health issue in the western world. It was also thought that in
conjunction with epidemiological studies, large-scale indoor radon studies might lead to
47
quantitative understanding of the low dose effects of radon exposures. As a result of these,
considerable amount of information is now available on the levels of radon gas and its
progeny in the indoor environment across the globe. Also there exists a good knowledge on
the total external radiation levels. Since it is estimated that nearly 50% of the total radiation
dose to the population, from naturally occurring radioactive materials (NORM), is contributed
by the inhalation dose due to radon isotopes and their progeny (UNSCEAR 2000), there is a
need to supplement the external exposure data with the inhalation component.
In contrast, there exist only a few studies related to measurements of thoron in the
environment since it is assumed that the inhalation dose to the general population from thoron
and its progeny is negligible as compared to radon and its progeny. But, later studies
indicated that this assumption is incorrect. For Indian conditions, where thorium is available
in the earth’s crust in abundance, the study (BARC-2003), revealed that the indoor thoron and
progeny concentrations and corresponding inhalation dose rates are found to be about half of
that due to radon and its progeny.
The concentration of radon and thoron and their daughters is often a site-specified one. It
depends on the uranium and thorium content in the soil, its permeability, building materials,
water etc. One square meter of typical soil containing radium at 0.03 Bq/g will release
between 1000 and 2000 Bq of radon to the atmosphere each day (UNSCEAR 1988). Other
sources of radon include ground water that passes through radium-bearing rocks and soils,
traditional building materials, uranium tailings, coal residues and fossil fuel combustion.
Besides, meteorological parameters such as ambient temperature, wind velocity, humidity and
rainfall would also influence the atmospheric radon gas content and hence the need for
collecting data from different regions.
Quantities and units for measurement of radon isotopes
As is well known, the dose to the lung is primarily contributed by the radon progeny species
deposited in the respiratory organs (Jacobi and Eisfield, 1980). This dose is a function of
several variables such as the activity-size distribution of the species, breathing parameters etc.
However, standard calculations have been made considering typical values for these
parameters. Dose conversion factors estimated by this mode, relate the default dose to the
lung per unit concentration in air. However, the radon and thoron progeny concentrations in
view of their short-lived nature are expressed in terms of a special quantity known as Potential
Alpha Energy Concentration (PAEC), usually expressed in the units of J/m3. PAEC of an
atom in the decay chain is defined as the total alpha energies emitted during the decay of this
atom along the decay chain starting from 218Po upto 210Pb or 208Pb for radon or thoron
respectively. PAEC of any mixture of short-lived radon or thoron progenies in air is the sum
of the potential alpha energy of all daughter product atoms present per unit volume of air.
Thus, The PAEC is expressed in SI units as 1 JM-3 (6.24 x 109 MeV per litre). For practical
purposes, it is expressed in another special unit known as the Working Level (WL), and used
for quantifying the PAEC for radiation protection purposes.
One WL =1.3 x 105 MeV/litre =2.08 x 10-5 JM-3
One WL corresponds approximately to the potential alpha energy of short-lived radon
daughters in air which are in radioactive equilibrium with a radon activity concentration of
3700 Bq/m3. One WL for the thoron daughters corresponds approximately to an equilibrium
thoron activity concentration of 275 Bq/m3. Radon, thoron progeny concentration exposure is
48
often expressed in terms of Working Level Month (WLM), which corresponds to an exposure
of one WL during a reference-working period of one month (2000 working hours per year / 12
months = 170 hours).
Health hazards
Radon and thoron are noble gases and are therefore non-reactive. Once formed they are free
to diffuse out of ground or dissolve in water without getting chemically bound anywhere along
the way. Since the inhaled radon and thoron have very small residence times in the lungs and
it is only the short-lived alpha emitting daughter products are of health concern as they can get
trapped in airways. The range of alpha particle from these isotopes is about 70 µm. The
epithelium where the stem cells are located is 40 µm thick only and as such they are within the
target range of the alpha particles and susceptible to cancer. International Commission on
Radiation Protection (ICRP) have found, as given in its publication #50 itself, that a positive
linear exposure-risk relationship is found with the available epidemiological and experiment
data on domestic radon and lung cancer from several countries.
Human exposure to radon and its progeny
There is a consensus among several organizations, including International Commission on
Radiological Protection (ICRP), International Agency for Research on Cancer (IARC), World
Health Organization (WHO), United Nations Scientific Committee on the Effects of Atomic
Radiation (UNSCEAR), and National Radiological Protection Bodies of Germany, the Nordic
countries, USA and UK, that there is a strong link between exposure to radon and the
occurrence of lung cancer. Based on the scientific information and cohort studies, ICRP
recommends that there is a need to limit the exposure due to radon and its daughter products.
When an individual is exposed to radon and its progenies, the part of the body that receives
the highest dose of ionizing radiation is the bronchial epithelium, although the extra thoracic
airways and the skin may also receive appreciable doses. In addition, other organs, including
the kidney and bone marrow, may receive low doses. Radon is soluble in water and its
solubility decreases rapidly with increasing temperature [(510, 230, 169 cm3/kg at 0˚C, 20˚C
and 30˚C respectively), (NCRP Report #97)]. Radon is extremely volatile and is readily
released from water. If an individual drinks water in which radon is dissolved, various organs
of stomach might also be exposed. Of late, more scientific research work is geared-up in many
countries to find out whether there is any correlation of stomach cancer vis-à-vis the radon
exposures due to ingestion. A case control study of stomach cancer in an area where there
were high concentrations of natural Uranium and other radionuclides in drinking water gave
no indication of an increased risk (Auvinen et al.2005). The following are the estimates of the
proportion of lung cancer attributable to radon in selected countries. The information is
provided by WHO. Other countries are still continuing their studies and yet to report the
values. As per WHO 2009 report, the worldwide average indoor radon concentration has been
estimated at 39 Bqm-3.
Country
Canada
Germany
Switzerland
UK
Mean indoor
Rn (Bqm-3)
28
49
78
21
% of lung cancer
attributed to Rn
7.8
5
8.3
3.3
49
Estimated deaths due
to Rn induced lung
cancer each year
1400
1896
231
1089
France
USA
89
46
5
10-14
1234
15, 400 – 21, 800
Radon in building materials
The building materials such as Cement, sand, Clay bricks, Concrete, Gypsum, Fly Ash,
Granite tiles, if they are rich in Uranium/Radium content will contribute significantly to
indoor radon concentrations. The indoor radon concentrations may vary depending on radon
entry and removal rates, occupancy patterns, as well as the frequency and duration of door and
window openings (ventilation). Radon exhalation rates (Bq.m-2.h-1) from the building
materials have been studied extensively.
Based on the results of a co-ordinated research project sponsored by DAE in eight universities
and a few research institutions in India, it was concluded that dwellings in India do not need
any remedial action against radon because they have adequate ventilation (BARC, 2003).
With respect to inhalation risk, several national and international organizations have
developed risk models based on epidemiological radiobiological data for radon. Risk
projections made using three of these models (NCRP Report #97, 1988, ICRP, 1987, and US
NAS, 1988) suggest that the average lifetime risk from inhalation exposure to radon daughters
is likely to be in the order of less than 100 cases per million WLM to perhaps 500 cases per
million WLM, with the lower value being applicable to females and non-smoking males and
the higher value being applicable to a mixed population of females and smoking and nonsmoking males (SENES, 1990).
Comparatively few epidemiological studies have investigated the exposure to natural
background radon levels and those that are available show no significant increase in lung
cancer death rate from inhalation exposure to normally occurring levels of radon and its
progeny (Cross et al., 1985; Letourneau et al., 1983 and Letourneau et al., 1994).
Radon in Water
The risk due to radon in drinking water derived from ground water is typically low compared
with that due to total inhaled radon. Underground rock containing natural uranium
continuously releases radon into water in contact with it (ground water). Ground water has
potentially much higher concentrations of radon than surface water and radon is readily
released from surface water. The average concentration of radon is usually less than 0.4
Bq/litre in public water supplies derived from surface waters and about 20 Bq/litre from
ground water sources. Higher concentrations were also found in certain cases. Countries like
India should have a program to monitor the radon in water as the ground water is the only
reliable source of water supply for a larger population.
ICRP’s concern on radiation hazards due to intake of radon
The following are the official statements released on radiological aspects of radon as approved
by ICRP in November 2009 (ICRP Ref: 00/902/09 Page 1 of 2) which are of very much
relevance to the topic. With the availability of more and more scientific inputs and increased
understanding of the new findings, the health hazards due to intake of radon isotopes appear to
have increased as per ICRP.
The Commission issued revised recommendations for a System of Radiological Protection in
2007 (ICRP, 2007) which formally replaced the Commission’s 1990 Recommendations
(ICRP, 1991) and updated, consolidated, and developed the additional guidance on the control
50
of exposure from radiation sources. The Commission has previously issued recommendations
for protection against radon-222 at home and at work in its Publication 65 (ICRP, 1993).
The Commission has now reviewed recently available scientific information on the health
effects attributable to exposure to radon and its decay products. The Commission’s full review
accompanies this Statement. As a result of this review, for radiological protection purposes the
Commission now recommends a detriment-adjusted nominal risk coefficient for a population
of all ages of 8x10-10 per Bq h m-3 for exposure to radon-222 gas in equilibrium with its
progeny (i.e., 5x10-4 WLM-1). The Commission’s findings are consistent with other
comprehensive estimates including that submitted to the United Nations General Assembly by
the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR,
2009).
Following from the 2007 Recommendations, the Commission will soon publish revised dose
coefficients for the inhalation and ingestion of radionuclides. The Commission now proposes
that the same approach be applied to intakes of radon and progeny as that applied to other
radionuclides, using reference biokinetic and dosimetric models. Dose coefficients will be
given for different reference conditions of domestic and occupational exposure, taking into
account factors including inhaled aerosol characteristics and disequilibrium between radon
and its progeny. Sufficient information will be given to allow specific calculations to be
performed in a range of situations. Dose coefficients for radon and progeny will replace the
Publication 65 dose conversion convention which is based on nominal values of radiation
detriment derived from epidemiological studies comparing risks from radon and external
radiation. The current dose conversion values may continue to be used until dose coefficients
are available. The Commission advises that the change is likely to result in an increase in
effective dose per unit exposure of around a factor of two.
The Commission reaffirms that radon exposure in dwellings due to unmodified concentrations
of radium-226 in the earth’s crust, or from past practices not conducted within the
Commission’s system of protection, is an existing exposure situation. Furthermore, the
Commission’s protection policy for these situations continues to be based on setting a level of
annual dose of around 10 mSv from radon where action would almost certainly be warranted
to reduce exposure. Taking account of the new findings, the Commission has therefore revised
the upper value for the reference level for radon gas in dwellings from the value in the 2007
Recommendations of 600 Bq m-3 to 300 Bq m-3. National authorities should consider setting
lower reference levels according to local circumstances. All reasonable efforts should be
made, using the principle of optimization of protection, to reduce radon exposures to below
the national reference level. It is noted that the World Health Organization now recommends a
similar approach (WHO, 2009).
Taking account of differences in the lengths of time spent in homes and workplaces of about a
factor of three, a level of radon gas of around 1000 Bq m-3 defines the entry point for applying
occupational protection requirements for existing exposure situations. In Publication 103, the
Commission considered that the internationally established value of 1000 Bq m-3 might be
used globally in the interest of international harmonization of occupational safety standards.
The Commission now recommends 1000 Bq m-3 as the entry point for applying occupational
radiological protection requirements in existing exposure situations. The situation will then be
managed as a planned exposure situation.
51
The Commission reaffirms its policy that, for planned exposure situations, any workers’
exposure to radon incurred as a result of their work, however small, shall be considered as
occupational exposure (paragraph 178 of ICRP, 2007).
Radon measurements
Active and Passive techniques are used for the measurement of radon. Instruments like
Alphaguard, Doseman, DosemanPro, Lucas Cells are widely used. Alpha spectrometry and
Gamma spectrometry are used for spectroscopic analysis of various materials. For the
measurement of integrated radon exposures, Solid State Nuclear Track Detectors (CR-39 and
LR-115 films) are being used. Radon in air can also be detected using charcoal canisters with
a detection limit 20 Bq/l (US EPA, 1986). For the measurement of radon in water, Liquid
Scintillation Counting system is used (Koziowska et al., 2010).
Remedial measures
In buildings, where indoor-radon activity concentrations have been identified as exceeding the
action level, it is prudent to implement remedial measures. The most obvious remedy is to
increase the ventilation of the home / building which allows the radon to escape. Another
method is to prevent the entry of radon into a building after taking into account the economic
feasibility, but it poses major difficulty in determining how the gas enters the building. The
mitigation technique that should be adopted is dependent on, for example, (i) whether it is a
new or existing building, (ii) building construction details, (iii) the requirement of magnitude
of the reduction in indoor radon activity concentrations and, (iv) the associated costs.
Following are some of recommended mitigation techniques.
Pumping of radon from underground to outside
This is particularly effective where large reduction in indoor radon activity concentration is
required. An electric or wind turbine-powered fan is used to suck the radon-laden air from
under the ground-floor of the building and eject it into the atmosphere well above the
uppermost window level.
Sealing the floors
This involves sealing of cracks and/or providing a continuous barrier to reduce the rate of
radon entry. Commercially available radon barriers are generally moisture resistant and hence
fulfill the role of damp-proof membranes. In order to be effective, radon barriers and sealants
should be:
Strong, tough, and resistant to damage
Flexible and able to accommodate the minor structural movements and cracks
without losing integrity
Thermally stable and resist cracking at lowest ambient temperatures
Easily installable (For example, the materials employed should be available in
suitable lengths and widths, and easier methods for providing reliable, and longlasting joints)
Positive pressure systems
Radon enters buildings because of the negative pressure-differential arising between the
internal and external environments, e.g. due to the presence of wind around the building or a
temperature difference between the internal and external environments. Positive pressure
52
reverses this process by maintaining a slight over-pressure within the building, thereby
inhibiting radon entry.
Removal of radon from water
There are two principal ways to remove radon from water supplies dependent on groundwater
sources (water drawn from surface supplies or temporary storage containers will not contain
any appreciable radon). Aeration, which forces radon from the water to the air, can be highly
effective; bubble plate aeration and diffused bubble aeration as point-of-entry units are
capable of achieving removal efficiencies in excess of 99% at loading rates of 185 Bq/l and
more (Dixon et al., 1991; Kinner et al., 1993). The low-tech spray jet aeration technique,
which is the most practical aeration method for small community water supplies, removes
between 50% and 75% of the radon (Dixon et al 1991; RCG/Hagler, Bailly Inc., 1991). One
concern with aeration is the possibility of creating a large source of air-borne radon.
Adsorption through granular activated carbon, with or without ion exchange, can also achieve
high radon removal efficiencies (upto 99.7%, depending on the loading rate) (Kinner et al.,
1993).
Besides above remedial measures, legal regulations for utilization of appropriate raw materials
during building construction on the basis of natural radioactivity content and a method of
certification of natural radioactivity present in the commonly used building materials may
have to be implemented. Such things are already in vogue in certain countries like Poland and
Italy.
Conclusion
Fifty percent of the total radiation dose to the population, from naturally occurring radioactive
material (NORM) is contributed due to intake of radon isotopes and their progeny. Due to the
intake of radon gas, the radioactive radon progeny species are deposited in the respiratory
organs and hence the lung organ receives almost the entire dose. There is a consensus among
internationally reputed scientific organizations which include, inter alia, ICRP, that there is a
strong link between exposure to radon and the occurrence of lung cancer and hence
recommended that there is a need to limit the exposure due to radon and its daughters.
Exposure to radon is essentially due to presence of naturally occurring primordial
radionuclides in open atmosphere, building materials and water. As per WHO, the worldwide
average indoor radon concentration has been estimated at 39 Bq.m-3. Most of the dwellings in
India do not warrant any action levels with respect to indoor radon and thoron due to good
ventilation prevailing in Indian dwellings. There is no significant increase in lung cancer
death from inhalation exposure to normally occurring levels of radon and its progeny. The
risk due to radon in drinking water derived from ground water is typically low compared with
that due to total inhaled radon. In spite of this, it is considered necessary to have more
monitoring programs to measure radon content in ground water. Taking into account the new
scientific findings, ICRP have revised the upper value for the reference level for radon gas in
dwellings from the earlier 600 Bq.m-3 to 300 Bq.m-3 and also likely to revise soon the dose
conversion values due to increase in effective dose per unit of exposure to radon and its
progeny around a factor of two. Legal regulations and certification of natural radioactivity
present in the commonly used building materials may have to be implemented.
Acknowledgement
Sincere thanks to Dr. P. Chellapandi, Director, Safety Group, IGCAR, Dr. B. Venkatraman,
Head, Radiological Safety Division (RSD), IGCAR for their sincere guidance and constant
encouragement and to Shri R. Santhanam, RSD, IGCAR for useful technical discussions.
53
References
Jacobi, W., and Eisfield K., 1980, Dose due tissues and effective dose equivalent by inhalation
of Rn-222 and Rn-220 and their short-lived daughters, GS Rep.No. S-626, Germany
Letourneau et al., 1983, Lung cancer mortality and indoor radon concentrations in 18
Canadian cities, Proceedings of 16th Midyear Topical Symposium on Epidemiology Applied
to Health Physics, Jan 10-14, 1983, Albuquerque, NM. Health Physics Society p470 (1983)
Cross et al., 1985, Health effects and risks from Rn-222 in drinking water, Health Physics
48(5): 649 (1985)
US EPA (1986), Implementation strategy for the radon/radon progeny measurement
proficiency evaluation and quality assurance program. EPA 520/1-86-03. Office of Radiation
Programs, Washington DC (1986)
ICRP, 1987, Lung cancer from indoor exposure to radon daughters. ICRP Publication 50,
Annals of the ICRP Vol.17, No.1 (1987)
NCRP Report No. 97, 1988, Measurement of radon and radon daughters in air.
US NAS (National Academy of Sciences), 1988, Health risks of radon and other internally
deposited alpha-emitters. BEIR IV Committee, National Research Council, Washington DC,
National Academy Press (1988)
SENES, 1990, SENES Consultants Limited. An evaluation of the risk of exposure to radon
daughters. Prepared for the American Mining Congress, April (1990)
ICRP, 1991. 1990 Recommendations of the International Commission on Radiological
Protection. ICRP Publication 60. Ann. ICRP 21 (1-3).
Dixon et al., 1991, Evaluating aeration technology for radon removal, J. Am. Water Works
Assoc., 83(4): 141 (1991)
RCG/Hagler, Bailly Inc., 1991, The cost of compliance with the proposed federal drinking
water standards for radionuclides. Pre-publication copy. Prepared for the American Water
Works Association, October (1991)
ICRP, 1993. Protection against Radon-222 at Home and at Work. ICRP Publication 65. Ann.
ICRP 23 (2).
Kinner et al., 1993, Using POE techniques to remove radon, J. Am. Water Works Assoc.,
85(6): 75 (1993)
Letourneau et al., 1994, Case-control study of residential radon and lung cancer in Winnipeg,
Manitoba, Canada. Am. J. Epidemiol., 140(4): 310 (1994)
BARC, 2003, Radon-thoron level and inhalation dose distribution patterns in Indian
dwellings, BARC/2003/E/026, 2003
54
Auvinen A et al. (2005), Radon and other natural radionuclides in drinking water and risks of
stomach cancer: a case–cohort study in Finland. – Int. J.Cancer, 10, pp 109-113
ICRP, 2007. The 2007 Recommendations of the International Commission on Radiological
Protection. ICRP Publication 103. Ann. ICRP 37 (2–4).
WHO, 2009. World Health Organisation (WHO). WHO Handbook on Indoor Radon: A
Public Health Perspective. WHO Press, Geneva, 2009.
UNSCEAR, 2009. UNSCEAR 2006 Report. Annex E. Sources-to-Effects Assessment for
Radon in Homes and Workplaces. New York: United Nations, 2009.
Koziowska et al., 2010, Radon in ground water and dose estimation for
inhabitants in Spas of the Sudety Mountain area, Poland, Applied Radn &
Isotopes 68 (2010), 854-857
55
Evaluation of submarine groundwater discharge in coastal
regions
Noble Jacob and K. Shivanna
Isotope Applications Division,
Bhabha Atomic Research Centre,
Mumbai – 400 085, India
[email protected]
Abstract
Submarine Groundwater Discharge (SGD) is an important component in the water balance of
coastal aquifers, which is often neglected because of the difficulty in its identification and
estimation. It has to be evaluated for the proper management of groundwater resources and
understanding the coastal pollution. This article briefly describes the various methods
available for the evaluation of SGD. Isotope techniques are found to be more powerful than
conventional methods in understanding the SGD, as they can provide information on coastal
processes occurring over a wide range of temporal and spatial scales. 222Rn is found to be an
excellent tracer for studying the SGD processes because of its conservative nature, easiness in
measurement, high abundance in groundwater compared to surface water, possibility for insitu measurement and its decay rate comparable with the time scales of many coastal
processes. The usefulness of 222Rn in SGD studies are highlighted with a case study.
Introduction
Submarine Groundwater Discharge (SGD) is one of the major components which are
often neglected in the regional water balance computation of coastal aquifers due to the
difficulty in its identification and estimation. Many aquifers in India are under severe stress
because of overexploitation and reduced groundwater recharge due to the recent variability in
the monsoons. Hence, evaluation of SGD is particularly important in the Indian conditions,
because of its long coastline (~7000 km) through which a large amount of utilizable
groundwater may be losing to the sea as submarine groundwater discharge.
Driving forces for submarine groundwater discharge
Coastal areas are natural sites for groundwater discharge from aquifers as they are lying in the
down gradient of the continental regions. Submarine groundwater discharge (SGD) can occur
whenever hydraulic head is above sea level and an aquifer extends beyond the shoreline to
crop out at the sea floor, or is hydraulically coupled with marine waters through permeable
bottom sediments.
SGD is defined as the flow of freshwater into the ocean from a coastal aquifer, or recirculated
seawater through the underlying sediments in the seabed, or a combination of both (Fig. 1).
The mechanisms and the driving forces for the above two components are quite different: the
terrestrial component depends on the hydraulic gradient of the groundwater, the extent and
hydraulic conductivity of the aquifer, recharge rates and many other factors, while the marine
component is controlled by the local oceanographic conditions such as wave set up, tidally-
56
driven oscillations, current induced pressure gradients and convective circulation of water
(thermal or density driven) from the bottom sediments (Taniguchi et al., 2002). Since these
terrestrial and marine driving forces are often superimposed, their relative contributions are
difficult to determine. Tidal effects on SGD can be especially pronounced. High tides, in
general, increase the hydrostatic pressure and resist SGD. Density-coupled modeling of the
saltwater interface indicates that seawater recirculation rates of about 60% or more can occur
due to dispersion and mixing within the aquifer even when wave and tidal effects are ignored.
Since wave-induced and tidal effects are rarely completely absent, a great deal of seawater
recirculation must occur on a global scale, while the local effects of freshwater SGD can be
dominant in nearshore environments. SGD rates are also affected by the groundwater
exploitation of coastal aquifers for domestic, agricultural and industrial purposes.
Fig. 1 Schematic sketch of processes associated with submarine groundwater
discharge (modified from Taniguchi et al., 2002).
Groundwater discharge, either in the form of concentrated or diffused discharge, could be as
large as half the total annual river flow (Moore, 1996). Highest freshwater SGD are generally
found close to the shore. In some places, a well defined seepage face is found, often in the
intertidal zone. In other cases, it is decreasing exponentially from the shoreline. In areas where
seepage occurs through permeable sediments, seepage may be very low and undetectable, yet
a small upward leakage over a wide area could make an important contribution to the coastal
ocean. Alternatively, in karstic aquifers where the flow is intensely channelized and focused,
SGD occurs in the form of springs in the beaches and submerged springs in the coastal waters.
Some times; geological conditions might be such that substantial flow occurs kilometers
offshore, as often observed in the form of underground springs. SGD occurring offshore
would be reflected by the hydraulic heads in the aquifer at the coast that are significantly
greater than mean sea level (with the exception of channelized aquifers, e.g., karst or volcanic
terrain). In such cases, the aquifer may be sealed above by an aquitard that prevents the SGD
at the coast and the saltwater interface which is an indicator of where SGD occurs can be
located offshore.
57
Implications of submarine groundwater discharge
SGD can be a major component of freshwater flow to the coastal zone in areas where surface
runoff is small or variable. The discharge of potable water across the sea floor may be
considered a waste, especially in water scarce regions. In such places, the detection of SGD
may provide as a new source of potable water for various purposes. SGD is also responsible
for limiting salt-water intrusion into the coastal aquifer.
SGD also acts as pathways of large concentrations of nutrients, metals, organic compounds
and inorganic carbon from the continents to the ocean. Even the SGD from the pristine
aquifers can be a source of nitrates and other nutrients to the oceans, as their concentrations
are much higher than seawater. Pollutant input via SGD has been a major cause for the
occurrence of eutrophication in many coastal waters. The effects of SGD in supplying
nutrients or, perhaps, merely reducing the salinity of the coastal waters may flourish the
growth of microalgae and thus causing algal blooms. Another use of better estimates of SGD
would be in the calculation of nonpoint sources of nutrients or chemical contaminants in the
coastal waters receiving SGD.
The magnitude of SGD and its generation mechanisms are indeed poorly understood mainly
because of the non-availability of accurate measurement techniques and many times, the
sources are invisible, slow moving and spatially and temporally variable.
Estimation of submarine groundwater discharge
As a residual term in the water balance computation
Freshwater SGD can be calculated directly as a residual term in the computation of
basin/aquifer scale water balance. Simple box models or hydrological budgets can quantify the
role of SGD in the coastal water balance. These estimates can be useful but they must be used
with caution because often the SGD is determined ‘by difference’ as the component that is
needed to balance the budget. The problem is that SGD is usually small in comparison with
the other parts of the water balance and the SGD estimate may be on the same order as the
errors of the other components. Thus, the combined uncertainties in the known terms
(precipitation, recharge rate, evapotranspiration, etc.) usually make the uncertainty in SGD
estimated by the water budget, unacceptably large. A case study on this method is given in
Smith and Nield, 2003.
Direct measurement of groundwater flow from nested piezometers installed in the
beaches
In this method, a nest of piezometers are installed in the beaches and measure the hydraulic
gradient of the groundwater and hydraulic conductivity of the sediments. By applying the
Darcy’s law, the groundwater flow to the coastal waters can be calculated. A case study on the
estimation of groundwater seepage rate into Lake Biwa, Japan is given by Taniguchi, 1995.
Mathematical modelling
Submarine groundwater discharge can also be quantified by the mathematical
modeling of groundwater flow conditions in a coastal aquifer. Care must be taken in
58
interpreting mathematical models (either analytical or numerical) for the following reasons.
Models typically are designed to forecast the characteristics of the potable water supply. As a
result, they often oversimplify the salt water/freshwater transition. They may assume a
discontinuous, stationary boundary, the saline groundwater may be assumed to be stationary,
or the boundary may be allowed to move and the salt water to flow but not to mix with fresh
water. Such assumptions are simplifications that can be more or less appropriate, depending
upon both the model's intended use and the actual geohydrological conditions. The
recirculation of seawater is an important component of the SGD but is rarely taken into
account in the hydrological models. Even if the model does allow the recirculation of
seawater, offshore data for calibration, verification, and driving the model is usually sparse.
Fortunately, new approaches to coastal hydrological models, with due consideration on both
salt-water intrusion and SGD, are being developed and show great promise. A typical
modeling study for the estimation of SGD in Baltic Sea is described by Kaleris et al., 2002.
Seismic surveys
SGD can be identified from the seismic profile record of the ocean floor. Exploratory
seismic reflection surveys are conducted by a seismic profiler mounted on a vessel. Several
features such as the shape of the SGD plumes and the discontinuation of the seafloor surface
can be delineated from the seismic profile records that may indicate offshore springs. Figure 2
shows a 3.5 KHz seismic profile record in Northern Harbor, Cockburn Sound, Australia
showing two possible ‘spring’ features (Smith and Nield, 2003)
Fig. 2 A seismic profile record in Northern Harbor, Cockburn Sound,
Australia (Smith and Nield, 2003).
Geophysical methods
Salinity decrease in the pore water of bottom sediments can be used to infer the flow of
freshwater causing this change. If the discharges of fresh groundwater are high enough, the
lowering of salinity in the coastal water alone may pinpoint the source of SGD. Conductivity
profiles can be taken by manually inserting a conductivity probe into the sand at various
depths. A conductivity profile record from Cockburn Sound, Australia is shown in figure 3.
These profiles show an along-beach gradient in pore water conductivity, with fresh
groundwater present at the eastern part of the beach. In the profile across the coastline, the
fresh groundwater seems to be restricted to a distance up to ~5 meters seawards from the
59
water line. Further offshore, conductivity appears to be that of seawater. SGD can also be
identified from high resolution resistivity profiles obtained by conducting multi electrode
resistivity surveys
Fig. 3 Conductivity profile across the coastline of Cockburn Sound,
Australia (Smith and Nield, 2003).
Remote Sensing
Remote sensing data can be used to identify SGD. It is based on temperature contrasts
between sea water and fresh water in regions where SGD were supposed to exist. Nowadays,
thermal sensors onboard of satellites such as LANDSAT 7 or ASTER can provide more
accurate information on temperatures contrasts. High spectral and ground resolution will also
help in the identification of geological features such as fault lines etc. prolonging into the sea.
Seepage Meter
SGD can be measured using a seepage meters (flow chambers vented to plastic bags
accompanied by volume and salinity measurements) Fig. 4. The seepage meter consists of a
steel drum (30 cm dia and 15 cm height) attached to a 4 liter capacity polythene bag (Lee,
1977). The polythene bag was pre-filled with a known amount of seawater (~500 mL) before
installing the seepage meter on the sea floor. Any increase or decrease in the volume of water
in the plastic bag after a period of time (1 hr) is measured as the groundwater (inflow or
outflow) flux to the coastal waters. However; this manual method is very labor-intensive and
most useful where the flow rates are relatively high. A few measurements at point locations
can be enlightening but the flow can be extremely variable in both time and space. As a result,
many measurements might be needed to get reliable averages.
60
Due to uncertainties in the estimation of water balance terms (Oberdorfer, 2003), limitations
in obtaining information about the aquifer heterogeneities in the case of hydraulic
measurements from piezometers and groundwater modeling and difficulties in accounting the
recirculation of seawater, the SGD rates obtained from these methods are sometimes
ambiguous; however, they could be used as first-hand estimates (Burnett et al., 2001).
Whereas, seepage meter measurements are labour intensive, difficult to install in rough sea
conditions and they provide only point information. Despite that, the use of seepage meters
showed consistent and reliable results in some studies. (Cable., et al., 1997). Tracer
techniques, including the use of naturally occurring isotopes, on the other hand, provide a
convenient way of assessing the SGD, as they integrate the water fluxes over various spatial
and temporal scales.
Fig. 4 Details of a simple seepage meter (Lee, 1977).
Geochemical tracers
Because of the distinction in geochemical constituents such as salinity, barium, cesium,
phosphate, ammonia etc. and some isotopic ratios such as 234U/238U, 87Sr/86Sr, 2H/1H, 18O/16O
etc. in groundwater and seawater, they could be used to identify fresh groundwater discharges.
The presence of excess methane in the coastal ocean or the absence of methane in sediments
with a high organic content is another indicator of SGD, which flushes the gas from the
sediments. A few studies conducted in the Bengal basin using barium, 226Ra and 87Sr/86Sr
indicates substantial groundwater discharge into the Bay of Bengal through GangaBrahmaputra river systems (Moore, 1997; Basu et al., 2001).
Radium isotopes
Measurements of isotopes in the uranium-thorium decay series have an additional advantage
of tracing and quantifying the brackish water SGD fluxes as well, which have more impact on
coastal environments. The most commonly used isotopes for SGD studies include the quartet
of radium isotopes having half lives ranging from 3.66 days (224Ra) to 11.4 days (223Ra) to 5.7
years (228Ra) to 1600 years (226Ra) which are derived from the radioactive decay series of
thorium in the sediments.
The observed enrichment of radium isotopes in groundwater relative to the surface water
bodies because of the more contacts with the sediments help to understand the groundwater –
seawater interactions occurring in a wide range of time scales. Radium tends to desorb from
61
the sediments with the increasing salinity whereas the parent thorium is retained, thus,
sediments act as a constant source of radium to the coastal waters. Accounting for the removal
of radium from the coastal regions by tidal flushing, the SGD rates can be estimated using
radium mass balance. Several recent studies used this approach to quantify SGD and its
seasonality (Moore, 2003), identify density driven SGD (Moore and Wilson, 2005) and
estimate radium fluxes to the oceans (Moore, 1997). In conjunction with biogeochemical
assays, they could be used to estimate nutrient (Hwang et al, 2005) and silicate (Kim et al.,
2005) fluxes associated with SGD, understand eutrophication and occurrence of algal blooms,
red tides (Lee., et al., 2007) etc. Since the flushing time of shallow aquifers are shorter
compared to the deep confined aquifers, they have more activities of 228Ra than 226Ra because
of the comparatively shorter half life, hence, measurement of these isotopes in the marine
waters help to quantify the relative contributions of SGD from the individual aquifers (Moore,
2003). Measurement of 224Ra and 223Ra are also useful in determining the horizontal eddy
diffusivity coefficients and coastal mixing rates (Moore, 2000a), shelf water residence times
(Moore, 2000b), identification of coastal plumes derived from rains and rivers (Moore and
Krest, 2004) etc.
Radon (222Rn)
Natural radon is an excellent tracer for identifying areas of significant groundwater discharge
because of its conservative nature, short half-life, high abundance in groundwater compared to
surface water and easiness in measurement. 222Rn activities in groundwater are often 2-4
orders of magnitude higher than those of seawater, hence, even after large dilutions in the
coastal waters, they can be detected at very low concentrations. 222Rn is particularly useful in
locating submarine freshwater springs as radium may not enrich under such conditions. One of
the limitations of 222Rn is that, being an inert gas, it evades into the atmosphere. From the
continuous monitoring of 222Rn in coastal waters, it is possible to quantify SGD (Burnett and
Dulaiova, 2003; Cable., et al, 1996).
Continuous monitoring of 222Rn in coastal water
Since the 222Rn concentrations in surface waters are very low and due to its short half life,
large volumes of water are required for the measurement of 222Rn and hence in-situ
monitoring is highly essential. A radon monitoring system developed by Burnett et al., (2001)
using a radon-in-air monitor is shown in figure 5a.
water
air
Moisture
a
Temperat
ure
Spray
cham
Pu
to
outl
Rado
n
inl
Desicc
BOA
Fig. 5 (a) Schematic sketch of surface water 222Rn monitoring system
62
b
Fig. 5 (b) In-situ measurement of
222
Rn in coastal water.
For the in-situ measurement of 222Rn in the coastal region, seawater is continuously pumped
from 1 m above the sea bed using a peristaltic pump and sprayed as a jet into an air-water
exchanger (Fig. 5b). The radon thus stripped out from the water is circulated through a closed
air-loop via a desiccant tube into a 222Rn counting system. The purpose of the desiccant is to
absorb moisture, since detection efficiency decreases at higher humidity. The equilibrium of
222
Rn between the liquid and gaseous phase is established within 30 min. The radon monitor
(RAD-7; Durridge make) uses a high electric field above a silicon semiconductor detector at
ground potential to attract the positively charged polonium daughters, 218Po+ (half life = 3.1
min; alpha energy = 6.00 MeV) and 214Po+ (half life = 164 µs; alpha energy = 7.67 MeV),
which are counted as a measure of 222Rn concentration in air. An air filter at the inlet of the
radon monitor prevents dust particles and charged ions from entering into the alpha detector.
The ions are collected in energy specific windows which eliminate interference and maintain
very low background. 222Rn activities are expressed in Bq/m3 (disintegration per second per
m3) with 2σ uncertainties. In order to get acceptable precision, 222Rn activity at each location
was counted for two hours (3 cycles of counting) after attaining equilibrium. At room
temperature, since the radon in air is about four times more than that in water at equilibrium,
the measured radon concentrations in air are corrected accordingly (Weigel, 1978).
Measurement of 222Rn in groundwater
Groundwater samples can be collected in 250 ml glass bottles using a low discharge sampling
pump. The 222Rn concentrations can be directly measured within 6 hours of sample collection
using a radon monitor (RAD-7; Durridge Make) by stripping radon via bubbling air in a
closed loop. 222Rn activity is then counted for 40 minutes (4 cycles of counting) after attaining
equilibrium. All 222Rn activities are corrected for the radioactive decay with respect to the
sampling time.
63
Computation of submarine groundwater discharge
Since 222Rn is highly enriched in groundwater, SGD can be calculated using 222Rn
mass balance in the coastal water. The general steady state 222Rn mass balance equation for
the coastal water can be written as:
ΨSGD × C GWRn × ABott + FDiff Rn = I CWRn × λ 222 Rn + C EX Rn × V B × λ Mix + FAtmRn
222
222
222
222
222
(1)
ΨSGD is the seepage rate of submarine groundwater (m/d); C GWRn is the average 222Rn
222
where
activity in groundwater (Bq/m3);
diffusive flux of
222
222
ABott is the bottom area of the bay (m2); FDiff Rn is the
222
Rn from the bottom sediments (Bq/day);
the coastal water (Bq);
λ
222
Rn
I CWRn is the 222Rn inventory in
is the radioactive decay constant of
222
Rn (day-1);
difference in 222Rn activity between coastal water and the open sea (Bq/m3);
3
of the coastal water (m );
222
C EX Rn is
VB is the volume
λMix is the exchange rate between coastal water and open sea (d-1);
222
FAtmRn is the atmospheric 222Rn evasion flux across air-sea interface.
The left side of the equation represents the various 222Rn input fluxes into the coastal waters
such as from the submarine groundwater flow and diffusion from the bottom sediments, while
terms on the right side represent the 222Rn outflow fluxes such as the loss due to radioactive
decay in the coastal waters, mixing with open ocean (offshore water) and evasion into the
atmosphere.
Temporal variation of 222Rn decay in the coastal water and diffusion from the bottom
sediments can generally be insignificant in many cases (Kim and Hwang, 2002). Other
components such as atmospheric evasion and mixing with offshore waters will vary depending
upon the local hydro-meteorological conditions such as wind speed, waves etc. Since the
water was sampled about 1 m above the seabed, the measured 222Rn activities may be less
affected by the fluctuation in wind speed and waves. Hence, in the absence of these data, the
above terms can be avoided to get a first estimate of SGD rates.
Therefore, equation (1) reduces to:
ΨSGD × C GWRn × ABott = I CWRn × λ 222 Rn
222
222
(2)
Radioactive decay constant of 222Rn is calculated as:
λ
where
222
222
Rn
=
ln 2
,
t 12
(3)
t 12 is the half-life of 222Rn (3.83 d).
Rn inventory in the coastal water is given by:
64
I CWRn = C EX Rn × ABott × y
222
where
222
(4)
y is the average depth of the coastal area.
222
C EX Rn is the unsupported or excess 222Rn calculated as the difference between the measured
222
Rn activity in the coastal water and the 222Rn derived from the in-situ decay of 226Ra.
Measured 222Rn activity in the open ocean can be taken as the 222Rn derived from 226Ra
(Burnett and Dulaiova, 2003). Equation (2) cannot be directly used to estimate SGD because,
in actual conditions, the 222Rn fluxes in the coastal water will vary with the tidal conditions
and steady state conditions are not valid. Burnett and Dulaiova (2003) continuously measured
222
Rn in the sea over a period of time and any change in the calculated time dependent 222Rn
inventories were converted into fluxes.
Therefore, for the transient condition, equation (2) can be modified as:
∆I Rn
= CW
∆t
222
ΨSGD × C
222
Rn
GW
× ABott
(5)
∆I CWRn is the change in 222Rn inventories between two consecutive measurements
and, ∆t is the time interval (generally 2 hour)
222
where
A case study – Estimation of SGD at Vizhinjam coast, Kerala
SGD is suspected from a hydrogeological and groundwater modelling study conducted in the
shallow aquifers of Vizhinjam, Kerala (Suresh Babu et al., 2009). Hence, an insitu 222Rn
monitoring study was carried out to identify the existence of SGD and semi-quantitatively
estimate its rate in the coastal waters of Vizhinjam, Thiruvananthapuram, Kerala.
Fig. 6 Location map of the study area showing the sampling points
65
The study area is located in the southwest coast of India between latitudes 80 191 - 231 N and
760 591 - 770 031 E which is about 16 km south of Thiruvananthapuram City in Kerala State
(Fig. 6). In the coastal region, water depth of about 9 -10 m is attained at less than 0.5 km
from the shore and this continental shelf is the steepest shelf in the west coast of India. It is a
microtidal coastal region having semidiurnal tides with tidal ranges less than 1 m. Bedrock
and lateritic cliffs are exposed at the shorelines, while irregular rock surfaces are encountered
at certain shallow depths. Beach profiles show change in morphology from sandy, lateritic to
rocky indicating sediment accretion and erosion at short distances alongshore. The average
annual rainfall exceeds 2300 mm, more than 70 % occurs during the summer monsoon (June
to September) and the remaining in winter monsoon (October to December).
The continental region is generally covered by Holocene unconsolidated sediments known as
red Teri sands and occasional exposures of Tertiary sedimentary sequence which is partially
covered by loose sand and clay intercalations of recent marine origin. They are conformably
underlain by Precambrian crystalline rocks comprising Khondalite suite of rocks. Neotectonic
disturbances are reflected by the development of sea cliffs and large scale deposition of red
beds over loose unconsolidated sands in the coastal region (Suresh Babu et al., 2009). The top
unconsolidated sediment sequences in the area forms the phreatic aquifer which is being
tapped for domestic purposes. Fracture flow occurs in the basement rock.
Two field monitoring surveys were carried out near Vizhinjam Harbour: (i) during November
2007 and (ii) during May 2008. The first survey was designed to understand the spatial
variability of SGD, hence, physical parameters (such as temperature, electrical conductivity
(EC) and pH) and 222Rn activity were measured at discrete points (Noble Jacob et al., 2009).
While, the objective of the second survey was to study the temporal variability of SGD and
the effect of tidal conditions. Hence, 222Rn was continuously monitored in the coastal waters
for 7 days at an interval of 1h following a spring tide (Noble Jacob et al., 2010). Groundwater
discharge fluxes were directly measured in the seabed for the same duration using a
conventional Lee type seepage meter.
Results of 222Rn monitoring survey carried out during November 2007
Electrical conductivity of coastal groundwater ranges from 180 to 1335 µS/cm. This shows
that the groundwater is fresh and the seawater interface is lying within the coastal zone.
Hence, presently there is no seawater intrusion in this region, which is mainly controlled by
the hydraulic gradient and the coastal topography (lateritic cliffs).
Vertical profiles of temperature and electrical conductivity at various locations in the
Vizhinjam coastal waters indicates that well mixed condition prevails in this region during the
winter season (Fig. 7). The observed northeast monsoon surges may also be responsible for
the horizontal and vertical mixing. The average electrical conductivity in the coastal region is
about 54100 µS/cm, which is 10000 µS/cm lower than those measured at Alappuzha coast
located at about 150 km north of the study area in the Arabian Sea (Noble et al., 2006). The
comparatively lower electrical conductivity in the Vizhinjam coastal water can be attributed to
submarine groundwater discharge as there is no major river exists in this area. The electrical
conductivity profiles show slight reduction in conductivity in the superficial waters at
locations S5 and S6 and in bottom waters at locations S3 and S4 (refer Fig. 7b). This reduction
in electrical conductivity could be an indication of existence of localized groundwater
discharge points.
66
o
Temperature ( C)
28
29
30
Electrical conductivity (µS/cm)
31
45000
0
0
1
S1
S2
S3
S4
S5
S6
S7
2
Water depth (m)
3
4
5
60000
2
3
4
5
6
7
7
8
8
9
9
10
10
a
55000
1
6
11
50000
11
b
Fig. 7 Vertical profiles of (a) water temperature and (b) electrical conductivity
in Vizhinjam coastal waters during November 2007.
Groundwater samples collected from the shallow aquifer exhibit higher 222Rn activities than in
coastal waters. They are enriched in 222Rn activities by two orders of magnitude. The average
222
Rn activity in the Vizhinjam coastal water is about 14.1±1.7 Bq/m3 where as in
groundwater it is about 4640±3830 Bq/m3. Considerable spatial variation in 222Rn activities
are observed in groundwater where as coastal water does not show significant variation in
activities. The large difference in activities among these end members helped in using 222Rn as
an indicator of groundwater discharge into the sea.
The relatively higher 222Rn activities in the Vizhinjam coastal waters compared to the offshore
value show significant SGD (Fig. 8a). Results of 222Rn and electrical conductivity
measurements indicate that the SGD in this region may be a combination of fresh groundwater
and recirculated seawater which is governed by the hydraulic gradient in the adjacent aquifer
and varying tidal conditions in the coastal waters.
Vizhinjam, being a micro-tidal coastal region, the tidal water-mass exchange time can be more
than 12 hours. Hence, terms on atmospheric evasion and offshore water mixing can be
neglected for a semi-quantitative calculation of SGD. Hence, the observed higher activities in
the coastal waters can be attributed only due to submarine groundwater discharge. Natural
222
Rn activity in the seawater comes from the radioactive decay of 226Ra present in the
sediment and the water. Measured 222Rn activity in the open ocean is generally taken as the
222
Rn derived from 226Ra. High natural radioactivity of 232Th and 226Ra in beach sediments of
Chavara, Kollam district, Kerala are reported (Shetty et al., 2005). However, we have not
observed any high concentrations of 222Rn in the coastal waters of Vizhinjam. Due to the
rough sea weather conditions prevailed during the field study, radon activities could not be
measured in the offshore waters. A radon survey conducted in the offshore waters of
Alappuzha, Kerala indicated an average 222Rn activity of about 5±5 Bq/m3 while the measured
average 222Rn activity in the shallow aquifer is 1850±600 Bq/m3 (Noble et al., 2006). Hence,
the 222Rn activity in the offshore waters of Alappuzha is considered as the background activity
in Vizhinjam.
67
SGD rate (cm/d)
222Rn activity (Bq/m3)
SGD rates were estimated using equation (5) and shown in figure 8b. The estimated average
SGD rate is found to be 10.9±6.1 cm/day. These are the absolute minimum values, as the
inclusion of atmospheric evasion term will only increase the SGD rates. The reported
significant SGD rates are typically in the range of 10-100 cm/day (Burnett and Dulaiova,
2003).
30
a
20
10
0
06:00
08:00
10:00
12:00
14:00
16:00
18:00
08:00
10:00
12:00
14:00
16:00
18:00
30
b
20
10
0
06:00
Time (hours)
Fig. 8 (a) Temporal variation of 222Rn (b) estimated SGD rates
in Vizhinjam coastal waters during November 2007
C
Fig. 8 (c) algal blooms observed during the field survey.
Algal blooms are visually observed in the Vizhinjam coastal waters during the field survey
(Fig. 8c). Occurrence of red tide of Noctiluca miliaris is reported in this region subsequent to a
‘stench event’ at the southern Kerala coast during 2004 which is suspected to be caused by
68
eutrophication followed by upwelling (Sahayak et al., 2005). Algal blooms and outbreak of
red tides are often associated with SGD, as they are the pathways of nutrients from the aquifer
to the coastal region. Hence, it can be conferred that the frequent outbreak of red tide in this
region could be because of SGD.
Results of 222Rn monitoring survey carried out during May 2008
Temporal variation of 222Rn activities in the Thiruvananthapuram coast during the 7 days of
continuous monitoring is shown in figure 9. It is seen that the 222Rn activities are not in steady
state but fluctuate with a period of 12 hours – a reflection of the semi-diurnal tides of the
region. Comparatively higher 222Rn activities varying from 15 ± 7 Bq/m3 during high tides to
50 ± 8 Bq/m3 during low tides are observed in this area indicating significant groundwater
inputs. With some exceptions, most of the peaks in radon activity occur at the lowest tides.
The observed cyclicity in the 222Rn activities may be because of the lower hydrostatic pressure
at low tides causing increased seepage and movement of recirculated seawater through the
shallow aquifer and sediments in response to tidal pumping.
1.4
80
222
Rn
tidal height
70
1.2
1.0
0.8
50
0.6
40
0.4
30
0.2
20
Tidal height (m)
222Rn (Bq/m3)
60
0.0
10
-0.2
0
May 5
May 6
May 7
May 8
May 9
May 10
May 11
-0.4
May 12
Date 2008
Fig. 9 Temporal variation of 222Rn activities in the Vizhinjam coastal waters,
5-11 May, 2008 (measurement interval – 30 minutes).
The average 222Rn activity in the coastal water is about 33.6.1± 8.7 Bq/m3 where as in
groundwater it is about 4150±3270 Bq/m3.Groundwater is enriched in 222Rn activities by two
orders of magnitude compared to those of coastal water. The large difference in activities
among these end members helped in using 222Rn as an indicator of groundwater discharge into
the sea.
A close observation of the groundwater 222Rn data along with the geological and structural
features of the area reveal that the high 222Rn activities (more than 3000 Bq/m3) are associated
with the location of lineaments (Fig. 6). Since the lineaments are geologically weak zones, the
222
Rn from the crust/mantle may be escaping into the atmosphere through these zones.
Therefore, these high 222Rn values are neglected and an average value of the remaining data,
i.e., 1530±520 Bq/m3 is considered as a representative 222Rn activity in the groundwater for
the calculation of SGD rates.
Assuming that the hydraulic gradient is more or less constant, tidal pumping and wave set up
result in the infiltration of seawater during high tide and draining water from the coastal
aquifer during low tide. Since draining the aquifer is typically slower than filling, the draining
69
500
222Rn inventory (Bq/m2)
450
400
350
300
250
200
150
100
May 5
May 6
May 7
May 8
May 9
May 10 May 11 May 12
Date 2008
Fig. 10 Variation of excess 222Rn inventories in Vizhinjam coastal waters, Kerala.
200
1.4
SGD rate estimated from 222Rn)
tidal height
1.2
1.0
0.8
0.6
100
0.4
0.2
50
0.0
Tidal height (m)
SGD rate (cm/day)
150
-0.2
0
May 5
-0.4
May 6
May 7
May 8
May 9
May 10
May 11
Date 2008
Fig. 11 Temporal variation of SGD rates estimated from the 222Rn mass balance
model.
waters may have a residence time quite a bit longer than the tidal cycle. This would explain
the observed time lag in the SGD rates compared to the tidal heights during low tides.
Figure 12 shows the temporal variation of seepage flux measured using a seepage meter. The
SGD rates estimated from the 222Rn activities were further confirmed by the same dynamic
seepage pattern observed by the seepage meter.
70
1.4
100
seepage flux
tidal height
1.2
0.8
0.6
60
0.4
40
0.2
0.0
20
-0.2
0
May 5
May 6
May 7
May 8
May 9
May 10
May 11
-0.4
May 12
Date 2008
Fig. 12 Temporal variation of seepage flux measured using a seepage meter
Conclusion
It is recognized that the submarine groundwater discharge (SGD) is an important component
in the catchment water balance and it need to be evaluated for the proper management of
groundwater resources in the coastal aquifers and understanding the coastal pollution. Isotopes
are found to be excellent tracers of SGD compared to the other conventional methods as they
can provide information on coastal processes occurring over a wide range of temporal and
spatial scales. 222Rn is particularly useful in SGD studies because of its conservative nature,
easiness in measurement, high abundance in groundwater compared to surface water,
possibility for in-situ measurement and its decay rate comparable with the time scales of many
coastal processes.
In general, submarine groundwater discharge studies help to plan for the optimum
groundwater exploitation of coastal aquifers keeping the seawater interface well within the
coastal zones. Also, ideal sites for the construction of subsurface barriers to arrest the
groundwater discharge could be explored.
Acknowledgement
The case study described in this article was carried out as a collaborative effort between
BARC, Mumbai and Centre for Earth Science Studies (CESS), Thiruvananthapuram, Kerala
and the help & support received from Dr. Suresh Babu, Scientist, CESS is gratefully
acknowledged. The encouragement and support by Dr.Gursharan Singh, Head, Isotope
Applications Division, Bhabha Atomic Research Centre (BARC) and Dr. V. Venugopal,
Director, Radiochemistry & Isotope Group, BARC during the period of the project is
gratefully acknowledged.
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Sahayak, S., Jyothibabu, R., Jayalakshmi, K. J., Habeebrehman, H., Sabu, P., Prabhakaran, M.
P., Jasmine, P., Shaiju, P., Rejomon, G., Thresiamma, J. and Nair, K. K. C. (2005) Red tide of
Noctiluca miliaris off south of Thiruvananthapuram subsequent to the ‘stench event’ at the
southern Kerala coast, Curr. Sci., 89, 1472-1473.
Shetty, P. K., Narayana, Y. and Siddappa, K. (2005) Vertical profiles and enrichment pattern
of natural radionuclides in monazite areas of coastal Kerala, J. Environ. Radioact., 86, 1-11.
Smith A. J. and Nield S. P. (2003) Groundwater discharge from the superficial aquifer into
Cockburn Sound Western Australia: estimation by inshore water balance. Biogeochem., 66,
593-608.
Suresh Babu, D. S., Anish, M., Vivekanandan, K. L., Ramanujam, N., Murugan, K. N. and
Ravindran, A. A. (2009) An account of submarine groundwater discharge of SW Indian
coastal zone, J. Coast. Res., 25 (1), 91-104.
Taniguchi M. (1995) Change in groundwater seepage rate into Lake Biwa, Japan. Jpn. J.
Limnol., 56, 261-267.
Taniguchi, M., Burnett, W. C., Cable, J. E. and Turner, J. V. (2002) Investigation of
submarine groundwater discharge. Hydrol. Process., 16, 2115–2129.
Weigel F. (1978) Radon, Chemiker Zeitung, 102, 287.
73
Application of Radon measurements in Uranium exploration
G.B.Rout
Atomic Minerals Directorate for Exploration and Research,
Department of Atomic Energy,
Beumpet,Hyderabad-500016
[email protected]
Abstract
Radiometric prospecting is being carried out using gamma ray measurement for Uranium
exploration. But gamma ray has limitation of only 50 cm from the surface of rock. If the area
is covered by soil, radiometric prospecting does not help much. In this situation, Radon
Emanometry is the tool which measures the radon concentration originating from few hundred
metres below and helps to great extent for Uranium exploration. Different radon measurement
techniques like ( i) Closed circuit technique (ii) Solid State Nuclear Track Detector (iii)
Radon on Activated Charcoal and (iv) Thermoluminescence Detector are being used for
Uranium exploration programme in AMD. Merits and demerits of different techniques are
being discussed with case study from field area.
Introduction
Radon isotopes are the direct daughter products of radium. The three natural radio active
series of Uranium, Thorium and Actinium have in each of them isotopes of radon, Rn222( half
life: 3.825 days), Rn220(Thoron) (half life: 56 seconds) and Rn 219(Actinon) ( half life: 4
seconds). All the isotopes of radon are gases under normal conditions of temperature and
pressure, they belong to the group of noble gases and do not react chemically with any of the
surrounding elements easily. Radon is a dense monoatomic gas, being the heaviest gas
member in group of noble gases. However they have a strong affinity for adsorption in liquids
and solids specially in organic materials. The adsorption, apart from many other factors, is a
steep inverse function of temperature. All isotopes are alpha active and have little of gamma
activity associated with them. As many as thirty one radioactive isotopes of radon are known
from Rn 198 through Rn 228, most of these produced artificially. The longest lived radon isotope
Rn 222 with a half life of 3.825 days, a daughter product of U 238 decay series is the only
isotope which has geophysical significance.
There are three possible steps involved in the migration of Radon.(i) recoil of the Rn atom by
disintegration of parent Ra atom. (ii) Diffusion (iii) Diffusion and transport. Rn222 atoms have
an initial recoil energy of 100keV, which corresponds approximately recoil ranges of 3X10-6
cm in rock, 10-5 cm in water and 6-9X10-3 cm in air. But with diffusion and transport
mechanism, radon gas can migrate long distance. Because of this inherent quality of diffusion
and migration that radon emanometry find wide application in Uranium prospecting,
particularly in finding out the extension of concealed Uranium bodies, delineation of fracture
and fault zones, and earthquakes prediction.
74
Theory of Radon Migration
A steady state diffusion equation of radon can be written as
D d2N/dz2 – λN =0
where, D is the diffusion co-efficient of radon in medium
N is the density of radon atoms per cc at z distance from source
λ is the decay constant
Boundary condition
N = N0 at Z= 0 and N=0 at Z=∝
N= N0e-(λ/D)1/2Z (Putting Boundary condition )
Taking D=0.03 cm2/sec (Typical value for sand stone)
λ= 2.098X10-6 sec-1.
N/N0 = 1/e, at Z = 1.2 m.(diffusion length)
Long distance Radon migration can be explained by transport component.
D d2N/dz2 –V/E dN/dz -λ N=0
V is velocity of Ground water
E is the permeability of medium.
Here the solution of the above equation comes out to be :
N=N0e[V/2DE-
√{ (V/2DE)2+λ/D}].Z
Depending upon the velocity of water, radon thus can be transported to hundred meters or
more distances from the source rock.
V=6X10-4 cm/s , velocity of Ground Water
D= 0.03 cm2/s( Diffusion Coefficient)
E =0.25(Permeability)
In AMD, different techniques like ( i) Closed circuit technique (ii) Solid State Nuclear Track
Detector (iii) Radon on Activated Charcoal and (iv) Thermoluminescence Detector(TLD) are
being used to locate concealed Uranium mineralization. The result of these studies proved to
be productive and serve as a guide in the exploration for Uranium.
Closed Circuit technique(CCT)
This technique was developed by the scientists of AMD (Ghosh and Bhalla,1966) to measure
the radon concentration in water and soil based on the alpha activity of radon and its daughter
products. The alpha detector is an inverted bell shaped nickel coated brass container. In this
container, the inner wall is coated with fine layer of phosphor material ZnS(Ag) with the help
of silicon grease. This is coupled to photomultiplier tube and counting system. When alpha
particle interacts with ZnS(Ag) phosphor, it produces scintillation, which is a measure of the
radon concentration. To estimate radon concentration in Ci/cc, the system is calibrated using
RaCl2 solution knowing strength of the solution. By making use of hand operated rubber
75
pump fitted with valves, the mixture of soil gas and radon gas is collected in the alpha
detector.
Fig: 1. Block Diagram of Closed Circuit Technique
Advantages
i. The radon concentration is measured in field itself.
ii. Radon and Thoron can be discriminated.
iii. Concentration as low as 10-14 Ci/cc can be measured
Disadvantage
i. The detector requires periodic calibration.
ii. Relatively smaller number of measurements are carried out per day.
Solis State Nuclear Track Detectors (SSNTD)
Solid State Nuclear Track Detectors are essentially insulating materials such as minerals
(quartz,mica) glass (pyrex, soda lime) and plastics(lexan,CN,Makrofol). Charged particles
passing through such insulating materials knock off electrons from the atoms in the crystal
lattice, leaving behind a number of positive ions which repel one another and violently disturb
the lattice structure. These damaged structure are more liable to chemical attack than its
immediate surrounding material. It can therefore subjected to preferential chemical etching,
which enlarges and “fixes” the damaged trails as hollow cylindrical tubes about 50X10 -8 cm
in diameter. Prolonged etching increases the diameter to the micron size. These are called the
“tracks” and upon scattering the visible light, can show up as dark lines when viewed in a
transmitted light under an optical microscope. Not all classes of detectors can register all kinds
of charged particles. Etch track detectors is characterised by a critical value of energy loss
rate. Those which give up energy in excess of the critical value register tracks with unit
efficiency, while those which deposit less energy than critical value cannot produce etchable
tracks.
Etching of Tracks
“Developing or etching” the tracks in a given substance depends on a variety
of conditions such as the nature of the etching chemicals, its concentration, the
temperature, as also presence of trace impurities. However, the basic principle
is that the particle trails, being disordered structures, associated with large free
76
energy are liable for preferential attack. Optimisation of etching condition can
be obtained by experimentation.
Advantages
Radon and Thoron can be discriminated
This is an integrated technique.
Concentration as low as 100-12 Ci/cc can be measured
Disadvantages
Data is not immediately available in the field.
Sometimes, SSNTD films are stolen.
Radon on Activated Charcoal
Radon on activated charcoal (ROAC) method is used for radon prospecting for uranium
exploration. Radon being gas is adsorbed by the activated charcoal pellet. After exposure, the
pellet is then kept in a brass container and resultant gamma rays emitted from the daughter
products(mainly Pb214 and Bi214) are counted using NaI(Tl) scintillation detector. The counts
obtained are proportional to the radon concentration emanated from the source.
The ROAC pellets should be robust and mechanically stable with optimum adsorption
properties for their effective performance. In addition, they should also be compact for
physical handling. The proportions of the components in the mixture of bakelite(binder) and
activated charcoal, pressure applied, duration of heating are the parameters involved in the
preparation of pellet. Variation in any one of them results in the change of adsorption
characteristics and consequently the response of the pellet.
Advantages
Data is available in the field.
This is an integrated technique.
Radon and Thoron can be discriminated.
Concentration as low as 10-12 Ci/cc can be measured
Disadvantages
If moisture is more in augur hole, the efficiency of ROAC pellet decreases.
Some times, ROAC pellets are also stolen.
Thermo luminescence Detector (TLD)
The detector used in TLD measurements is CaSO4(Dy). A thin layer of CaSO4(Dy) powder is
deposited on Kanthal strip for making it predominantly sensitive to alpha particle.
Advantages
i This is an integrated technique.
ii Radon and Thoron can be discriminated.
77
Disadvantages
i Some times, TLD detectors are stolen.
ii The detectors cannot be reused.
Radon Surveys in Lotapahar area of District Singhbhum, Jharkhand
Solid State Nuclear Track Detector and Radon and Activated Charcoal method
and Thermoluminescence Detectors were used in Lotapahar area of
Singhbhum District, Jharkhand . The detectors were exposed to soil gas for a
period of three weeks in aluminium cylinders with its open end at the bottom
of an auger hole of one metre depth and 5 cm diameter. The data obtained
from the survey are presented in the form of contour maps. All these results
have been published.
Fig:2 Contour map of Radon conc. by SSNTD
Fig.3 Contour map of Radon conc. by ROAC
Radon Concentration in hot springs.
The closed circuit technique was used to measure the radon content of various hot springs in
India.
Patna district(Rajgir)
i. Brahma kund:
5.84X10-12 Ci/cc
ii. Sapta dhara:
2.54X10-12Ci/cc
iii. Makhdoom kund: 3.62X10-12 Ci/cc
iv. Suraj kund:
4.06X10-12 Ci/cc
v. Tapowan(Gaya district): 3.53X10-12Ci/cc
78
Conclusion
The above contour map are based on SSNTD and ROAC surveys and assume similar pattern
and gives the direction of radioactive zone. If Radon is not in equilibrium with Uranium,
conclusion cannot be drawn about concealed Uranium mineralisation. Therefore Radon survey
is a indirect and cost effective method for Uranium Prospecting. Closed Circuit Technique
also measures radon concentration in spring water wchich helps to find the origin of Uranium
source.
Acknowledgements
The author is highly indebted to Dr.Anjan Chaki, Director, AMD, for permission to present
this paper at the workshop organised by Central ground Water Board, Bangalore on
26.03.2010 and also Shri K. Umamaheswar, Additional Director(R&D) for encouragement,
grateful to Dr.S.S.raghuwanshi, Head Physics Group for critical suggestion and improvement
of paper.
References
Alekseev V.V.,Grammckou, AG. Tafeev G.P.,(1959)- Radiometric Methods in the
Prospecting and Exploration of Uranium Ores. Translation Series USAEC- 3730, Book-2.
Bhargava R.C., Sethuram S., Rout G.B.(1990) A comparative study of Solid State Nuclear
Track Detectorand Thermoluminescence as tools for Uranium exploration.Exploration and
Research for Atomic Minerals,Vol.3. pp.155-159
Butt C.R.H., and Gole M.J.,(1984) Helium emanometry under Australian conditionsPreliminary results-jour.Geo.Chem.Exp.Vol.22,pp.359
Cohen B.L., and Nason R.,(1986)- A diffusion barrier charcoal adsorption collector for
measuring radon concentration in indoor air. Health Physics, Vol.50,pp.457-463
Durrance Eric Michael (1986)- Radioactivity in Geology, Principles and Applications.
Pub.Ellis Hoorwool Ltd. Eng.
Fleischer, R.L. and Antonio Mogro- Campero (1978). Mapping of integrated radon emanation
for detection of long distance migration of gases within the earth: Technique and Principles,
Jou. Of Geophy.Res. pp.3539-3549.
Ghosh P.C., and Bhalla N.S., (1966) – A close circuit technique for radon measurement in
water and soil with some of its applications. All India Symposium Radiactive Metrol.
Radional Proc. Pp.226-239, Bombay.
Ghosh P.C.,(1974)- Radon Survey: Instruments and techniques of interpretation – Lecture
notes for IAEA Training Course on Uranium and Thorium Prospecting and evaluation, NovDec., 1974.
G.B.Rout. (2009). Optimisation of exposure time for Radon on Activated Charcoal(ROAC)
pellet in the field environment. Sixteenth National Symposium on Solid State Nuclear Track
Detectors and their Applications(SSNTD-16), Amritsar.
79
Hambleton –Jones B.B., and Smit M.C.B., 1980 ROAC- A new Dimension in Radon
Prospecting.
Iyer R.H..,(1976) Lecture note (unpublished) for training programme on SSNTD techniques
in Uranium Exploration, Bombay.
King C.V., (1986)- Gas Geochemistry Applied to Earthquake Prediction, Jr of Geophysical
Research, Vol.91, No. B 12,pp 12269-12281
P.K.Sharma, K.N.Srinivas, G.B.Rout, M.Sooundararajan and J.N.Gupta.(1994) Various
Emanometric Techniques in Exploration for Uranium: A comparative study. Proc. Vol. 3rd
ICRGG. pp. 100-110
Sethuram S. and Bhargav R.C. (1990)- Optimisation of activated charcoal pellets and their
response to radon atmosphere and application to Uranium prospecting.Expl. and Res. For
Atomic Minerals, Vol.3, pp. 161-167
Tanner A.B.,(1966)- Radon migration in the ground, a review, Natural Radiation
Environment, Ed Adams and Lowder.
Tewari,S.G.,Ghosh,P.C. and Bhatanagar,A.S.(1968) A closed circuit Technique for the
measurement of Radon/Thoron ratio in soil-gas, Ind. Jour. Of Pure and Applied Physics,
Vol.6, No.1 pp.33-36
80
Groundwater dating using 14C, 4He and 4He/222Rn methods – A
case study of the North Gujarat Cambay region, India
R.D. Deshpande∗ and S.K. Gupta
Physical Research Laboratory, Navrangpura, Ahmedabad 380 009 India
[email protected]
Abstract
The basic theoretical concepts and equations involved in the ground water dating using 14C,
4
He and 4He/222Rn methods are discussed in this paper. A case study of the North Gujarat
Cambay region is presented in which 14C, 4He and 4He/222Rn methods were applied
simultaneously to estimate the groundwater ages, identify groundwater recharge area and to
determinate direction and rate of groundwater movement in the regional aquifer system. The
field sampling and storage procedures for above dating methods are discussed and important
observations related ground water ages are presented.
Introduction
In an aquifer system, ensemble of water molecules arriving at a particular location within the
aquifer comprises molecules that spend various time durations between recharge and their
reaching a particular location. The concept of groundwater dating involves estimating the
average time spent by the molecules before reaching a given location. The age of
groundwater at a particular location is the estimated average time spent by the water
molecules in the aquifer since the time of recharge and before arriving at that location.
Depending on the conceptual mathematical model of the aquifer system, the age can give
different additional information about the aquifer system. The various conceptual models in
common use are: (i) the Piston Flow Model (PFM); (ii) the Well-Mixed Reservoir Model
(WMRM); and (iii) the Dispersion-Advection Model (DAM).
The most commonly used groundwater flow model for estimating groundwater age assumes
that as groundwater moves away from the recharge area, there are no flow lines of different
velocities and that hydrodynamic dispersion as well as molecular diffusion of water molecules
are negligible. Thus, water moves from the recharge area very much like a parcel pushed by a
piston with mean velocity of groundwater (Piston Flow Model; PFM). This implies that a
radiotracer which appears at the sampling point at any time “t” has entered the system at a
time “t-T”, and from that moment its concentration has decreased by radioactive decay during
the time span “T” spent in the aquifer. Therefore:
Cout(t) = Cin(t-T) . exp (-λT)
Eq. 1
81
This equation describes a dynamic system and is mathematically equivalent to describing the
concentration of a radioisotope in a static water parcel separated since its recharge, whereby:
Ct = C0 . exp (-λT)
Eq. 2
Here, “t” is the radiometric age of the water and corresponds to “T” of the dynamic system. If
“x” is the distance from the recharge boundary, T = x/u can be used to estimate the flow rate
(u) of groundwater in the aquifer.
Ct = C0 exp (-λx/u)
Eq. 3
Unlike in the PFM, if it is assumed that the recharge flux completely mixes with the entire
volume of the reservoir before outflow, we get another extreme situation and the model is
known as Well Mixed Reservoir (WMR) model. In applying this model to an aquifer system,
it is assumed that the well-mixed reservoir comprises the entire volume between the recharge
area and the sampling point. Under this condition for a radiotracer,
Ct = C0 / (1+λτ)
Eq. 4
In Eq. 4, “ ” is the radioactive decay constant and “ ” is the ratio of the reservoir volume to
the flux into the reservoir and represents the estimated mixing time (or the mean residence
time) between the recharge area and the sampling location. It is seen that estimated from the
tracer data actually represents a dynamic parameter – the mixing time.
τ
=
1  C0

λ  C t
−

1

Eq. 5
The phenomenon of mixing accompanying the movement of water molecules through porous
media can also be described by a diffusion-advection equation in which the diffusion
coefficient is replaced by a dispersion coefficient (Scheidegger, 1961; Gupta, 2001). For a
radiotracer, the one dimensional continuity equation in groundwater flow system may be
written as (Guymon,1972):
δC
δt
=
(
δ D δC
δx
δx
− uC
)
+
W1
−
W2
Eq. 6
In the above equation, “D” is the diffusion coefficient of the tracer, and as in the case of PMF,
“x” is the distance from the recharge boundary, “u” is the bulk flow velocity, “W1” and “W2“
are the rates of the introduction and removal of the tracer respectively. With further
assumptions of u and D not being function of x and in case of steady state (i.e. dC/dt = 0), the
above equation reduces to:
D
∂ 2C
∂C
−u
W1 − W2
2
∂x
∂x
=
Eq. 7
0
In case of radioactive tracers, the term W2 will include, in addition to radioactive decay, loss
of tracer from water due to non-radioactive processes (e.g. adsorption on the aquifer matrix).
82
Dealing with the case of loss of tracer by radioactive decay alone and for W1 = 0, the above
equation can be rewritten as:
D
∂ 2C
∂x
2
−u
∂C
− λC
∂x
=
0
Eq. 8
This equation for the case of D = 0, and the boundary condition C = C0 at x = 0 give the
solution for the piston flow (Eq. 3).
In case of finite dispersion, the solution of for
C = C0 at x = 0 and C = 0 at ∞, is given by Gupta et al (1981).
C
=
C0
1
 xu 


 2 
exp
1 − 1 + 4λD u  
2
 2D  


the
boundary
conditions
Eq. 9
The tracer concentration decreases exponentially with distance similar to the case of PFM.
Therefore, a simplistic application of the PFM would give an apparent velocity:
ua
=
u  

1 −  1 + 4λD 2 
u 
2  
1
2

Eq. 10


There are several other mathematical models in use that conceptualize the flow of
groundwater in aquifer system differently depending on specific understanding of the
geometry, and flow paths. But for confined aquifers with a definite recharge area, the above
models are most frequently used to interpret the environmental isotopic data and to determine
the groundwater age evolution from recharge to discharge regions. This, though, is still a
challenging task for hydro-geochemists because sampling locations are often randomly
distributed over an area where water from an aquifer is pumped from various depths or where
springs bring water to the surface. Several environmental tracers (including radio nuclides)
find application in determining direction and magnitude of groundwater flow, hydrogeological parameters of the aquifer and age of groundwater (Andrews et al., 1989; Cserepes
and Lenkey, 1999).
In regional aquifer systems, groundwater ages may range up to 103 ka and more. Radiocarbon,
with half-life (t1/2) of 5.73 ka, can be used for groundwater dating up to ~35 ka (Geyh, 1990).
The other available radio nuclides such as 36Cl (t1/2 = 3.01x 102 ka; Andrews and Fontes,
1992), 81Kr (t1/2 = 2.1x103 ka; Lehmann et al., 1991) and 234U (t1/2 = 2.45x102 ka; Fröhlich and
Gellermann, 1987) enable groundwater age determination well beyond the radiocarbon dating
range. On the other hand, the range of groundwater ages that can be estimated by radiogenic
4
He is 1–100 ka (Torgersen, 1980; 1992; Mazor and Bosch, 1992; Clark et al., 1998; Castro et
al., 2000). When combined with 222Rn activity measurements, the 4He/222Rn systematics also
provides estimation of groundwater age in the range 1–1000 ka (Torgersen, 1980; Gupta et al,
2002). Another advantage of both 4He and 4He/222Rn methods is that measurements of 4He and
222
Rn are relatively simple. However, some complications also exist.
The basic theory of four dating methods namely, 14C decay, 4He accumulation, 4He/222Rn ratio
and CFC concentration, is discussed in the following.
83
Theory
14
C Dating Method
The radiocarbon (14C) dating method for groundwater is based on measuring the residual
activity of 14C, in total dissolved inorganic carbon (TDIC) of a given groundwater sample. The
groundwater age is estimated using:
Age( t )
=
−
A
1
. ln t
λ 14
 A0



Eq. 11
In
Eq. 11, 14C activities are expressed in terms of percent modern
carbon (pmC). The activity of ‘modern Carbon’ is defined as 95% of the 14C activity in 1950
of the NBS oxalic acid standard. This is close to the activity of wood grown in 1890 in a
fossil-CO2-free environment and equals 13.56 dpm/g carbon. Thus, expressed in pmC, .At
refers to 14C activity of TDIC in groundwater at the time of sampling and A0 the initial activity
in the recharge area; λ14 is the radioactive decay constant for 14C and t is the estimated age.
Two key assumptions made in this method are: (i) A0 is known and has remained constant in
time; and (ii) the system is closed to subsequent gain or loss of the parent, except by the
radioactive decay. In the case of radiocarbon dating of vegetal remains, A0 can be taken as
equal to 100 pmC since the only source of 14C is atmospheric CO2. However, in the case of
ground waters, A0 in TDIC depends on contribution from soil CO2 and from soil carbonates
which can have any value of 14C activity between zero (radioactively dead carbon) to 100
pmC. Depending on the contribution from soil carbonates, A0 can have any value between 50–
100 pmC. This is because, of the two carbon atoms in the molecule of Ca(HCO3)2 in TDIC,
[CaCO3 + H2O + CO2 ↔ Ca(HCO3)2], one is derived from soil CO2 and the other from soil
carbonate. There is further complication due to isotopic exchange between TDIC and the soil
CO2 and carbonate material. There are several models that attempt to estimate the contribution
of soil carbonates to the TDIC and estimate the applicable value of A0. This is done either
through a stoichiometry approach for the various chemical reactions involving carbon or by
estimating dilution of active carbon using an isotope mixing approach based on 13C content of
each species involved or a combination of the two approaches. Various methods for estimating
A0 can be found in Mook (1976) and Fontes and Garnier (1979). The error due to incorrect
estimation of A0, however, is <t½ of 14C, except in some special case of carbonate aquifers
where continuous exchange between TDIC and the aquifer matrix may reduce A0 to <50 pmC.
Since most of the chemical and isotope exchange occurs in the unsaturated soil zone during
the process of groundwater recharge, and between TDIC and the soil CO2, the A0 in several
ground waters has been found to be 85±5 pmC (Vogel, 1967; 1970).
In the present investigation, the theoretical value of A0 after equilibrium between soil CO2, soil
carbonate (at 14C = 0 pmC; δ13C = 0‰) and infiltrating water has been achieved, is estimated
using the following equation (Münnich, 1957; 1968):
A0
=
δ13 C TDIC
100
δ 13 C soil − ε
Eq. 12
Where, δ13CTDIC is the δ13C value of the groundwater TDIC, δ13Csoil is the δ13C of soil CO2 (~–
22‰) and ε is equilibrium fractionation between the soil CO2 and the TDIC of groundwater
84
(~–9‰). This is done with the knowledge that application of any other model would give
radiocarbon ages differing by < ± 2ka. Also because, in regional aquifers, the difference in
groundwater ages between any two locations, after the confinement of the groundwater in the
aquifer becomes effective, depends little on the applicable value of A0.
4
He Dating Method
The Helium-4 dating method for groundwater is based on estimating the amount and rate of
accumulation of in situ produced radiogenic 4He in groundwater (Andrews and Lee, 1979;
Stute et al, 1992a). If secular equilibrium and release of all the produced 4He atoms in the
interstitial water are assumed, the groundwater ages can be calculated from the annual 4He
production rate estimated as (Torgersen, 1980):
0.2355 x 10 −12 U *
=
'
JHe
Eq. 13
where,
U*
=
[U] {1+ 0.123 ([Th] /[U] − 4)}
'
JHe
4
3
–1
Eq. 14
–1
= production rate of He in cm STP g rock a ; [U] and [Th] are concentrations (in
ppm) of U and Th respectively in the rock/sediment.
Accumulation rate (AC’He) of 4He in cm3 STP cm–3 water a–1 is therefore, given by:
'
AC He
=
'
JHe
.ρ.Λ He .(1 − n) / n
Eq. 15
Where, ΛHe = helium release factor; ρ = rock density (g cm–3); n = rock porosity.
Since the helium measurements were actually made on equilibrated headspace air samples, in
the present study, the dissolved helium concentrations (cm3 STP cm–3 water) are expressed in
terms of Air Equilibration Units (AEU) which expresses the dissolve helium in terms of the
corresponding equilibrium dry gas phase mixing ratio at 1 atmospheric pressure and 25°C. As
a result, water in equilibrium with air containing 5.3 ppmv helium is assigned dissolved
concentration of 5.3 ppmAEU. Water of meteoric origin will have a minimum helium
concentration (4Heeq) of 5.3 ppmAEU, acquired during its equilibration with the atmosphere.
Excess helium (4Heex) represents additional helium acquired by groundwater either from in
situ produced radiogenic 4He or any other subsurface source.
Using a dimensionless Henry Coefficient (Hx) of 105.7 for helium at 25°C (Weiss, 1971), 5.3
ppmAEU corresponds to a moist air equilibrium concentration of 4.45x10–8 cm3 STP He. cm–3
water. Therefore,
AC He
=
'
AC He
. 10 6 . H X . (T T 0 ). P0 (P0 − e )
Eq. 16
where, AC He = accumulation rate of 4He in ppmAEU a–1; T0 = 273.15° K; P0 = 1 atm and e =
saturation water vapour pressure (0.031 atm) at 25°C, Hx = 105.7.
For an average [U] = 1 ppm for alluvial sedimentary formations and [Th]/[U] = 4 (Ivanovich,
1992), n = 20 %; ΛHe = 1; ρ = 2.6 g cm–3, in situ 4He accumulation rate ( AC He ) of 2.59 x 10–4
ppmAEU a–1 is obtained. Therefore, from the measured helium concentration of the sample
(4Hes), the age of groundwater can be obtained by using:
85
Age ( t )
=
4
He ex / AC He
Eq. 17
here, 4Heex is obtained by subtracting 4Heeq (= 5.3 ppmAEU at 25°C and 1 atmospheric
pressure) from the measured concentration in groundwater sample (4Hes).
The above formulae, however, ignores ‘excess air’ helium (4Heea) due to supersaturation of
atmospheric air as the groundwater infiltrates through the unsaturated zone. Various models
have been proposed for estimating this component in groundwater studies (Aeschbach–Hertig
et al., 2000; Kulongoski et al., 2003) based on measurement of other dissolved noble gases.
Since measurement of dissolved noble gases has not been made in this study, it has not been
possible to correct for this effect. However, from other studies (Holocher et al., 2002) it
appears that 4Heea can be up to 10–30% of 4Heeq giving a possible overestimation of
groundwater age up to ~ 6 ka.
Further, (4Hes) can contain other terrigenic helium components (Stute et al., 1992a) that can
cause overestimation of groundwater age. These terrigenic components are: (i) flux from an
external source e.g. deep mantle or crust, adjacent aquifers etc. (Torgersen and Clarke, 1985);
and (ii) release of geologically stored 4He from young sediments (Solomon et al., 1996).
Depending upon the geological setting, particularly in regions of active tectonism and/or
hydrothermal circulation, the contribution of these sources may exceed the in situ production
by several orders of magnitude (Stute et al., 1992a; Minissale et al., 2000; Gupta and
Deshpande, 2003a). Additional measurements/ data (e.g. 3He/4He, other noble gases) are
required to resolve these components. For recent reviews on terrigenic helium, reference is
made to Ballentine and Burnard (2002) and Castro et al. (2000). However, in many cases it
seems possible to rule out major contribution from terrigenic He sources since the helium flux
may be shielded by underlying aquifers that flush the Helium out of the system before it
migrates across them (Torgersen and Ivey, 1985; Castro et al., 2000). According to Andrews
and Lee (1979), with the exception of a few localised sites and for very old ground waters,
‘excess He’ in groundwater is due to in situ production only and is often used for quantitative
age estimation within the aquifer if the U and Th concentrations of the aquifer material are
known. But, in case, there is evidence of deep crustal 4He flux (J0) entering the aquifer, the
Eq. 17 gets modified to (Kulongoski et al, 2003):
Age ( t ) = 4 He Ex [(J 0 n Z 0 ρ w ) + AC He ] / 8.39 x10 −9
Eq. 18
where, Z0 is the depth (m) at which the 4He flux enters the aquifer and ρw is the density of
water (1 g cm–3).
4
He/222Rn Dating Method
Since both 4He and 222Rn have a common origin in groundwater, being produced by the α
decay of U and/or Th in the aquifer material, their simultaneous measurements in groundwater
can also be utilized for calculating its age (Torgersen, 1980).
As in case of 4He, the 222Rn accumulation rate (ACRn) in cm3 STP cm–3 water a–1 is given by:
AC Rn
=
'
where, JRn
'
JRn
.ρ.Λ Rn .(1 − n) / n
Eq. 19
= 1.45 x 10 −14 [U]
Eq. 20
86
'
and JRn
= production rate of 222Rn in cm3 STP g–1 rock a–1 and [U] = concentration ( in ppm)
of U in the rock/sediment.
Thus, computing accumulation rate ratio of 4He and
groundwater can be calculated as follows:
Age( t )
=
(Λ Rn
Λ He )(AC Rn AC He ) (C 4 A 222 )
222
Rn (= ACHe/ACRn), the age of
Eq. 21
where, ΛRn/ΛHe is the release factor ratio for radon and helium from the aquifer material to
groundwater; C4 is concentration (atoms l–1) of 4He and A222 is activity (disintegration l–1 a–1)
of 222Rn in groundwater. From
Eq. 13 to
Eq. 15 and
Eq. 19 to
Eq. 21 it is seen that 4He/222Rn ages are independent of
porosity, density and U concentration, but do require a measure of [Th]/[U] in the aquifer
material. The ratio ΛRn/ΛHe depends upon grain size and recoil path length of both 222Rn
(~0.05µm) and 4He (30–100µm) (Andrews, 1977). Release of 222Rn by α–recoil from the
outer surface (~0.05µm) of grain (~2–3mm) has been estimated to be ~0.005% (Krishnaswami
and Seidemann, 1988). Apart from α–recoil, both 222Rn and 4He can diffuse out from rocks/
minerals through a network of 100–200Å wide nanopores throughout the rock or grain body
(Rama and Moore, 1984). Radon release factor (ΛRn) ranging from 0.01–0.2 has been
indicated from laboratory experiments for granites and common rock forming minerals
(Krishnaswami and Seidemann, 1988; Rama and Moore, 1984). On the other hand, Torgersen
and Clarke (1985), in agreement with numerous other authors, have shown that most likely
value of ΛHe ≈1. Converting C4 (atoms l–1) to CHe (ppmAEU) units and A222 (disintegration l–1
a–1) to A’222 (dpm l–1) units,
Eq. 21 can be rewritten as:
Age (t) = 4.3x108.(ΛRn/ΛHe).(ACRn/ACHe).CHe /A’222
Eq. 22
Here, 1 ppmAEU 4He concentration corresponds to 2.26 x 1014 atoms of
4
He l–1 of water. Another inherent assumption of 4He/222Rn dating method is that both 4He and
222
Rn originate from the same set of parent grains/rocks and their mobilization in groundwater
is similarly affected.
Andrews et al. (1989) used following one–dimensional equation for calculating diffusive
transport of 222Rn in granites:
Cx =
C 0 exp(− λ D . X )
Eq. 23
where, C0 and Cx are concentration of 222Rn from an arbitrary x = 0 and at x = x respectively;
D = diffusion coefficient in water (~10–5 cm2 s–1); and λ = decay constant for 222Rn. They
calculated that Cx/C0 = 0.35 at a distance equal to one diffusion length (X = [D/λ]1/2 i.e. 2.18
cm). Therefore, even at high 222Rn activity its diffusion beyond few metres distance is not
possible. The average radon diffusion co–efficient in soils with low moisture content and
composed of silty and clayey sand is even lower ~2x10–6 cm2 s–1 (Nazaroff et al., 1988).
Therefore, 222Rn measurements of groundwater depend essentially on U in the pumped aquifer
horizons in the vicinity of the sampled tubewell.
Therefore, for ground waters that may have a component external to the aquifer, the measured
Rn because of its short half–life (t1/2 = 3.825 d) is indicative of local mobilisation only.
222
87
Whereas 4He, being stable, might have been mobilized from the entire flow–path. In such
cases, the resulting 4He/222Rn ages for groundwater samples having high ‘excess He’ might be
over estimated.
The three groundwater dating methods discussed above have been applied in several of
hydrological studies across the world. In the following, a study in the regional aquifer system
of North Gujarat Cambay (NGC) region is discussed in which all the three methods were
simultaneously applied for dating the confined ground waters.
Case Study in North Gujarat Cambay Region
The North Gujarat - Cambay (NGC) region (21.5°–24.5°N; 71.5°–74°E; Figure 1) of Gujarat
State in Western India is characterised by a unique combination of geological, hydrological,
tectonic and climatic features, namely, (i) two major bounding faults, defining the Cambay
Graben, and several other sympathetic faults parallel and orthogonal to these; (ii) more than 3
km thick sedimentary succession, formed by syndepositional subsidence in the Cambay
Graben, acting as a reservoir for the oil and gas at deeper levels and a regional aquifer system
at shallower depths; (iii) higher than average geothermal heat flow; (iv) intermittent
seismicity; (v) emergence of thermal springs; and (vi) arid climate with high rate of
evapotranspiration. A detailed description of the above features in the NGC region is given by
Deshpande (2006).
A multi-parameter geohydrological investigation was undertaken (Deshpande, 2006) in the
NGC region to understand the roles of (i) geohydrological; (ii) palaeoclimatic (iii) topographic
and (iv) tectonic processes/ features in controlling the chemical and isotopic composition of
groundwater, its interaction with aquifer matrix and movement in the regional aquifer system.
Some of the specific objectives of the investigation were:
To estimate groundwater ages employing 14C, 4He and 4He/222Rn methods and to understand
their relationship with various ionic and isotopic properties of groundwater from the recharge
towards the discharge area.
To identify the regions of groundwater recharge to the regional aquifer system of NGC region.
To determine direction and rate of groundwater movement within the regional aquifer system.
Based on general topography, geology, the lithologs of drilled tubewells and the water level/
piezometric level data, recharge area of the regional aquifer system in the study area was
identified in the foothills of the Aravalli Mountains in the NE. However, this inference of
groundwater recharge and discharge areas, based on topographic and lithological parameters,
was required to be strengthened and refined by determining the age of groundwater, its
direction and rate of movement in the aquifer. Since radiometric dating methods provide
information on age and residence time of groundwater, exchange between shallow and deep
aquifers and interaction with the aquifer matrix groundwater dating employing 14C decay, 4He
accumulation and 4He/ 222Rn methods, was undertaken. Laboratory and field procedure for
water sample collection, carbonate precipitation, storage and analyses were developed for
various groundwater dating methods.
88
Experimental
Groundwater samples were collected from tubewells, hand pumps, dug wells and thermal
springs ranging in depth from 3m to 350m. Prior to sampling, the hand pumps and tubewells
were purged long enough (>3 well volumes) to flush out any stagnant water. The standard
water quality parameters like temperature, pH and electrical conductivity (EC) were measured
in the field during sampling using the µ-processor based water analysis kit.
Detailed procedures for sampling, storage and analytical techniques have been discussed
elsewhere (Deshpande, 2006), however, some of the important aspects are presented in the
following.
14
C Dating Method
For 14C dating, about 100 litre of groundwater was piped directly into a collapsible high
density PVC bag through a narrow opening. The PVC bag was kept in the folded condition in
a stand (Figure 2) designed specifically for this purpose and assembled from its prefabricated
parts at the site. The PVC bag unfolds only when the groundwater gets filled into it. Before
piping in the groundwater, a few pellets of NaOH (~10g) were added to the PVC bag to raise
the solution pH to >10 for immobilising the dissolved CO2 in the form of CO32– and its
eventual precipitation as barium carbonate. At pH greater than 10.3 most of the dissolved CO2
is in the form of CO32– since at this pH, activity of HCO3– drops and activity of CO32– rises
rapidly (Drever, 1997).
Depending upon the alkalinity and sulphate concentration of groundwater samples (measured
in the field), a pre-determined amount of barium chloride (BaCl2) was then added to the
‘groundwater-NaOH’ solution to ensure complete precipitation of dissolved carbonates (Clark
and Fritz, 1997). Following vigorous stirring, the mixture was left undisturbed for precipitates
to settle in the conical base of the PVC bag (Figure 2). It usually takes 4-5 hours for the
precipitates to settle. After decanting the supernatant liquid, precipitates were transferred to
glass bottles and sealed by capping the bottle with a bromobutyl synthetic rubber stopper and
triple aluminium protective cover on it using a hand held crimping tool. Care was taken to
prevent/minimise sample exchange with atmospheric CO2 during the entire field procedure.
On reaction with orthophosphoric acid, barium carbonate precipitates liberate CO2. The
liberated CO2 was first converted to acetylene and then trimerised into benzene (C6H6) and the
14
C activity in the benzene counted by liquid scintillation spectroscopy (Gupta and Polach,
1985). A small aliquot of the sample CO2 was sealed in glass ampoules for δ13C measurement
using SIRM (PDZ Europa Model GEO 20-20). The processing and analyses of the CO2
liberated from the groundwater samples was done by Dr. M.G. Yadava in the 14C laboratory of
PRL.
4
He Dating Method
Groundwater samples were collected directly from the pump outlet using a PVC tube (Φ = 8
mm) to divert the water flow and to transfer the same to the bottom of a 1.2-litre soda-lime
glass sample bottle, pre-rinsed with groundwater from the same source being sampled. It was
ensured that the tubewell was pumping at least for half an hour prior to sampling. In case of a
hand pump, it was operated at least for 15 minutes before sampling. After allowing the sample
bottle to overflow for a while and when no bubbles were visually seen, the PVC tube was
withdrawn and the sample volume was reduced to a factory -marked position on the sampling
bottle leaving 98 ml of air above water surface. The bottle was then sealed within few seconds
with a rubber stopper and triple aluminium protection cap using a handheld crimping tool.
89
Wherever pumping facility did not exist (e.g. a dug well), the sample was collected by
immersing an inverted (mouth down) empty 2-litre glass bottle inside the water with the help
of a rope and suitable weight attached to keep it inverted. After the bottle reached the required
sampling depth, it was reverted (mouth up) for water sampling. The collected sample was then
transferred to a 1.2-litre soda-lime bottle and sealed as above.
The bottles were stored in inverted position to minimise any loss of helium from the stopper.
The air in the bottle got equilibrated with water by movement during transportation and
additionally by shaking for sometime in laboratory before analysing helium concentration by
the standardised procedure (Deshpande, 2006, Gupta and Deshpande, 2003).
4
He/222Rn Dating Method
For 222Rn measurements, groundwater from pump outlet was piped directly into 630-ml PVC
bottles (Brand: Tarson) by a PVC tube and allowed to overflow for >> 3 bottle volumes to
minimise atmospheric contamination during sampling. The bottles were completely filled up
to the brim and capped immediately and sealed with parafilm (Brand: AMERICAN Can
Company) to prevent escape of dissolved gases. 222Rn was measured, within 5 days of sample
collection, by counting 609 keV gamma rays produced by the decay of its short-lived daughter
214
Bi using a high purity germanium (HPGe) gamma ray spectrometer (Agarwal et al, 2006).
Results and Discussions
The complete results from this multi-parameter geochemical and isotopic investigation has
been reported elsewhere (Deshpande, 2006; Agarwal et al, 2006; Gupta and Deshpande,
2003). Some important observations are presented in the following.
The isoline map of radiocarbon ages is shown in Figure 3. It is seen that the groundwater 14C
ages progressively increase from <2 kaBP in the recharge area (foothills of Aravalli
mountains) to a limiting value of >35 kaBP in the LRK-NS-GC tract. Within the Cambay
Basin, 14C age isolines are nearly parallel to each other and the horizontal distance between
successive 5 kaBP isolines is nearly constant. The low lying LRK-NS-GC tract is the zone of
convergence for the surface drainage (Figure 1). Presence of several free flowing artesian
wells found in the LRK-NS-GC tract and continuity of the aquifers with those from the
Cambay Basin confirm that their recharge area is in the foothills of Aravalli Mountains.
From the compilation of lilthologs of various tubewells it was found that succession of sand/
silty-clay layers in the multilayered aquifer system is roughly inclined parallel to the ground
surface and the sampled tube wells tap nearly the same water bearing formations across the
NGC region. Since the tubewells tap all the water bearing horizons intercepted within their
maximum depth, these can be treated as pumping a single aquifer system for which the 14C
ages increase progressively in the flow direction. The above geo-hydrological model appears
reasonable because, the unconfined aquifers in the region have almost completely dried up (as
evident from several dried up and abandoned dug wells). Therefore, the groundwater in all the
parallel layers of confined aquifers is recharged largely from the sediment-rock contact zone
in the foot hills of Aravalli Mountains, and after the confinement becomes effective, moves
with nearly the same velocity. The narrow range of δ13C values of TDIC of groundwater
indicates that the dead carbon dilution factor for the various samples is not significantly
different from each other. As a result, the relative age difference between the samples can be
relied upon.
90
Within the Cambay Basin, the age isolines are nearly parallel to each other and the horizontal
distance between the successive 5 kaBP isolines is nearly constant for the groundwaters older
than 5 kaBP. This gives a regional flow velocity in the range 2.5 – 3.5 m a–1 for the prevailing
hydrostatic gradient of 1 in 2000 (GWRDC, unpublished data), which is comparable to an
earlier estimate of ~ 6 m a–1 (Borole et al, 1979) for a small part of the Vatrak-Shedhi subbasin (marked by an ellipse in Figure 3). The Vatrak-Shedhi sub-basin is closer to the recharge
area, therefore, both the permeability and the hydraulic gradient are expected to be relatively
higher than that for the regional estimates of flow velocity (2.5 – 3.5 m a–1) from this study.
The isoline map of the excess 4He and corresponding estimated 4He ages of the groundwater
in the NGC region is shown in Figure 4. The 4He ages are calculated based on the measured
concentration of Uranium (U = 1.07 ppm) and Thorium (Th = 7.54 ppm), and assuming the
value of density (ρ) = 2.6 g cm–3, porosity (n) = 20% and helium release factor (ΛHe) = 1.
With these values, the 5 ppmAEU Helium Excess corresponds to groundwater 4He age of ~15
kaBP. If, however, the value of He is assumed to be 0.4, the 5 ppmAEU Helium Excess
would correspond to 4He age of ~37 kaBP and would be in close agreement with the
groundwater 14C age isoline of >35 kaBP (Figure 3).
It is seen from Figure 4 that 4He concentrations rapidly increase west of the 5 ppmAEU
Helium Excess isoline that almost coincides with WCBBF. This could be due to (i) rapidly
increasing residence time of groundwater resulting from decrease in the transmissivity as a
result of a general decrease in grain size away from the sediment source; and/or (ii) increase in
the influence of deep crustal flux of helium in the aquifer. The latter possibility is supported
by very high (>50 ppmAEU) Helium Excess pockets on both eastern and west flanks of the
Cambay Basin. The east flank is the major recharge area of aquifers in the Cambay Basin
where groundwater pockets of very high helium are associated with the thermal springs of
Tuwa and Lasundra. Considering the observed Helium Excess as in-situ produced will result
in groundwater age >100 kaBP, which is not tenable hydro-geologically. The presence of
extraneous 4He in these pockets is also indicated by the overlapping of high groundwater
temperature (>35°C) pockets suggesting hydrothermal venting of deep crustal fluids in such
pockets. Therefore, 4He method of dating is not applicable for groundwater in such pockets.
However, it is possible that observed high Helium Excess (and consequently, higher 4He ages)
in certain groundwater inliers are due to injection from a deeper source that derives helium
from much larger aquifer/ rock volume.
The isoline map of estimated 4He/222Rn ages of the groundwater in the NGC region is shown
in Figure 5. The 4He/222Rn ages are calculated based on Th/U concentration ratio = 7.1; ρ =2.6
g cm–3; n = 20%; and release factor ratio ( Rn/ He) of 0.4. A gradual WSW progression of
4
He/222Rn ages, away from the recharge area, is seen in Figure 5 similar to 14C age progression
(Figure 3).
The groundwater ages derived from simultaneous application of above three methods in the
NGC region not only provided the information about the direction and rate of sub-surface
movement of ground water but also provided useful insights about origin of high fluoride,
high excess helium and high temperature in the ground waters. In addition, the 4He and
4
He/222Rn ages provided information about how tectonic framework of the region influences
the groundwater temperature regime. The groundwater ages also provided information about
the role of palaeoclimatic changes in influencing the ionic and isotopic characteristics of
groundwater in the NGC region. A variety of new scientific information generated from
geochemical and isotopic investigation in conjunction with ground water ages is presented by
91
Deshpande (2006), Agarwal et al (2006), Gupta and Deshpande (2003 a and b) and Gupta et al
(2005)
Summary
Basic concepts of the ground water dating using 14C, 4He and 4He/222Rn methods are
discussed. These three methods were simultaneously applied in the regional aquifer system of
North Gujarat Cambay region. All the three methods provide comparable ages. The accuracy
of 4He and 4He/222Rn ages depend to a large extent on the value of the release factors of He
and Radon considered in the age equations. The estimated regional groundwater flow velocity
in the Cambay Basin is in the range 2.5 – 3.5 m a–1.
Acknowledgements
The case study reported here is a part of a research project entitled “Estimation of Natural and
Artificial Groundwater Recharge Using Environmental, Chemical and Isotopic Tracers and
Development of Mathematical Models of Regional Aquifer Systems in North Gujarat”.
Authors acknowledge the Gujarat Water Resources Development Corporation Ltd.,
Gandhinagar (GWRDC) for sponsoring this project and for sharing the unpublished
hydrogeology data from the study area. Authors also thank Dr. M. Agarwal, DR. M. G.
Yadava, Mr. Bhaswan Raval and Late Shri M.H. Patel who were research team members and
contributed significantly to the field and laboratory activities.
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14
C – Gehaltes von hartem Grundwasser.
Münnich KO. (1968). Isotopen-Datierung von Grundwasser, Naturwissenschaften, 55, 158–
163.
Nazaroff WW, Moed BA, Sextro RG. (1988). Soil as a source of Indoor Radon: Generation,
Migration and Entry. In: Nazaroff, W.W. and Nero, A.V. Jr.,(Eds.),Radon and its decay
Products in Indoor Air, John Wiley & Sons, NY.
94
Rama and Moore WS. (1984). Mechanism of transport of U-Th series radioisotopes from
solids into groundwater. Geochim. Cosmochim. Acta, 48, 395–399.
Scheidegger AE. (1961). General theory of dispersion in porous media. J. Geophy. Res.,
66,3273–3278.
Stute M, Schlosser P and Clark JF. (1992a). Palaeotemperatures in Southwestern United
States derived from the noble gases in groundwater. Science, 256, 1000–1003.
Stute M, Sonntag C, Deak J and Schlosser P. (1992b). Helium in deep circulating groundwater
in the Great Hungarian Plain: Flow dynamics and crustal and mantle helium fluxes. Geochim.
Cosmochim. Acta, 56, 2051–2067.
Torgensen T and Ivey GN. (1985). Helium accumulation in groundwaters, II. A model for the
accumulation of the crustal 4He degassing flux. Geochim. Cosmochim. Acta, 49, 2445–2452.
Torgersen T and Clarke WB. (1985). Helium accumulation in groundwater, I: An evaluation
of sources and the continental flux of crustal 4He in the Great Artesian Basin, Australia.
Geochim. Cosmochim. Acta, 49, 1211–1218.
Torgersen T. (1980). Controls on pore-fluid concentration of 4He and 222Rn and the calculation
of 4He/222Rn ages. J. Geochem. Explor. 13, 57–75.
Vogel JC. (1967). Investigation of groundwater flow with radiocarbon. In Isotopes in
hydrology, IAEA, Vienna, pp 355–368.
Vogel JC. (1970). Carbon-14 dating of groundwater. In Isotope hydrology. IAEA, Vienna, pp
225–237.
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241.
95
Figure 1 Map showing the North Gujarat Cambay (NGC) region and the geographical locations of some
important landmark features such as major rivers, towns, lakes etc. The inset map shows location of the
NGC region.
Figure 2 Picture showing a specially designed foldable stand with conical aluminium base which holds the
high density PVC bag filled with 100 litre of water sample. The supernatant water is decanted by piercing
the bag after the carbonate precipitates settle in the conical base of the bag.
96
Figure 3 Isoline map of groundwater Radiocarbon ages, along with sampling locations. Sampling locations
of an earlier study (Borole et al, 1979) are enclosed in an ellipse.
97
Figure 4 Isoline map of estimated 4He ages of groundwater from the NGC region (for helium release
factor; ΛHe = 1). Isoline of 15 kaBP runs almost along the Western Cambay Basin Bounding Fault
(WCBBF). This isoline will correspond to the 4He age of ~37 kaBP for ΛHe = 0.4, and would be in close
agreement with the groundwater 14C age (for example, compare along line BB’). Dots indicate sampling
locations. L, T and Z respectively indicate the locations of thermal springs at Lasundra, Tuwa, and the
tubewell in Zinzawadar.
98
Figure 5 Iso-line map of groundwater 4He/222Rn ages in the NGC region for Th/U = 7.1; Rn/ He = 0.4; ρ
=2.6 g cm–3 and n = 20%. Samples with >10ppmAEU ‘excess He’ and/or >2000 dpm/l 222Rn in the
recharge area were exclude during contouring because these indicate preferential pathways for
addition of water from deeper sources.
99
Radioactive isotope tracers for groundwater recharge studies
A Shahul Hameed
Scientist and Head, Isotope Hydrology Division, CWRDM, Kozhikode
[email protected]
Abstract
Groundwater, a vital resource for the mankind is primarily recharged through precipitation. The demand
for this prime resource is ever increasing warranting thorough understanding of the system for better
development and judicious management. Tracers of different kinds help to study the movement of subsurface water and understand the system effectively. The basic requirement of a best tracer is that it should
behave in the same manner as the material to be traced. In this context, water would be an ideal tracer for
water in the hydrological cycle. Isotopes in the water molecule behave as a best tracer in this context.
Isotope based technique form a part of modern tool serving as compliment to the conventional methods in
the groundwater investigations.
Isotope techniques are largely based on the tracer concepts which
generally aims to directly trace the movement of water molecules in any part of the hydrological cycle and
derive information on the transport processes and, study how such processes are affected by other factors.
Environmental and artificial radioactive isotopes are found to be best tool for groundwater investigations.
Artificial tracer methods have been extensively used in India and in other countries to estimate rates of
infiltration in the unsaturated zone for determining the direct rainfall recharge to the groundwater. The
method involves tagging a horizontal layer at a certain depth below the root zone with a suitable tracer
followed by monitoring the tracer profile at regular intervals. Radiotracers are usually used as they have
good detection sensitivity and hence can be introduced in small quantities without large disturbance to the
soil or its moisture content.
With this in view, the details on isotope based techniques with special reference to the application of
radioactive isotopes for groundwater recharge measurements are discussed in this manuscript.
Introduction
The concept of tracers in hydro-geological investigations is not entirely new. The basic requirement of a
best tracer is that it should behave in the same manner as the material to be traced. Tracers are substances
such as salts, dyes or chemical compounds which may behave exactly similar to the materials to be traced
but differ from them by a particular property. Differences in mass, conductance, colour, temperature or
radioactivity are some of the properties used to employ a particular substance as a tracer. Isotopes as tracers
perform key roles for solving many hydrological problems including investigations pertain to groundwater.
Isotope based techniques largely follow the tracer concepts which generally aim to directly trace the
movement of water molecules in any part of the hydrological cycle. Isotopes in the water molecule
available in nature and those produced artificially in reactors are effectively used as potential tracers in
hydrology and have been widely employed in the recharge assessment of groundwater in many parts of the
world.
In this context, a brief description on the basics of isotopes, their different types and their role in
groundwater recharge investigations are presented in this manuscript with a case study.
Isotopes – environmental and artificial
Isotopes are atoms of the same element having same atomic number but different mass numbers as
they differ in the number of neutron in the nucleus. In general, the isotopes are grouped into two,
namely, stable isotopes and radioactive isotopes. Further, based on their origin, they can be classified as
100
environmental (or natural) and artificial isotopes. Environmental isotopes are those isotopes, both stable
and radioactive, which occur in the environment in varying concentrations as a result of cosmic ray induced
nuclear reactions involving elements like nitrogen and carbon.
The environmental isotopes of interest in hydrological studies are:
2
H, 18O, 13C, 15 N, 34S, 37Cl
Stable isotopes
3
H, 14C, 137Cs, 32Si, 210Pb
Radioactive isotopes
Among this, Deuterium(2H), Oxygen-18(18O) and Tritium(3H) being the constituents of the water molecule,
are conservative and hence ideal water tracers. The isotopes like Carbon-13, Carbon-14 etc., which occur
as dissolved compounds are non- conservative i.e. their physico-chemical behaviour is not identical to
water. Other isotopes which are also used in a few laboratories include Silicon-32, Argon-39, Krypton-81,
Krypton-85 and Chlorine-36.
The artificial isotopes are those produced in the reactor or laboratory as per the requirement. These
isotopes are introduced into the system by the investigator to trace the movement of water and its
components. The common artificial isotopes in use in hydrological investigations are: 3H, 14C, 82Br, 60Co,
58
Co, 51Cr, 131I, 22Na, 198Au.
Isotope applications in groundwater investigations
Isotope data in combination with hydrological, hydro-geological and geo-chemical data help to solve many
environmental problems. The isotope based techniques mainly involve the use of either environmental
isotopes or artificial isotopes. In groundwater studies, isotope techniques are broadly employed in the
following fields.
Age determination
Occurrence and recharge mechanism
Origin, identification of recharge sources and areas
Soil moisture movement in unsaturated zone
Inter connection between groundwater bodies
Pollution source and mechanism
Environmental Tritium as a tracer
The origin and the age are two important parameters concerning groundwater which can be determined by
techniques based on isotopes. Whereas stable isotopes throw some light on the origin of groundwater, the
radioactive isotopes are useful for determining the age/residence time. Tritium (3H ) and Carbon-14 (14C)
are the two isotopes that are in use for the determination of groundwater residence time. Tritium and 14C
are produced in the atmosphere by the interaction of secondary neutrons produced by cosmic rays with
elements like nitrogen as per the following nuclear reaction:
1
14
( 15N )
14
1
14
( 15N )
12
n +
n +
N
N
C*
C
1
+
+
H
3
H*
The radioactive isotopes 3H and 14C produced in the atmosphere are converted to their oxides and reach
the earth through precipitation which serves as the vehicle. The environmental radioactive isotopes undergo
decay as a function of time following the first order kinetics described by the equation,
A = A0 e-λ t
101
where A is the observed activity, A0 is the initial activity when water entered into the aquifer, λ is the
decay constant and t, the age of groundwater. Tritium has a half life of 12.3 years and is applicable for
estimating the groundwater residence time up to 50 years. On the other hand, 14C with a half life of 5730
years can be used to find out the age of groundwater spanning several hundred to 50,000 years.
The details on the recharge measurements using environmental and artificial isotope tracers are presented
in the following sections.
Qualitative and quantitative recharge using environmental tritium
Environmental isotopes help in studying the problems concerning groundwater recharge qualitatively to
understand whether groundwater recharge occurs or not i.e. whether a particular water is a renewable
resource, which is particularly important in arid and semi arid regions. Besides, it also helps to determine
quantitatively the annual recharge of precipitation to the aquifer in a particular region.
Qualitative evidence of recharge
Nuclear bomb tests, which began in 1952 in the northern hemisphere have resulted in large amount of
tritium in the atmosphere. They reached a peak in 1963, with up to 10000 TU in a single monthly rain in
the United States. The international treaty stopped surface nuclear bomb tests in 1963 and tritium
concentrations in precipitation decreased steadily afterwards. If a particular groundwater body contains
tritium significantly above 5 TU (1 TU is 3.2 pCi/l or 0.12 Bq/l) then it has been recharged in the post
thermonuclear era (i.e. after 1952). Tritium can hence be used to detect recharge which occurred in the past
three decades. However, in arid areas due to scarcity of rainfall and high potential evaporation and low
infiltration through the unsaturated zone, the shallow groundwater may not receive any modern recharge.
This is indicated by low tritium content in groundwater. Environmental Carbon-14 could be used to date
very old waters up to an age of 50,000 years. A number of hydrological studies carried out in arid zones
using isotope techniques have shown that the deep groundwater is very old.
Quantitative estimation of recharge
The distribution of tritium in precipitation has strongly varied with time particularly during 1963-1970 due
to thermonuclear tests. Hence, determination of depth at which the tritium peak of 1963 is present in the
unsaturated zone soil water could be used for estimating groundwater recharge. The total water content
above the depth at which the tritium peak of 1963 was found, is the recharge over the intervening years.
The environmental tritium method has been used by many researchers. The method cannot be used if the
1963 peak has already reached the water table. This is mostly the case, except in arid regions.
Direct recharge of precipitation to the groundwater
Artificial tracer methods have been extensively used in India and in other countries to estimate rates of
infiltration in the unsaturated zone for determining the direct rainfall recharge to the groundwater. The
method involves tagging a horizontal layer at a certain depth below the root zone with a suitable tracer
followed by monitoring the tracer profile at regular intervals. Radiotracers are usually used as they have
good detection sensitivity and hence can be introduced in small quantities without large disturbance to the
soil or its moisture content.
The vertically downward movement of moisture is very slow and the moisture flows in such way that if any
fresh water is added to the top surface of the soil, the infiltrated layer of the water pushes the older layer
downward in the soil system till the last layer of the moisture reaches the saturated zone. The moisture
movement through the soil in this way is termed as piston flow, the added water replacing the older water
further downward but in the same amount thus keeping the affective moisture content constant.
Tritium as tritiated water is the most commonly used tracer, although others like Cobalt-60, a gamma
emitting isotope, as K3Co(CN)6, have also been used to take advantage of in-situ detectability in the
field.
102
The radiotracers used for soil moisture movement studies should have the following
characteristics.
i.
ii.
iii.
iv.
v.
vi.
it should follow the movement of water as closely as possible
it should be easily detectable
it should not change the hydraulic characteristics of an aquifer
it should not be absorbed/adsorbed by the medium through which it will be transmitted along
with water
its natural concentration must be much lower than the intended dose
its half life should be optimum and long enough for the duration of the experiment, but short
enough so that radioactive contamination is minimum
Tracers suitable for moisture movement studies
Among the radioactive tracers available for the investigation of soil moisture movement, tritium which is
the radioactive isotope of hydrogen is considered to be an ideal tracer. It has all the desirable qualities
mentioned in the preceding section. Besides, it has the following additional advantages.
i.
ii.
iii.
iv.
it behaves similar to ordinary water
it is pure beta emitter of low energy, the energy level being 18.6 Kev, and belongs to the
lowest radio-toxicity class
it can be measured with high detection sensitivity
it has comparatively long half-life (12.43years) and hence useful for soil moisture movement
studies.
However, tritium as a tracer is not entirely free from some disadvantages. For example, being a soft beta
emitter, it cannot be measured in the field in-situ. The application of artificially injected tritium will
interfere with the use of naturally occurring tritium in hydrological investigations.
The recharge to groundwater is determined by finding the tritium peak shift and multiplying it by average
field capacity in the tritium peak shift region. This recharge is the recharge to the groundwater during the
time interval of tritium injection and sampling. Groundwater recharge is also found out by multiplying the
tritium peak shift by the average of volumetric moisture content in the peak shift region. This later
procedure is claimed to be better method by some workers. Mathematically, the percentage of recharge to
groundwater can be found using the relationship.
Av . d
----------- x100
P
R
=
R
= Percentage of recharge to groundwater
Av
= Average volumetric moisture content in the tritium peak region
d
= Shift in tritium peak (cm)
P
= Precipitation or irrigation (cm)
Where
In order to get meaningful results on recharge by the tritium injection technique, it is essential to take
utmost care while carrying out the field experiment and laboratory analysis of the tracer. Selection of
representative field site, marking of the site for relocation, quantification of radiotracer and injection of the
tracer at the particular depth are some of the points to be dealt with carefully. Regarding the site of
injection, it should be representative of the area of investigation; it must have permanent marking points
like trees, electric poles, roads etc to identify the site later for soil sample collection.
103
CASE STUDY
General features of the experimental sites
Field experiments were conducted at two locations viz. Mooliyar and Cheemeni in Kasargod District of
Kerala (Fig.1). These sites fall under the Chandragiri and Kavvayi river basins, respectively. Both the sites
are located equidistant from Arabian Sea Coast and have similar hydro-meteorological characteristics
typical of humid tropics. The area receives an uneven
Fig.1 Location map of the recharge experiment sites at Kasargode
distribution of rainfall in different months. The average annual rainfall in Mooliyar is 3800mm and that in
Cheemeni is 3890mm. The number of rainy days during the year of the investigation in the selected areas
are 110 days with a monthly rainfall of the order of 1000 to 1500mm in the months of July to August and
practically no rain in the months from January to April. The maximum temperature recorded during the
year of the study was 39º C and the minimum was 19º C. The annual potential evaporation was 1332mm.
Both the study areas form part of the midland region with an elevation of 7.6m to 76m above MSL in the
northern part of Kerala. The locations have a general slope towards the west. The morphological unit of
the area is residual flat topped hills with gentle slopes and concave depositional valleys. The laterite form
as the capping over the mounds and in low lying areas they occur as detrial formations. The depth of
laterite over burden in the topography is about 10m in both the areas. The area is represented with
granulitic rocks of Precambrian age. The area is devoid of much vegetation except for the presence of
grass and trees like cashew, which are scattered. The laterites are hard in nature which occurs to a depth of
up to 11m. However, the loose soil occurs only to a depth less than a metre. Below the laterite zone, a
lithomarge zone containing clays is observed. The kaolin is underlain by weathered basement rocks.
104
Tracer Injection
A total of seven sites (4 in Mooliyar and 3 in Cheemeni) in Kasargod district were chosen for the tracer
application. Tritium activity equal to 100µCi was prepared for each site and taken to the sites. At each
site, 5-10 holes equidistant along the perimeter of a circle of radius 5cm were driven to a depth of 50 to
75m using an iron rod, ensuring the absence of vegetatitive roots. In each hole, tritiated water with tritium
of specific activity of 20µCi/ml was introduced using a syringe through small copper tubes. The openings
in the hole were closed by filling with the loose soil to prevent the loss of the tracer due to evaporation.
Iron nails were plugged at the site of injection to locate the site of injection. Necessary identification
marks available in the location were recorded. After the monsoon season, the soil samples were collected
at an interval of 5-10 cm depths beyond the depth of the injection point to ascertain the downward
movement of the tracer. Necessary precautions were taken while collecting and transporting the soil
samples to the laboratory for the analysis. It is better to keep the soil samples either in polythene bags or
in the air tight polythene containers to avoid loss of moisture during transit. The tritiated water was
extracted from the soil by vacuum distillation method. The moisture content of the soil before the injection
of the tracer as well as at the time of collection of soil samples were also determined. Moisture
determination using an infrared moisture meter is always preferred to that by gravimetric method.
Fig.2 Depth wise distribution of measured tritium activity at the test sites
The tritium activity in the extracted moisture was determined by counting in Liquid Scintillation Counting
system. Each sample is counted for a minimum period of 2 minutes depending upon the total number of
counts to reduce any statistical error. The tritium counts per minute (cpm) obtained are plotted against
depth and the center of gravity of the tritium peak is calculated (Fig.2). By subtracting the depth of
injection from the center of gravity of the tritium peak, the shift in the tritium peak can be calculated.
Then, the percentage of recharge to groundwater was calculated using the relationship mentioned earlier
section.
Site of injection indicated that there was very little vertical movement of tritium even after the second
monsoon was over. The downward movement of moisture through the laterite is generally low as the
hydraulic conductivity in the vertical direction is considered to be less compared to that in the horizontal
direction
References
Athavale, R.N, Murty C S., Chand R., 1980. Estimation of recharge to the phreatic aquifers of the Lower
Manar Basin, India by using the tritium injection method. Journal of Hydrology, 45:185.
Athavale, R.N. 1983. Injected tracers in studies on the unsaturated zone.
Hydrology, BARC, December, 13-14, 1983 : 179-197..
105
Proc. Workshop on
Clark, I.D
328pp.
and Fritz, P., 1997. Environmental isotopes in Hydrogeology. Lewis Publishers, New York.
CWRDM, 1998. Application of nuclear techniques to hydrological problems of Kerala. Final Report to
Board of Research in Nuclear Sciences (BRNS), Department of Atomic Energy, BARC, Mumbai. 69pp.
Datta P S, Gupta S K, Jayasurya A. 1978. Soil moisture movement through Vadose zone in alluvial plains
of Sabarmatic Basin. Report No.HYD-78-0333, Physical Research laboratory, Ahmadabad, India.
Goel P.S,Detta P.A,Rama,Sangal S.P, Hans Kumar Prakash bahadur,Sabharwal R.K, and Tanwar
B.S.1975 . Tritium tracer studies in ground water recharge in the alluvial deposits of Indo-Gangatic plains
of Western U.P., Panjab and Haryana Proc Indo-German Workshop on Approaches and Methodologies for
Development of Ground water Resources, Hydrabad, India.
Nair A.R, Pendharkar A.S, Navada S.V, and Rao.S.M. 1979. Ground water recharge studies in
Maharashtra. Development of Isotopes Techniques and Field Experiences Proc. Symp Isotope Hydrology
,Vienna :503
Zimmerman U,Munich.K.O , Roether. N,Kreutz .W,Schubech.K, and Siegel.O 1996. Tracers determine
movement of soil moisture and evapo-transpiration. Science,152
106
Estimation of groundwater recharge using isotope and geochemical
tracers
D.V. Reddy# and P. Nagabhushanam
National Geophysical Research Institute (Council of Scientific and Industrial Research),
Uppal Road, Hyderabad – 500606, India.
#
[email protected]
Abstract
Groundwater recharge is an important parameter in the sustainable development and management of
groundwater resources. Groundwater recharge occurs in two modes based on the hydrogeology of the area,
i.e., piston flow of recharge and preferential flow of recharge. Isotopic and tracer methods play an
important role in the recharge estimation. Injected tracer method provides short duration seasonal values or
one or two years experimental period recharge values, while the environmental tracers provide long-term
recharge values. The injected tracer method provides minimum recharge value as this method is based on
the piston flow recharge model. Moreover, this method will not consider the preferential flow, which is
more dominant in several geological conditions. Environmental chloride tracer method based on the
chloride mass balance (CMB) provide reliable and long-term recharge values, and can also be used for
estimation of total recharge (piston flow plus preferential recharge). The CMB method has also
demonstrated its potential in the assessment of the performance of percolation tanks as artificial recharge
structures.
Introduction
In arid and semi arid regions, where the rainfall pattern is quite erratic and the annual average rainfall spans
a range of ~200 to 800 mm, and the potential evaporation is more than the rainfall, the groundwater
recharge can be considered as zero to few percent (~120) of total rainfall. Generally, the recharge is
facilitated by some extreme rainfall events in a day or few days in a year. To measure the temporally
variable and also the small quantities of recharge reliable methods are needed.
Rainfall recharge can be expressed in terms of cm of water or percentage of rainfall recharged to
groundwater. Several conventional methods used for the above purpose (Lerner et al., 1990) require data
like specific yield, surface runoff, evapotranspiration etc. Generally such data are neither available nor
reliable. Gee and Hillel (1988) observed large errors in recharge estimates for arid and semi-arid sites
estimated using water balance methods, soil water flow models etc. The recharge estimates based on fixed
factors and annual precipitation are generally high and misleading due to inherent limitations and
uncertainties (Scanlon et al., 2002). To overcome these problems, geochemical and isotopic tracer methods
came into existence. The stable isotopes, Deuterium (2H) and Oxygen-18 (18O); radioactive isotope of
tritium (3H) and geochemical tracer, chloride ion, have been employed extensively in groundwater recharge
estimation. Several studies are conducted in various hydrogeologic environs: aeolian deposits, alluvium,
sedimentary deposits (Coastal aquifers) and crystalline rocks in India (granites, gneisses and basalts) (
Kumar et al., 1980; Navada et al, 1986; Datta et al., 1996; Sukhija et al, 1996 a & b; Sukhija et al., 1997;
Rangarajan and Athavale, 2000; Sukhija et al, 2003)
This paper summarises the studies carried out in the estimation of natural recharge and artificial recharge
using various isotopic (natural and artificial), geochemical tracer (chloride) and neutron probe.
107
Groundwater recharge estimation methodologies
Fig. 1 Different techniques used for groundwater recharge
Tracer definition
A tracer is a substance, which is present naturally in water, introduced in to it
intentionally, or un-intentionally, follows the flow path of water faithfully. Ideally a
tracer should be such that it travels with same velocity and direction as the water does,
and does not interact with solid material, should be non-toxic and relatively inexpensive.
Further the natural concentration of the substance used as tracer should be as low as
possible and easily detectable. The tracer should not modify the hydraulic properties of
the material being studied. Some of the earlier tracers used have been dyes and salts
which were used in Karst system. Since 1960 naturally occurring radioisotopes and stable
isotopes have proved invaluable tracer tools and later environmental chloride (Cl) has
become more popular
Soil moisture flow mechanism in the unsaturated zone
Generally, increase in groundwater levels found immediately after rainfall and the belief
is that rainwater is being added to the water table with in no time. In reality, it is not
correct; the rainwater takes several months to years to reach the water table. The
movement of soil moisture in the unsaturated zone explained below.
108
Piston flow mode of infiltration/recharge
Soil water movement in a homogeneous medium like sand, silt and clayey soils is in the
layered form. The infiltrated rainwater below the ground surface forms as a layer and
subsequent rains form different layers over the previous layer and it never crosses the
earlier layer. From ground surface to water table several such layers can be formed based
on the depth to water table and the rate of moisture movement. Such layered movement
of soil moisture in the unsaturated zone is called piston-flow movement (Fig. 2a). In such
conditions present day rainfall takes several years to reach the water table. The time taken
to the rainwater to reach the water table is called turn over time.
In contrast to the piston flow movement, recharge into the groundwater is often rapid, through conduits and
zones of preferred pathways. These conduits are fissures and or cracks filled with coarse material. In most
cases, water descends through the aerated zone in combined mode, that is, through inter granular pores as
well as through conduits (Fig. 2b).
Fractures
Piston flow
Preferential flow
Piston flow
Fig. 2 (a). Piston flow of water infiltrating into homogeneous granular soil and (b.) Infiltration of
recharge water in a combined mode (piston flow through uniform
grain size soils and preferential flow though fractures)
Estimation of natural recharge using environmental isotopes
Munnich and his co-workers (Zimmermann et. al. 1967) made pioneering contributions to the development
of tracer methods. From their experiments using environmental and injected tritium as tracers, they
postulated that, in case of homogenous soil without many cracks and fissures, the bulk of water movement
from unsaturated zone to saturated zone takes place in layered form, and hence, developed the concept of
“Piston Flow” movement of soil moisture. This concept provided a convenient method of evaluation of
recharge processes and recharge estimates using the thermonuclear tritium peak of 1963-64 in precipitation
(due to testing of nuclear explosions in the atmosphere) which could be easily identified in the unsaturated
zone.
Following this concept, many workers (Datta et. al., 1973; Sukhija and Rama, 1973; Sukhija and Shah,
1976; Verhagen et. al., 1979; Athavale and Rangarajan, 1988; Sukhija et. Al., 1996 a & b) have utilized the
environmental and injected tritium to estimate the groundwater recharge in semi-arid and arid
environments.
109
Environmental Tritium
There are two different methods in which environmental tritium can be used for average recharge
evaluation (i) by tracing the movement of precipitation carrying peak tritium concentration of 1963-'64
(Fig. 3), (ii) by establishing mass balance of the entire fall out of tritium.
.
Peak tritium method
In the 'Peak Method' (Munnich, 1968), the 1963-64 tritium peak in precipitation (Fig. 3) is made use of that
exhibits layered movement. It is possible to identify the tritium peak at some depth, though it might have
become rather broad due to dispersion and scatter. The amount of 3H input with in a given time interval can
be followed by making successive measurements. Recharge computations are made by measuring the total
amount of soil moisture up to the tritium peak position by taking soil moisture samples at regular intervals
(say 30 cm) by means of hand auger. The water is extracted from the soil moisture by simple vacuum
distillation apparatus and the moisture content is measured gravimetrically.
The recharge 'r' is computed following the relation,
r = (S/P)/100
----------- (1)
Where r = recharge as a percentage/fraction of rainfall.
S = soil moisture (cm) in the column from the surface to depth where tritium peak occurs.
P = total precipitation (cm) since 1963 to the time of investigation.
Total tritium method
`The other method i.e., 'Tritium Integral Method' (Munnich, 1968) aims at finding out the total amount of
bomb tritium in a profile underground (unsaturated and saturated zones) as a fraction of the total tritium
precipitated since the onset of thermonuclear era (1952). The total amount of tritium fallout is computed
adding the yearly production of the natural rainfall and its average tritium concentration. The tritium
levels if not available for some time intervals can be constructed from the world data.
110
Mathematically:
T = Σ Ai Pi
---------------------------- -
(2)
where T = total tritium fallout (T.U. cm)
Ai = weighted mean annual tritium concentration (T.U.) of precipitation in the year 'i'
Pi = precipitation (cm) in the year 'i '
And the amount of tritium in the profile is obtained by adding the products of moisture content by its
tritium concentration to a depth where tritium content becomes negligible. The total amount in the profile is
obtained by the following relation;
t = Σ aj × mjd
-------------- (3)
g
where t = total amount of tritium (T.U. cm) present in profile from ground level (g) to depth 'd'.
aj = tritium concentration (T.U.) in soil water of segment 'j'
mj = moisture column (cm) of soil segment 'j'
The ratio of the total tritium observed underground to the total tritium fallout 'T' in precipitation provides a
measure of the fraction of precipitation that goes to recharge the groundwater.
Mathematically, recharge percentage 'r' is given by:
r = (t /T)/100
--------------- (4)
In this case also soil samples are taken by hand auger at an interval of 30 cm till water table reaches.
Rock
type
Area
Aeolian
deposits
Thar desert Rajasthan
Alluvium
Gujarat
U.P.
Punjab
Haryana
Pondicherry
Semiconsolidated
Basalts
Consolidated
rocks(Granit
e/Gneiss)
Neyveli
Kukadi Basin
Godavari-Purna
Basin
Jam Basin
Lower Maner (S.S)
Marvanka Basin
Manila Water shed
Vedavati Basin
Noyil Basin
Vattamali-Karai
Basin
Ponnani Basin
Chitravati Basin
Kunderu Basin(S.S)
Auropally Water
shed
Recharge
averaged
during
1982-83
Ave. annual
rainfall
Recharge
Percent
Method
used
Source
328
3-10
Inj. 3H &
conventional
1a &
1b
1952-69
1971-72
1972
1973
1980-85
1984-85
1985-87
688
990
460
470
1200
1080
1200
612
652
3-11
22
18
17
19
28
15-27
7.5
8.6
Env. 3H
Inj. 3
“
"
Env.Cl/
Inj. 3H
“
“
“
2
3
4
5
6
7
8a & 8b
9
10
1988-91
1976
1979
1986
1978
1979
1979
866
1250
550
390
565
715
460
9-12
8
19
6
2-7
10
13
“
“
“
“
“
“
“
11
12
13
14
15
16
17
1979
1981
1982
1320
615
615
5
4
5
“
“
“
18
19
20
1984-85
573
3-6
“
21
111
It is obvious that peak method could be applicable better in areas where the soil is homogeneous and
movement of water is strictly layered. While the total tritium method is not seriously subject to this
restriction, it requires an accurate knowledge of tritium fallout. Also it is based on as much as the tritium
fallout has variation with time i.e., the results are dominated by the actual contribution of recharge during
1963-'65. For example, in England, Smith et al. (1970) used the method for estimation of groundwater
recharge in chalk and clay formation; Sukhija and Rama (1973) and Sukhija and Shah (1976) used it for
alluvial tracts of Gujarat (Fig. 4); Dincer, et al. (1974) used it for Kalahari sands. Based on two
environmental tritium methods estimated natural recharge in alluvial tracts of Gujarat varies from 3.3
to10.9% of the local rainfall (Table 1).
However, because of environmental tritium levels becoming less and less, and the tritium peaks reaching
greater depths, the environmental tritium method is becoming difficult to be used.
Table 1 Average annual groundwater recharge in different rock types in India using tracers
(Sukhija et al., 1996a).
1a - Sharma & Gupta (1985); 1b - United Nations (1976);
2 - Sukhija & Shah (1976); 3,4 & 5 - Goel et al.,(1975);
6 & 7 - Sukhija et al., (1988);
8a - Rangarajan et al.,(1989); 8b - Sukhija et al.,(1996b)
9&10-Athavale et al.,(1983); 11-Rangarajan et al.,(1994)
12-21 - Athavale & Rangarajan (1988).
Injected Tritium Method
The injected tritium technique is based on the piston flow model of soil moisture movement in the soil
matrix. In the tritium injection technique, the moisture at a certain depth in the soil profile is tagged with
tritiated water. The tracer moves downward along with the infiltrating moisture due to subsequent
precipitation. A soil core is collected from the injection site after a certain interval of time and the moisture
content and tracer concentration are measured from various depth intervals. The displaced position of the
tracer is indicated by a peak in concentration. The peak may be broadened because of factors such as
diffusion, irregularities in water input and streamline dispersion. The centre of gravity of the profile is
assumed to correspond to the displaced position of the tagged layer. Moisture content of the soil column,
between injection depth and displaced depth, is the measure of recharge to groundwater over the time
interval between injection of the tritium and collection of the soil profile (Rangarajan and Athavale, 2000).
112
Fig. 4: Tritium conc. at Ahmedabad rainfall and b) movement of environmental tritium conc. in soil
moisture in repeated sampling
Fig.5 depicts the injected tritium profile with the displacement of tritium signal from injection point (IP at
70 cm) and the centre of gravity (CG) of the profile at 210 cm depth. The moisture content between the
depths of CG and IP is the moisture displaced during the experimental period or recharge occurred during
the period. The recharge estimates using injected tritium along with environmental tritium and chloride for
various geological environs are summarized in Table 1.
Qualitative assessment of natural recharge using 14C
Carbon-14 dating method can be used to qualitative assessment of groundwater recharge. In general, for
phreatic aquifer conditions, the cabon-14 age indicates the modern or present day recharge. However, the
deeper aquifer or confined aquifer waters show some ages indicating the aquifer is not directly connected to
the present day recharge conditions.
113
Fig 5. Environmental Tritium profile in the sand stone (Pondichery)
Recharge evaluation using stable isotopes (Deuterium (D) and Oxygen-18(18O )
Stable isotopes of 2H and 18O which form part of water molecule can be used to estimate the groundwater
recharge. Mass balance technique is used to compute the recharge. In this technique stable isotope values of
storm water, pre and post storm groundwater are used by Shivanna et al (1994) for recharge estimation in
the Bagepalli, Kolar Dist, Karnataka, and found 19 to 27% of storm water recharge to groundwater.
Using the stable isotopes (D and 18O), qualitative evaluation of recharge were carried out by Kumar et al.
(1980) in hard rock Lower Maner Basin (LMB), (AP); Navada et al., (1986) in Rajasthan, Sukhija et al.
(2002) in Hyderabad conglomerate; irrigation return flow in IARI farm, Delhi (Chandrashekar et al,1990);
ground water mixing phenomenon in Delhi (Datta et al., 1996).
Environmental chloride as a tracer to estimate the natural recharge
The chloride ion (Cl) in water is conservative; simple to measure, has gained the popularity as a natural
tracer for estimation of groundwater recharge as well as for understanding the recharge processes. Chloride
is introduced in to the soil from the rain water and dry fallout. It travels along with the moving soil
moisture front. However, due to evaporation it gets enriched in the top soil zone and the same will move
into the moisture in the vadose zone along with the percolation water to the groundwater system. Similar to
tritium method, the chloride mass balance (CMB) method has been utilised in the unsaturated as well as in
the saturated zone to estimate the total recharge to the groundwater system (Errikson and Khunakasem,
1969; Allison and Hughes, 1978; Edmunds and Walton, 1980; Sharma and Hughes, 1985; Sukhija et al.,
1988, Cook et al., 1992; Native et al., 1995; Sukhija et al., 1996a; Wood et al., 1997; Davidson et al., 1998,
Reddy et al., 2009).
Assumptions in chloride usage in recharge estimates
The method has the following assumptions
1. The chloride concentration is constant in the precipitation over time and this concentration
averaged for a number of years to provide long term recharge.
2. There is no additional chloride input to soil through fertilizers, animal and human waste.
114
has been
3. There is no change in chloride storage below zero flux plane (ZFP) due to action of plant or soil or water
rock interaction.
4. There is no major change in land use.
5. The recharge is essentially through vertical flow over the area at the surface and at least 4-5 meters
below.
6. There is no significant runoff.
Estimanation of natural recharge using environmental chloride method
The simple CMB can be used to estimate recharge (Edmunds and Walton, 1980):
P × Clp = R × Clgw
(5)
R = (P*Clp) / Clsw
(6)
where:
R = recharge (mm/a)
P = mean annual precipitation (mm/a)
Clp = chloride in rain (mg/l)
Clsw = chloride concentration in soil water below the active root zone in the unsaturated zone (mg/l)
and Clsw = Clgw
(7)
where: Clgw = the chloride concentration of groundwater.
The above equation (7) is valid only in the piston flow regime, which is derived as a downward vertical
diffuse flow of soil moisture, is assumed. However, this assumption may not be invalid if the flow occurs
through preferred pathways that exist in the unsaturated zone. In general, the flow through the unsaturated
zone is bimodal (i.e. diffuse plus preferred flow), and the Cl content in the soil water in the unsaturated
zone will be different than the Cl content in the groundwater. If the Cl in the groundwater is less than the Cl
in the soil water, preferential flow paths exist. Sharma and Hughes (1985) have expressed this as:
R = Rp + Rm
(8)
where:
Rp and Rm represent the recharge (mm) from preferred pathways and the matrix, respectively. The total
recharge (R) can be estimated as:
R = Clp/Clgw
(9)
Further preferential flow through macro pores is assessed as the difference between total recharge and
matrix flow.
A typical chloride profile from Pondicherry is shown in Fig. 6, and the recharge results of the method are
shown in the Table 2.
115
Figure 6. Environmental chloride profiles in the sandstone in Pondicherry
Table 2: Comparision of recharge rates as percentage of mean annual rainfall (115
mm)for the repeated sampling by two approaches and the estimation of turn over time
after excluding the top 90 cm of the profile
Site
L.R. Palayam
Murattandi
Idayanchavadi
Ariyur
Tirubhuvani
Conc. method
Rc1 Rc2 Mean
27.5 23.7 25.0
17.7 13.4 15.5
11.4 15.3 13.4
15.8 12.9 14.4
13.8 9.5
11.7
Rc
29
18
16
17
13
Flux method
Rf1 Rf2
29.3 24.8
18.8 14.9
14.1 17.8
18.1 17.5
23.2 12.2
Mean
21.1
16.8
15.9
17.8
17.7
Rf
31
19
18
20
20
t1
12.9
11.7
11.0
4.1
7.5
t2
16.8
17.2
9.3
9.8
5.0
Mean
~15
~15
~10
~4
~6
•
•
•
•
•
•
Rc1 = % of recharge for the first collection (1984)
Rc2 = % of recharge for the second collection (1985)
Rc = mean recharge in cm/yr by concentration method
Rf1 = % of recharge for the first collection (1984)
Rf2 = % of recharge for the second collection (1985)
Rf = mean recharge in cm/yr by flux method
• t1 and t2 are turnover time (yr) determined for 1 and 2 collections
Recharge ranges estimated using environmental chloride for both piston flow and preferential flow
processes in three studied geological provinces are shown in Table 3.
Table 3. Recharge ranges estimated in 3 geological provinces using environmental
chloride method.
116
Preferential flow Preferential flow
Piston flow
Total recharge
recharge
recharge (% of
recharge
range (average)
Formation
(mm)
range (average)
range
total recharge)
(average) (mm)
(mm)
Consolidated
20-40
70-170
50-130
75
(Granites)
(30)
(120)
(90)
Semi-consolidated
170-300
260-440
90-140
32
(Sandstone)
(235)
(350)
(115)
Unconsolidated
13-66
13-50
-----(Alluvium)
(39.5)
(31.5)
As can be seen from the table 3, preferential flow has dominant role for the realistic assessment of total
natural recharge (sum of piston flow recharge and preferential flow recharge) especially for the
weathered/fractured granites and semi-consolidated sandstones where the preferential flow component of
recharge is generally unaccounted for in the estimation of total natural recharge in these formations.
Assessment of piston flow and total recharge in the groundwater flow direction in a
granitic watershed using CMB method.
The piston flow component of recharge can be estimated based on the soil moisture chloride and soil
moisture profiles. However, to estimate the total recharge, it is necessary to have Cl concentration of local
recharge. In general, the Cl concentration increases in the groundwater flow direction which includes local
vertical flow component and horizontal flow component. To asses the Cl concentration due to local vertical
recharge, Reddy et al. (2009) attempted a discrete model to assess the total rainfall recharge in different
zones, using the CMB (Fig. 7).
The Wailapally study area was divided in to 4 zones based on the geomorphic and hydrochemical
conditions. If the aquifer is a confined aquifer with specific recharge zone, whatever Cl concentration
measured in the zone I should also be same in the zones II, III and IV as there is no input in the individual
zones (Fig. 7a). However, if the aquifer is unconfined, there are Cl inputs from the rainfall recharge from
every point. The Cl inputs at a given point are vertical recharge through matrix flow and preferential flow
and also horizontal inflow (groundwater flow). Matrix flow can be calculated through measured soil
moisture chloride and rainfall chloride (equations 6-9). In order to estimate the total rainfall recharge for
individual zones representative groundwater chloride is needed. To estimate the long time reliable total
recharge for each zone here they have assumed the lateral flow as, in flow to the zone is equal to that of out
flow from the zone (quantity of groundwater). However its chemical characteristics vary and generally out
flow concentrations are more than the inflow in the flow direction. Input Cl is same as that of previous zone
output Cl and the output Cl concentration is generally higher due to the precipitation recharge (matrix flow
+ preferential flow) in that zone (Fig. 7b). In order to estimate the matrix and preferential flow in the
different zones the following equation is derived:
Matrix flow (%) in a zone (Rm(%) = (Clgwn – Clgwn-1) / Clsmn X 100
(5)
Preferential flow (%) in a zone (Rp(%) = (1-(Clgwn – Clgwn-1)) / Clsmn X 100
(6)
All the parameters are defined above and the n is n-1 stand for zones in the flow direction.
117
Fig. 7. Chloride mass balance model: (a) for confined aquifer; and (b) for unconfined aquifer. A discrete
model for different zones consisting of multiple flows (horizontal, matrix and preferential) with different
chloride concentrations (Reddy et al., 2009).
Based on the discrete model actual vertical input Cl in groundwater is calculated and further total rainfall
recharges in different zones are calculated (Table. 4). The study indicates, the total recharge and also
preferential recharge decreases in the flow direction with increasing piston flow recharge. This can be
explained based on the soil characteristics of the area. The exposed fracture system has highly pervious
pediment zone in the west favouring the preferential recharge, and the fine grained clayey soils in the zone
IV favoured the piston flow recharge.
Table 4. Average chloride concentration in the well water and the computed average total groundwater
recharge, piston flow recharge and preferential recharge in different zones of the watershed.
Zones
No. of
groundwater
samples
Ave.
Cl. conc.
(mg/l)
Well
water
Cl input
to GW
from Rf
recharge
total Ave. soil
rech. moisture
(%
Cl
Rf) (mg/l)
Matrix
flow
rechar
ge
(% Rf)
Matrix
flow
recharge
(% total
recharge)
preferent
ial
recharge
(% total
recharge)
Zone-I
Zone-II
Zone-III
ZoneIV
56
111
24
13
13.4
43.8
84.3
116.7
13.4
30.4
40.5
32.4
16
7
5
7
0.9
1.4
0.9
1.5
6
20
20
22
94
80
80
78
118
245
159
235
143
Fig. 8 Natural Recharge measured using Neutron
Moisture Probe in a depth profile
Measurement of total recharge using neutron moisture probe
Neutron moisture probe can be used for the determination of recharge by measuring the travel of moisture
through a soil column. The technique is based upon the existence of a linear relation between neutron count
rate and moisture content (% by volume) for the range of moisture contents generally exists in the
unsaturated soil zone. The mixture of Beryllium (Be) and Radium (Ra) is taken as the source of neutrons.
Another method is the gamma ray transmission method based upon the attenuation of gamma rays in a
medium through which it passes. The extent of attenuation is closely linked with moisture content of the
soil medium.
Repeat measurement of soil moisture during pre- and post-monsoon, where the raise in water table exists
indicates the total recharge during that season. Fig. 8 shows the example of moisture profiles during pre and
post monsoon periods. Actual amount of water added to the aquifer is computed using variation in moisture
content between respective water levels at the zone of saturation (Ramesh Chand et al., 2005). Further it
can also be assessed the effective storativity value using the quantum of water added to the system divided
by the water table rise. As can be seen in the Fig. 8, groundwater table during pre-monsoon period is 7.2 m
and during post-monsoon it is rose to 5.8 m. The difference is 1.4 m. Moisture content during pre-monsoon
time is about 10% and it is increased to 35% during post-monsoon when it is saturated. On an average 25%
of the 1.4 m is the total recharge occurred during that particular season.
Assessment of percolation efficiency of percolation tanks using the CMB method
Ever increasing groundwater exploitation resulted in the lowering of groundwater levels. To augment the
groundwater recharge, artificial recharge structures such as percolation tanks and check dams are the most
popular. To assess the efficiency of the percolation tanks CMB method is quite useful.
Quantitative evaluation of performance of the percolation tanks is shown in figure.9, where C1, C2 are
chloride concentrations in the tank water at time t1 and t2, Cp is weighted average chloride concentration,
‘f’ is fractional loss of water by evaporation and 1-f is percolation fraction. Assumptions for this study are,
there are no additional sources (like insignificant dry fallout) or sinks of chloride in the percolation tank
after impounding, and loss of water is through evaporation and/or percolation (seepage from the dam,
119
if any, should also be accounted), the mass balance of chloride in the tank water can be used in estimating
the percolated fraction of impounded volume of water (Sukhija et al., 1997).
Two tanks in Granite-gneisses located in Southern India (close to Hyderabad, Andhra Pradesh) and the
other two tanks situated in different hydrogeological terrains of basalt and sandstone but in similar climatic
conditions (Surendra Nagar Dist., Gujarat) were studied and the results are mentioned in table 5.
Comparison of efficacy of the studied tanks suggests that the effectiveness of percolation tanks is
controlled more by geology than climate. Also soil characters of tank bed and ayacut area are equally
important in the selection of sites for percolation tanks. Moreover the effective hydrogeologic regime such
as higher groundwater exploitation in the downstream of a tank also plays an important role in enhancing
the tank efficiency.Even stable isotopes were employed to estimate the Hingonigoda percolation tank
contribution, (Pune Dist, Maharashtra) to ground water (Shivanna et al., 1994). The tank water contribution
was 50 %
Fig. 9. Schematic diagram showing chloride mass balance in tank water for evaluation of percolation efficiency of
tanks.
120
to the near by wells and gradually decreased to 25% to the far away wells. Nair et al. (1979) qualitatively
evaluated the contribution of Shindawave percolation tank (Pune Dist, Maharashtra) to the groundwater
using Deuterium stable isotope content of tank, water canal water, and rain water.
Table 5. The average percolation efficiencies of studied tanks determined using chloride and water
balance methods.
Tanks
Percolation Efficiencies (% of impounded water)
Chloride Mass Balance
Water Balance Method
Method
In Granites
1.
2.
Kalwakurthy
Singaram
45
25
38
49
28
56
63
68
In Basalts
Lakanka
In Sandstones
Saper
Conclusion
Various techniques are available to estimation the groundwater recharge; however, choosing appropriate
techniques is often difficult. Isotopic techniques are more reliable than the conventional techniques.
However, most of the time, the values acquired by different methods are site specific and time bound.
Important considerations in choosing a technique include space/time scales, range, reliability, and
reproducibility of estimated recharge values. The range of recharge rates that can be estimated using
different approaches should be matched to expected recharge rates at a site. As the isotopic techniques are
expensive and needs trained manpower and sophisticated equipment; the environmental chloride method,
which is less expensive and more reliable than the other can be adopted.
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Wood WW, Rainwater KA, Thompson DB (1997) Quantifying macropore recharge: examples from a semiarid area. Ground Water 35: 1097-1106.
Zimmerman U, Munnich KO, Roether W (1967) Downward movement of soil moisture traced by means of
hydrogen isotopes. Geophys. Monogr., Am. Geophys. Union, 11, 28-36.
123
Use of natural isotopes in delineation of multi - aquifer system and
intermixing of waters – A case study
S.Suresh1, K.Shivanna2, S.Tirumalesh2 and J.F.Lawrence3
1.Central Ground Water Board, SCER, Chennai, 2. Isotope Division, BARC, Mumbai,
3. Department of presidency College, Chennai
Abstract
The population growth coupled with near utilization of surface water potential has increased the stress on
groundwater system. The coastal areas are the centres of development from British Raj Days and the trends
continue in Independent India also. In general, coastal areas are characterized by multi aquifer system
comprising many layers of sand clay alteration. The groundwater development through dug wells or filter
points or shallow tube wells is carried out depending on the depth of water level in the area. In planning for
a sustainable strategy, it becomes pertinent to know the degree of interconnection between the different
layers and intermixing of water. In the broader perspective, it also becomes necessary whether to treat the
different layers as a single layer or multi layered system, while planning for groundwater development.
The principles of hydrogeology and hydrochemistry have been used conventionally to decipher source of
recharge or salinity or intermixing. The use of naturally occurring isotopes, to track the source and
movements of water and solutes, to assess water budget and geochemical/mathematical models has been
gaining enormous importance. Environmental isotopes, both stable (2H & 18O) and radioactive isotopes (3H
& 14C) are generally used to study different hydrological aspects like, source and origin of groundwater,
mechanism of groundwater salinisation, source and origin of groundwater pollutants, aquifer–aquifer
interconnections, surface water groundwater interconnection. However, use of isotope signatures in
conjunction with the hydrogeology and hydrochemistry is a powerful tool for solving hydrological
problems.
A collaborative study was taken up by CGWB with BARC, Mumbai to study the coastal aquifers of
Southern part of Chennai Metropolitan Area and the paper discusses results and conclusions of the study.
Introduction
The development of technology has resulted in the betterment of engineering products and the growth of
population has resulted in the increase in demand. In order to meet the demand, the production has to
increase and the water being the sustainer of life, its demand has been ever increasing. The near utilization
of surface water potential has diverted the stress to groundwater system to meet the increase in demand of
water. The coastal areas are the centres of development from British Raj Days and the trends continue in
Independent India also. In general, coastal areas are characterized by multi aquifer system comprising
many layers of sand clay alteration. The groundwater development through dug wells or filter points or
shallow tube wells is carried out depending on the depth of water level in the area. In planning for a
sustainable strategy, it becomes pertinent to know the degree of interconnection between the different
layers and intermixing of water. In the broader perspective, it also becomes necessary whether to treat the
different layers as a single layer or multi layered system, while planning for groundwater development
plans.
The
principles
of
hydrogeology
and hydrochemistry have been used conventionally to
decipher source of recharge or salinity or intermixing. The use of naturally occurring isotopes, to
track the source and movements of water and solutes, to assess water budget and
geochemical/mathematical models has been gaining enormous importance. Environmental isotopes, both
124
stable (2H & 18O) and radioactive isotopes (3H & 14C) are generally used to study different hydrological
aspects like, source and origin of groundwater, mechanism of groundwater salinisation, source and origin
of groundwater pollutants, aquifer–aquifer interconnections, surface water groundwater interconnection.
However, use of isotope signatures in conjunction with the hydrogeology and hydrochemistry is a powerful
tool for solving hydrological problems.
An attempt has been made to study the possible interconnections between different layers of multi aquifer
system in the southern part of Chennai Metropolitan Area, Tamil Nadu. The work presented in the paper
has been done jointly with Isotope Application Division, BARC, Mumbai.
Background Information
The study area has Bay of Bengal as the eastern boundary and Kovalam Creek as the southern boundary
while Tiruvanmiyur area was taken as northern boundary and about 1 – 3 km west of Buckingham canal
was taken as western boundary. The Buckingham Canal, which extends through the area from north to
south, parallel to the coast bifurcates the area and it was used for navigation earlier and at present is used as
a Channel for sewage disposal (Fig 1).
In general, the aquifer system comprises fine to coarse-grained sand, sand – clay admixtures and weathered
basement comprising Charnockites and Granitic Gneisses. Geophysical studies carried out by Central
Ground Water Board have been considered while compiling the information for the study area. The study
has shown that the depth to basement varies over the area from 14 to 30 m bgl and in general increases
towards the coast. The geophysical survey has indicated two to three layers of sand and sand/clay
admixtures overlying the weathered and fractured charnockites. Thus aquifer system can be divided into
two zones, viz., consolidated and unconsolidated formations. The unconsolidated formation can be further
segregated locally into layers comprising Grey sand- clay intercalation (Zone III), Grey sand with
argillaceous intercalations and characterised by shells (Gastropods and Lamellibranches) (Zone II) and
brown/red sand (Zone I). The weathered and fractured consolidated formation has been designated as Zone
IV (Fig 2).
Isotope Techniques
The variation of isotopic composition starts in hydrologic cycle itself and it is termed as isotopic
fractionation. The physical and chemical processes during process of hydrologic cycle result in isotopic
fractionation. The physical process includes evaporation and chemical processes are resultant of the
variation in strength of the chemical bonds.
125
Stable isotope measurements can be used as an indicator for intermixing of waters from more than one
source. In the last two decades, the application of isotopes has been used to seek solutions for the
hydrological problems in India some of which are quoted below.
126
Navada & Rao (1991) have studied Ganga river – groundwater interaction using environmental oxygen –
18. Navada et al (1993) have used isotope techniques for carrying out groundwater recharge studies in arid
regions of Rajasthan. Nachiappan (1998) has successfully used isotope techniques to study the lake –
aquifer interactions. Shivanna etal (2000) carried out isotope hydrological investigations in arsenic infested
areas of West Bengal to study the origin, source, residence time and dynamics of arsenic contamination.
In the present study, water samples were collected for isotopes measurement along with the samples
collected for chemical analysis for major, minor and trace elements from the study area during June 2000
and December 2000. The measurement of stable isotopes (2H, 18O) was carried out using 602E VG
ISOGAS mass spectrometers.
A perusal of data of pre and post monsoon samples indicates that the effect of isotopic
fractionation in the precipitation water has resulted in the seasonal changes. In general, the
aquifer system comprises fine to coarse-grained sand, sand – clay admixtures and weathered basement
comprising Charnockites and Granitic Gneisses. Geophysical studies carried out by Central Ground Water
Board have been considered while compiling the information for the study area. The study has shown that
the depth to basement varies over the area from 14 to 30 m bgl and in general increases towards the coast.
The geophysical survey
The stable isotope plot (δ2H Vs δ18O) reveals that samples form three groups, viz., Group I (around Local
Meteoric Water Line (LMWL)), Group II & III (slightly enriched). The samples from Backwater and
Marshy Land (north western part of the study area) are highly enriched due to evaporation effect. The
closeness of the cluster to LMWL suggests that precipitation is the main source of recharge. All samples of
Sandy Aquifer (Zone I, II & III) except Sholinganallur (Central Part of the study area) form a single cluster
(Group I). The samples from both sandy and weathered aquifers at Sholinganallur are found to be slightly
enriched indicating an additional source of recharge from surface water bodies other than precipitation. The
occurrence of sample from both the aquifers together as a cluster, further suggests a possibility of
interconnection (Fig 3).
The plot of δ18O Vs Cl indicates that samples can be grouped into freshwater, brackish water and saline
water categories (Fig 4). The points are also found scattered on either side of the dilution line indicating
that the source of brackishness is not due to mixing of sea and fresh waters but it is due to leaching of salt
from the formation. The samples from the sandy aquifers on the eastern side of the study area fall under
fresh water category. In addition, samples from weathered & fractured aquifer on the western side
(Sitlapakkam & Jaladampettai) also fall under fresh water category. The brackishness of the sandy aquifer
at Uthandi (adjacent to Buckingham canal) on the eastern side of the study area suggests a recharge from
canal and precipitation. The closeness of the plot of samples of sandy aquifer
127
and weathered & fractured aquifer at Uthandi suggests a possibility of interconnection. The brackishness of
groundwater in weathered & fractured aquifer in the eastern and northwestern side indicates a lateral
hydraulic connectivity across the study area. The saline nature of groundwater at Mettukuppam on the
northwestern part of the study area and its position away from the dilution line suggests that modern
seawater is not the source of salinity. The comparison of stable isotope plot with δ18O Vs Cl plot
indicates a possibility of mixing of entrapped seawater and water rock interaction.
128
Fig 5 shows the plot of Tritium Vs δ18O and it indicates that the samples can be grouped into three
categories, viz., Group I (0-5 TU), Group II (5-10) and Group III (10-15 TU). The groundwater in the
sandy aquifer on eastern as well as the western side of the study area falls under Group II indicating a
modern and single source of recharge through precipitation. In addition, weathered & fractured aquifer at
Kottivakkam (on the eastern side of the study area) and Sitlapakkam and Jaladampettai, (western side of the
study area) also fall under Group II, indicating the similar source of recharge. The rest of the samples from
weathered & fractured aquifer fall in Group I indicating comparatively a longer residence time and lateral
hydraulic connectivity across the study area.
14
C dating of the groundwater was carried out at places where tritium content was less than 3 TU. It
revealed that groundwater in both sandy and weathered & fractured aquifers at Mettukuppam is 6900 years
old indicating interconnection as well as mixing of meteoric water with entrapped water. The age of 11000
years of the groundwater in weathered & fractured aquifer at Nilangarai and Uthandi suggests entrapment
and isolation from the rest of the area.
Hydrochemical methods
The overall hydrochemistry of the coastal aquifers indicates that the groundwaters are dominated by cations
Ca 2+, Mg 2+ and Na+ and anions HCO3-, SO4 2- and Cl-. In comparison of the water types of these aquifers
during pre monsoon and post monsoon, Ca 2+, HCO3-and SO4 2- have increased in the post monsoon
samples thereby indicating annual replenishment to these aquifers.
In the plots of Ca, Mg, Na, K, Cl Vs Total Dissolved Ions (TDI), the samples collected from all the aquifers
from the study area falls as a single cluster, except at Mettukuppam (northwestern part) (Fig 6). Further, the
cluster plots away from the samples collected from both the aquifers at Mettukuppam, Sea (Peria
Nilangarai) and Backwater (Muthukadu) indicating source of recharge other than sea to these aquifers.
Further, closeness of plot of samples collected from Uthandi (located in the central part of the study area
and adjacent to Buckingham canal) and Buckingham canal indicate a possible recharge
component from canal.
129
In the plot of Na, K and Ca Vs Cl, most of the samples (except samples from Mettukuppam) form a single
cluster falling near the fresh end member (Fig 7). In all the plots, the brackish groundwater of this cluster
fall above the mixing line, which indicate the water rock interaction. The formation of single cluster
indicates a single major source of recharge for both the aquifers, which may be supplemented by the
recharge from other sources at different places.
Fig: 6
The groundwater from both the aquifers at Mettukuppam falls above the mixing line indicating the
enrichment of magnesium, sulphate and bicarbonate. This combined with high salinity show that there is a
possibility of water - rock interaction in addition to the mixing of entrapped seawater.
Fg:7
The ionic ratio of ((Ca+Mg)/(Na+K)) indicates that the ratios in general do not compare with that of
seawater samples indicating that modern seawater has not mixed with the groundwater.
A study of the ratio of Na/Cl shows that most of the samples from all the zones in the study area is
different from that of seawater sample and does not support the mixing of seawater.
130
In general, Cl/ (CO3+HCO3) ratio varies from 0.2 to 2 in Zone I & II 2-5 in Zone III and 1-5 in Zone IV.
Br/Cl ratio is very low, thereby indicating that seawater has not mixed with groundwater.
The water samples collected at Mettukuppam (north western part of the study area) are different from the
samples collected at other places. Though, the ratios of Na/Cl. and Br/Cl are comparing well with the
seawater sample, Cl/ (CO3+HCO3) ratio is not closer to the seawater sample. Further, Mettukuppam falls
on the western side and samples collected from locations in between sea and Mettukuppam do not show
any correlation to the seawater sample thereby indicating that modern seawater may not be the source for
salinity at Mettukuppam.
In general the studies of various ionic ratios indicate that most of the samples do not correspond to the ionic
ratio of seawater collected at Peria Nilangarai. The high ionic ratio in the formation water is due to ion
exchange process and/or dissolution of aquifer material.
Hydrogeological Methods
The sub strata encountered in different piezometers drilled in the project revealed that the clay horizon is
not continuous thus facilitating the interconnections at the places devoid of clay horizon. The presence of
clay-sand admixtures in varying proportions throughout the area between top sandy aquifer and weathered
& fractured aquifer deems it possible to have interconnection between the two aquifers. Further, a similar
response of the different layers of the aquifer system with the rise in water level or piezometric surface
after the monsoon indicates the interconnection between the different layers of the aquifer system. A
similarity in the groundwater flow pattern in the top sandy aquifer and lower fractured crystalline aquifer
also indicates a possible interconnection.
Corroboration of Results
The presence of clay-sand admixtures in varying proportions throughout the area in the top of the aquifer
indicates a possibility of treating sand clay alterations as a single layer. The weathered and fractured rock
can be treated as a separate layer and thus the aquifer system can be treated as two aquifer system with top
sandy layer and bottom crystalline aquifer. A similarity in the response of the water levels in the sandy and
crystalline aquifers to the monsoon and similarity in the groundwater flow pattern in the two aquifers
indicates a limited to complete interconnection between the aquifer (Suresh, 2006).
The samples from different zones at the same location plot together as clusters in the plots of various ionic
species against Total Dissolved Ions & Cl. Further, the ionic ratios of the samples from different layers also
indicate a similarity. The chemical constituents in the samples collected from different sandy layers
indicate that the different sandy layers can be clubbed together as a single unit from the regional point of
view. The plot of the samples together as a single cluster also indicates intermixing of water.
The plots of δ2H Vs δ18O, δ18O Vs Cl and Tritium Vs δ18O, indicate three groups or three clusters and the
clusters do not segregate the samples from different depths The samples collected from different layers at
the same location fall within the clusters there by indicating a possible interconnection. Thus it can be
inferred that the interconnection between the aquifers varies from limited to a complete interconnection.
Conclusion
The isotope technique has corroborated the findings of conventional methods in conceptualization of the
coastal aquifers. It can be inferred that the multi layered sandy aquifers can be grouped into a single aquifer
unit in the regional scale and crystalline aquifer beneath the sandy aquifer can be considered as a separate
aquifer unit. The results of isotope technique has also supported the interconnection of the aquifer unit
varying from limited to complete interconnection, there by resulting in the mixing of recharged water. The
study has also proved the efficacy of isotope technique in solving the hydrological problem, there by
removing the ambiguity in the conclusion. It is also to add that no single method can be effective in
isolation but in combination with the conventional methods of hydrogeology and hydrochemistry, the
isotope technique can be very effective in providing solutions to the hydrological problems.
131
Acknowledgement
The authors express their gratitude to The Chairman & Member (SA&M), CGWB for according permission
to publish the work in workshop. The project has been conceived at the behest of Dr S.M.Rao, Associate
Director (Retd.), BARC & Shri M.B.Raju, Regional Director (Retd.), CGWB and authors gratefully
acknowledge their valuable technical and moral support during the project work. The moral support and
technical guidance extended by the Regional Director, CGWB, SECR is gratefully acknowledged. We take
this opportunity to thank the scientists of CGWB, SECR & Isotope Application Division of BARC for their
kind cooperation and innumerable discussion during the analysis period.
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Inc. New York, 413 p.
Nachiappan, R.P., 2000. Surface water and groundwater interaction studies using isotope techniques.
Unpublished Ph. D. Thesis, University of Roorkee, Roorkee, India, 191 p.
Navada, S.V. & Rao, S.M., 1991. Study of Ganga River- Groundwater interaction using environmental
oxygen-18. Isotopenpraxis, 27 91991) 8, pp 380-384.
Navada, S.V., Nair, A.R. and Rao, S.M., 1993. Groundwater recharge studies in arid regions of Jalore,
Rajasthan using isotope techniques. J. Arid Engg., 24, pp 125-133.
Rao, S.M., Jain, S.K., Navada, S.V., Nair, A.R. and Shivanna, K., 1987. Isotope Studies on seawater
intrusion and interrelation between water bodies: some field examples. Proc. Symp. on Isotope Techniques
in Water Resources Development, IAEA, Vienna, pp 403-427.
RITES, 1996. Report on Hydrogeological and Seawater Intrusion Studies between Thiruvanmiyur and
Muttukadu aquifer. A consultancy Report submitted to Madras Metropolitan Water Supply and Sewerage
Board. p 150.
Shivanna, K., Navada, S.V., Nair, A.R. and Rao, S.M., 1993. Isotope and Geochemical evidence of past
seawater salinity in Midnapore groundwaters. Proc. of an Intl. Symp. on applications of isotope techniques
in studying past and current environmental changes in the hydrosphere and atmosphere, IAEA, Vienna.
Shivanna, K., Navada, SV., Sinha, UK., 1998. Application of isotope techniques to investigate groundwater
pollution in India. IAEA – TECDOC –1046, International Atomic Energy Agency, IAEA, pp167184.
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Shivanna, K., Sinha, U.K., Joseph, T.B., Sharma, S. and Navada, S.V., 2000. Isotope hydrological
investigation in arsenic infested areas of West Bengal, India. ICIWRM-2000, Proc. of Intl. Conference on
integrated Water Resources Management for Sustainable Development, New Delhi. pp 490-500.
Shivanna, K., Suresh, S. and Tirumalesh, K. 2006. Environmental Isotope Hydrochemical Investigation for
Characterisation of Groundwater in Tiruvanmiyur coastal aquifer, Tamil Nadu, India. Second Intl.
Conference on Groundwater for Sustainable Development: Problems, Perspective and Challenges (IGC2006), Volume of Abstracts and Souvenir, pp6-7.
Srinivasan, R. 1980. Evaluation of Groundwater Resources in coastal aquifers of South Madras
(Tiruvanmiyur-Covelong Basin), Part I. Progress Report for the Field Season 1974-78. Geological Survey
of India, Tamil Nadu Circle, Madras 96 p (Unpublished).
Suresh, S. 2006. Hydrodynamics of coastal aquifers in southern part of Chennai Metropolitan Area, Tamil
Nadu, India. Unpublished Ph.D Thesis, University of Madras, Madras 148p.
Suresh, S. and Lawrence, J.F., 2006 (a) – Conceptualisation of coastal aquifers in southern part of Chennai
Metropolitan Area, Chennai. Presented in National Conference on Disaster Mitigation and Management
using Space Technology (DIMMS, 2006). Bharathidasan University, Trichy.
Suresh, S. and Lawrence, J.F., 2006 (b) – Hydrodynamics of coastal aquifers in southern part of Chennai
Metropolitan Area, Chennai. Presented in XXV National Seminar on Hydrology with special colloquium
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133
Isotopic technique for evaluating artificial recharge to groundwater - A
case study from Maharashtra
B. K. Purandara1, Bhishm Kumar2, M. S. Rao2 and S. K. Verma2
1
Regional Centre, Belgaum of the National Institute of Hydrology
2
National Institute of Hydrology, Roorkee – 247667
[email protected]
Abstract
In recent years, the depletion of ground water table at an alarming rate in many of the urban area has drawn
the attention of water resources managers. To mitigate the increasing shortage of ground water, artificial
recharge measures are taken up by constructing earthen bunds, injection wells and roof top rain water
harvesting. However, the effectiveness of these programs has not been assessed in many parts of the
country. Therefore, in the present study an attempt is made to evaluate the effectiveness of artificial
recharge structures by employing environmental isotopes (3H, 18O and D) in two watersheds of
Maharashtra (hard rock watershed - Ozhar, Nasik and alluvial watershed – Bamnod, Jalgaon) having two
different hydrogeological set up. Isotope indices (18O) are determined for the end members and the
contribution of artificial recharge is evaluated with its variation in space and time. The environmental
tritium is used to determine flow velocity, direction of flow and recharge areas. 18O and 3H isotopes in
Ozhar watershed revealed that the surface water impounding by earthen bunds or structures contribute
significantly to groundwater recharge. However, the contribution varies from month to month, i.e., the
contribution was found to be minimum during the month of July while about 100% in the month of
October. In Bamnod watershed, it is observed that the rainfall is the primary source for recharge to ground
water. Further, it revealed that there is a significant influence of recharge structures on ground water
recharge.
Introduction
Artificial recharge is the process by which human action is responsible for the transfer of surface water to
the ground water system (Driscoll, 1986; Freeze and Cherry, 1978). Many methods of artificial recharge
have been developed such as water spreading, recharge through pits, wells, and shafts etc. Artificial
recharge is considered as the tool for water management whereby water is introduced in the aquifers and is
stored there and when demand for water increases the additionally stored water is withdrawn.
Benefits from the use of the artificial recharge strategy are obtained through improved rural water quality
and supply and come as an enhanced quality of life. Economic benefit is realized in the rural community,
since resources previously devoted to the acquisition of water become available to other options. The key to
the strategy is that the ground water resource is essentially renewable. The strategy promotes conjunctive
use or management of surface and ground water supplies. Increasingly, governmental agencies, particularly
in India are recognizing the importance of regulating the use of both ground water and surface water in
coordinated manner. This type of water management regulation would provide an increase in the efficiency
and cost effectiveness of use and reliability of supply. Depletion of groundwater table at a faster rate in
most of the urbanized areas has drawn the attention of water resources managers. This situation has also
arisen in areas where surface water bodies such as rivers, canals and natural or artificial lakes/ reservoirs do
not exist. In order to mitigate the increasing shortage of groundwater, artificial recharge of groundwater by
making earthen bunds, through injection wells or roof top rainwater harvesting programs have been given
priority by the concerned organizations and individuals. However, the effectiveness of these programs has
not been assessed at the desired scale as it is difficult to do so using conventional techniques. Isotope
techniques have the potential to assess the effectiveness of these programs using environmental isotopes.
Darling (2002), Li Ma et al (1996), Nevada et al (1991) and Sukhija et al (1997) successfully used the
isotopic techniques to ground water recharge studies. In this connection three watersheds have been
selected under varied hydrological conditions in the State of Maharashtra for evaluating the impact of
134
artificial measures on ground water recharge during the Hydrology Project Phase I. The present paper
highlights the ground water recharge estimates in two watersheds which represent two different
hydrogeological set up, i.e. a hard rock watershed (Ozhar, Nasik district) and an alluvial watershed
(Bamnod, Jalgaon district).
A detailed hydrogeological investigations were carried out in two watersheds namely, Ozhar and Bamnod.
The local information was also collected during the survey, which revealed that the depth of wells,
increased considerably from 18m (which was normal depth during 70’s) to 30 m during the recent past.
The declining trend of water levels apparently led to take the conclusion that unless additional water is
added to the present groundwater system, the withdrawal of groundwater at the present rate would become
unfeasible. Therefore, GSDA, Pune initiated projects on recharging groundwater by artificial means i.e., by
storing surplus irrigation water in Bandharas (small section by making earthen bunds) was adopted in hard
rock areas and by adopting well injection technique in Bamnod watershed using disused open dug wells.
Hatnur canal water was supplied for this purpose with the assistance of Bamnod Lift Irrigation Society of
farmers. The results of the pilot studies were found encouraging as the unsaturated aquifers tapped in open
wells were found to have very good intake capacity.
Study Area
The Ozhar with latitude 200 04' to 200 05' N and longitude 760 54' to 760 56’ E is located in the watershed
GV 7 (designated and numbered by GSDA) of Godavari basin. The area is approachable and connected by
road and lies on Mumbai-Agra Highway at a distance of 20 Km from Nasik. Mahatma Phule Society and
Jai Yogeshwar Societies operate in the watershed for taking measures required for artificial recharge to
aquifers.
The Bamnod area lies between the North latitude 210 8’ 210, 10’ and East longitude 750 48’ 750 50’. It falls
in Yawal Taluka of Jalgaon district, Maharashtra. It is located in the extreme South-East corner of
watershed TE-11 (designated and numbered by GSDA) of Tapi basin. The wells pertinent to the recharge
study are within the command area of Bamnod Co-operative Lift Irrigation Society, Bamnod. Total area of
the village is 1373 Ha. The area is famous for banana cultivation. The study area indicating all the three
watersheds separately is shown in Figure-1.
Hydrogeology
The Ozhar watershed comprises of weathered, fractured, brownish, Amygdaloidal basalt. This is followed
by hard compact massive basalt. Hence only phreatic aquifer is predominating in this area. The maximum
thickness of the aquifer is 15 m. This aquifer saturates during only depending upon the amount of
precipitation. As the monsoon recedes, desaturation of the aquifer resumes. The area is drained by river
Banganga that flows through the northern part. It flows from west to east and its river-bed is rocky. It is
also drained by two drains. One flows from Mahatma Fule Society area while other flows through Jai
Yogeshwar Society. The elevation between these two through which the river flows ranges from 601m to
588 m. On the left bank of river, Banganga Society area exists while on the right bank, Mahatma Fule
Society and Jai Yogeshwar Society areas are located. The river as wells as drains are seasonal. The average
annual rainfall in Ozar watershed is only 427 mm.
135
Banganaga
Mahatma
Fule
Jay
Yogeshwar
Jalgaon
Ozar Watershed, Nashik
Nashik
Pune
Scale
0
4 km
Scale
0
1 km
Lift Irrigation
pipe line
Districts Jalgaon, Nashik & Pune
In the state Maharashtra, India
Hatnoor Canal
Bamnod Watershed Jalgaon
BM-60,
PUNE
Figure 1: Location of Ozar, BM-60 and Bamnod watersheds in Maharashtra State.
Bamnod is the part of Tapi valley that comprises of mainly alluvium with clay, silt, sand, gravel and
pebbles. The alluvium is broadly divided into younger alluvium occurring up to 80 meters below ground
level and older alluvium beyond 80 m. The principal water bearing formation in Bamnod is alluvium where
granular zones are encountered at various depths. Groundwater in the area generally occurs under
unconfined conditions at shallow depth and under semi-confined to confined conditions at deeper levels. In
alluvium, multi aquifer systems are present where each aquifer is separated by confining clayey formation.
The thickness of granular zone varies from 4 to 15 m. There are 215 irrigation wells in the project area of
which 120 wells are in disused condition due to lowering of groundwater table. The depth of disused wells
ranges between 20 to 30 m. The wells in operation are in the depth range between 30 to 50 m. The static
water levels ranges between 27.5 to 48 m b.g.l. The area receives rainfall from southwest monsoon, which
begins from middle of June and lasts up to the end of September. The area receives average annual
precipitation of about 768 mm. Good network of pipelines for water conveyance is available in the
command area of Bamnod under Co-operative Lift Irrigation Scheme. An experiment of well injection was
carried out for 9 days in area during February 1997, where about 12945 m3 of surface water was injected
into the sandy aquifers at the rate of 900. The experiment was considered successful because the water
injected continuously for 9 days was absorbed by the sandy shallow aquifers.
Methodology
Environmental isotopes such as deuterium, oxygen-18 and tritium were used to understand the contribution
of different recharge sources and to identify the recharge zones of aquifers. Groundwater, rainwater, canal
and impounded water (in earthen channels) samples were collected for oxygen-18, deuterium and
environmental tritium isotopes analyses. The isotope indices for groundwater, precipitation and canal water
were determined separately for each watershed. The variation of oxygen-18 values with time and location
was studied in each watershed. The environmental tritium contours were also plotted for Bamnod
watershed to understand the flow direction and recharge areas.
136
Results and discussion
Ozhar watershed, Nasik district
The distribution of δ18O in groundwater and the groundwater flow pattern in the Ozhar watershed is shown
in the Figure 2. The groundwater samples collected for environmental tritium analysis were carried out by
liquid scintillation counting and were found in the range of 14-16 TU. These values are comparable to the
surface water bodies (~12.6 TU), which is used for artificial recharge through earthen channels, and much
different from the rainfall in the area which show a typical value within the range of 5 to 6 TU. Therefore,
it is inferred that in Ozhar watershed, the source of recharge to groundwater is from surface water which
was impounded by using earthen bhandaras in various parts of the study area. The results clearly indicated
that the rainfall recharge is comparatively very less. The results showed that the most depleted and enriched
δ18O values of the groundwater in Ozhar watershed are –2.2‰ and 0.0‰ respectively. A comparison of the
monthly average values of δ18O (neglecting spatial variations) of groundwater with the surface
impoundment, it is seen that in the month of June, the groundwater is more depleted than the surface water
(Figure 3). This could be attributed to the evaporation loss of surface water due to which there will be an
enrichment of isotopic composition. In the month of July, the isotopic values of both the systems (surface
and ground water) were quite comparable indicating the impact of impounded water on the ground water
recharge ( Fig. 3).
The evaporation effect further enriched isotopic contents of small quantity of water stored in channels due
to no supply from canal in the month of August, while the groundwater also show a little enriching trend
due to delayed contribution from precipitation and channel water.
In the month of September, the input from surface water/canal is provided to the channels and therefore the
isotopic contents of channel water was comparatively less. However, the δ18O of groundwater in the month
of October is found identical to that of the channel water which clearly indicates that major recharge to the
groundwater is from the impoundments across the channels locally called as Bandharas. The contribution
from rainfall or canal (channel) to groundwater is determined using the following relation based on δ18O
values of end members.
mch = (δ18Ogw - δ18Op) /(δ18Och - δ18Op)
or mp = (δ18Ogw - δ18Och) /(δ18Op - δ18Och)
Where, mp and mch are the contributions of precipitation and channel water to groundwater respectively
while δ18Ogw, δ18Op, and δ18Och are the corresponding oxygen-18 values of groundwater, precipitation and
channel water (Table 1)
137
-1.5
B-1
B-3
-1.6
-0.93
B-CB
-0.93
M-11
J-6
-2.2
-1.7
-1.7
M-1
J-1
-1.1
J-CB
-1.5
MF-CB
J-8
-1.2
-0.99
J-4
Water Sampling Sites
Tritium Injection Sites
Observational Wells
Figure 2: Groundwater level contours in the post monsoon season of 1997, stable isotope (
composition (‰) of groundwater in the Ozhar watershed, Nasik
June
July
Aug
Sept
18
O)
Oct
δ18O (‰ )
1
-1
Canal
-3
Rain
GW
-5
Figure 3: Variation of δ18O of channel water, groundwater and precipitation with
time in Ozhar watershed, Nashik
Table 1: Recharge percentage due to canal water supplied through channels and rainfall in different months
Recharge in (%) due to
June
August
September
October
Canal or Channel (Mch)
80
74
73
100
Rainfall (Mp)
20
26
27
0
138
Figure 4 shows the percent contribution of rainfall to groundwater in Ozhar watershed. The percent of
artificial recharge to groundwater through channels can be estimated by subtracting percent of rainfall
recharge from 100 as shown in Fig. 4. A straight line relation between the amount of rainfall and δ18Op
values also enables to determine the percent contribution of rainfall to groundwater.
120
Canal recharge
Recharge (%)
100
Rainfall recharge
80
60
40
20
0
0
50
100
150
200
250
Rainfall (mm)
Figure 4: Percent recharge to groundwater due to rainfall and canal water in Ozar
watershed, Nasik
Bamnod watershed, Jalgaon
The sample of groundwater, precipitation and canal water collected from Bamnod watershed were analysed
for environmental tritium contents. The tritium data of the Bamnod watershed showed wide variations in
tritium contents indicating different ages (residence time) of groundwater. The highest value of tritium
content is about 15.7 TU (April) found near the well no. OBW8, while the lowest value is about 1.29 TU
(June) near the well no. OBW14. This indicates that the groundwater occurring near the well no. OBW8 is
the most modern while that occurring near the well no. OBW 14 is comparatively older in the watershed.
The high TU values observed once in the watershed may be due to the supply of canal water used during
the past for artificial recharge through injection wells which has high tritium contents (~12.6 TU). The
environmental tritium distribution plot (Figure 5) shows that the general groundwater flow direction is from
northwest to southeast. Since the tritium input function is not available, a value of 8 TU (June) may be
considered as the initial value of environmental tritium for estimating the uncorrected age of the
groundwater. Therefore the age of the water encountered at Well no. OBW14 with a tritium value of 1.29 is
then about 32 years. However, this estimated age has to be used with caution as the initial activity has been
assumed as 8 TU, which may not be exact. If we adopt the piston flow model with the assumption
applicable, then the average groundwater flow velocity may be computed as the ratio of age difference to
the distance traveled. Since the distance between the well no. OBW8 and OBW 14, is about 2.8 km, the
groundwater flow velocity is calculated as 0.24 m/d or 2.77x10-6m/s.
139
OBW- 8
OB -1(3.3
OB
W -2)
W
(3.4)
OB
(2.79
W
)
OB -1
W(3.362
)OB -6
(1.56
W
-5
)
OB -1
W (2.7
3
)
(1.29) OBW-
0
14km
Lift Irrigation
(Di
pipe - use
s
d)
Hatno
can
or
al
Scal
1
e
- lin
e
Figure 5: Environmental tritium contours in Bamnod watershed, Jalgaon
Conclusions
The stable isotopic data (δ18O) of ground water and surface water of Ozhar watershed indicated that the
ground water is dominantly recharged from surface waters (canal/channels) bodies and the rainfall recharge
component is much lower. Further, the data reflect that the groundwater is young with negligible aquifer
storage. The isotopic values of both surface and groundwater systems showed closer resemblance during
July, 2000, probably indicating the higher influence of surface water bodies on ground water recharge than
the rainfall. The effect of evaporation was also evident as there were enriched isotopic contents of small
quantities of water stored in channels due to no supply from the canal in the month of August. However,
the ground water showed a little enrichment trend due to delayed contribution from precipitation and
channel water. In September, input from the canal is provided to the earthen channels as shown by the
depleted isotopic contents of the channel water. Further, the δ18O of groundwater in October is found to be
identical to that of the earthen channels in the Ozhar watershed. This clearly indicates that major recharge
to the ground water is from the impoundments built across the channels called bandharas. In Bamnod
watershed, large variations were noticed in the tritium data (15.7 TU to 1.29 TU). Based on the results of
isotope analysis, the general groundwater flow direction was from northwest to southeast. Therefore,
the recharge area lies in the northwest side of the watershed. It is also important to note that in Bamnod
140
watershed, the major source of recharge is precipitation. However, it is opined that, the canal water which
is used for artificial recharge plays a significant role to enhance the groundwater recharge.
Acknowledgement
The study was conducted as a part of the first phase of the Hydrology Project. We thank members of World
Bank Team and Officials of the Ministry of Water Resources, Govt. of India for technical and financial
support from World bank. Authors are highly grateful to GSDA, Pune for providing necessary data and
also for assisting in field investigations. Authors also thank authorities of BARC, Mumbai for their
continuous support. The guidance and help rendered by Dr. Navada and Dr. Swathi is gratefully
acknowledged. Our sincere thanks are also due to the Director, NIH for his continuous encouragement.
References
Driscoll, F. G., 1986. Ground water and Wells, Johnson Filtration Systems Inc.,St Paul, Minnesota.
Darling, W. G.,. Burgess, W. G, and Hasan, M. K.(2002) Isotopic evidence for Induced river recharge to
the Dupi Tila aquifer in the Dhaka urban area, Bangladesh, IAEA- TECDOC-1298.
Digney, J. E. and J. A. Gillies, 1995. Artificial Recharge in Saskatchewan: Current Developments. Water
Resources Bulletin, Vol. 31, No.1, pp. 33-42.
Freeze, R. A. and J.A. Cherry, 1978. Ground water, Prentice Hall, Inc., Englewood Cliffs, New Jersey.
Li Ma and Roy F. Spalding (1996) Stable isotope characterization of the impacts of artificial groundwater
recharge, AWRA, V. 32, No. 6, p. 1273- 1282.
Navada, S. V. and Rao, S. M. (1991) Study of Ganga river- groundwater interaction using environmental
oxygen-18, Isotopenpraxis 27, 380-384.
Sukhija, B.S., Reddy, D.V., Nandakumar, M.V. and Rama (1997) A method for evaluation of artificial
recharge through percolation tanks using environmental chloride. Groundwater, 35 (1): 161-165.
141
Natural recharge and irigation return flow evaluation using Tritium
tracer technique in a granite watershed, Midjil mandal, Mahboobnagar
district, Andhra Pradesh, India
Pandith Madhnure1, P.N. Rao1, Rangarajan. R,2 Shankar. G.B.K,2 Rajeshwar.K2 and
A.D.Rao1
1
Scientists, Central Ground Water Board, GSI Post, Bandlaguda, Hyderabad-68
2
Scientists, National Geophysical Research Institute, Uppal, Hyderabad-606
Abstract
Sustainable development and proper management of groundwater resources in hard rock areas requires
scientific assessment of natural recharge and irrigation return flow factors. Tracer technique, which is a
direct method for estimation of recharge, is adopted in Madharam watershed to asses both natural recharge
and irrigation return flow. The watershed is spread over an area of 95 km2 in Mahabubnagar district, where
irrigation water requirement is mainly met through groundwater. The normal annual rainfall in the area is
about 620 mm. The depth to water level during the study period (2009) varies between 8 to 30 mbgl during
pre-monsoon and 6 to 24 mbgl during post-monsoon seasons. Tritium tracers were injected into the ground
at eleven places in the year 2009 before the sowing of khariff crops and samples were collected from seven
places in the post harvest period. The study indicates that the natural recharge caused by rainfall is about
15% in recharge area (upper reaches) and about 2% in discharge area (lower reaches) with a mean of 8.4 %.
The recharge by irrigation return and effective rainfall together is about 19.4 % in recharge area and 39 %
in transient area with a mean of 26.8 %. The net average recharge by return groundwater irrigation in the
area is about 18.4% of applied of ground water/acre (5750 m3).
Introduction
The groundwater recharge is estimated by indirect conventional, chemical and isotopic methods like
groundwater balance, lysimeteric, resistivity, isotopic and tracer techniques (Sukhija and Rama, 1973,
Athavale, R.N. et al., 1980, Edmonds et al., 1980, Sharma, M.L., 1987, Rangarajan et al. 2000, Pradeep
Raj, 2001, Jacobus et.al. 2002, Healy and Cook, 2002, Mondal and Singh, 2004, Israil et al., 2006). The
tracer method provides direct method for estimation of groundwater recharge based on piston flow model
concept (Zimmermann et al., 1967, Munich, 1968).
Central Ground Water Board (CGWB) in association with National Geophysical Research Institute (NGRI)
has carried out limited type of studies in different hard rock terrains spanning the last 35 years during the
execution of work in different projects viz., SIDA assisted ground water project in Noyil, Ponnani and and
Amaravati river Basins (CGWB, 1983), Vedavati River Basin Project (VRBP) (CGWB, 1988) of (Table-1).
However, NGRI has done extensive work in different terrains of India (Sukhija and Rama, 1973, Athavale,
R.N. et al., 1980, Sukhija et al. 1996, Rangarajan and Athavale, 2000, Kumar and Sethapathi, 2002,
Marchal et al., 2006). Similar studies were also taken in Rampatna watershed in the Kolar Semi-arid
Regions of Karnataka State (http://nrdms.gov.in).
142
Table 1: Results of recharge measurements (by tritium tagging method) in various projects during the
studies.
Name of the Basin
Recharge rates Remarks
(%)
Noyil, river Basins
7
CGWB in collaboration with NGRI
Ponnani river Basins
2.7 to 7.2
Do
Vedavati
River
Basin
Do
Project
Upper reaches
10
Do
Lower reaches
4
do
Rampatna watershed in the 5
http://nrdms.gov.in
Kolar Semi-arid Regions of
Karnataka State
In the absence of adequate field tested point values, GEC 1997 recommended a range of values for
different lithological units / depth to water levels, based on findings/results, obtained in different basin
studies, which are limited in nature (CGWB, 1998) (Table-2).
Table 2: Norms for return flow factor for irrigation water applied by ground water irrigation (after GEC-97
Methodology).
Crop
Depth to water
Return flow
Crop
Depth to
Return flow
type
level range
factor as a
type
water level
factor as a
fraction
range
fraction
(mbgl)
(mbgl)
Paddy < 10
0.45
Non< 10
0.25
Paddy
Paddy 10 to 25
0.35
Non10 to 25
0.15
Paddy
Paddy > 25
0.20
Non> 25
0.05
Paddy
Considering the importance of natural recharge and return irrigation flow factor data base in ground water
resource estimation, Central Ground Water Board, Southern Region took up a study area to estimate ground
water resources by adopting field-tested values and to fine-tune the groundwater resources estimation
norms in typical hard rock terrain in Andhra Pradesh. The studies were initiated during Annual Action Plan
(AAP) 2007-08 and will continue till AAP 2010-11.
Study Area
The study area is part of Madharam watershed (95 km2) of main MBNR-D-44- Tarnikal watershed,
Mahabubnagar district of Andhra Pradesh (Fig.1). The areas is underlain by granites of Archaean age and
covered by red sandy soils, silty clay soils with a weathering depth of 6 to 22 m and receives an average
annual rainfall of 620 mm. However during the study period (2009) about 659 mm of rainfall is received.
The depth to water level varies between 8 to 30 mbgl during pre-monsoon and 6 to 24 mbgl during postmonsoon seasons of 2009 (table-3). The area is totally dependent on ground water (excluding some parts
below
143
Location
Nearby
obser.
wells
Table 3: Location of recharge test sites.
Depth
of Crop type Soil
Estimated
weathering
type
Natural
recharge
DTW
(mbgl)
Bommarajpally 17.80
(mbgl)
Bommarajpally 21.40
14.5016.0
Urukonda
15
17.50
Urukonda
39.15
39.15
18 to 22
Castor
(rainfed
crop)
Paddy
to
Fallow
land
Irrigated
Mucharalapally 27.8
15
to
17.50
18 to 20
Timmanapally
23.80
18 to 20
Irrigated
Madharam
15.30
14.5021.50
Fallow
land
Irrigated
Average recharge for rain fed crops and follow land
Average recharge by irrigation return for paddy crops
Red
sandy
soil
Red
silty
loam
Red
sandy
soil
Do
Red
silty
loam
Red
sandy
soil
Red
silty
clay
(mm)
%
Total
Input of
irrigated
ground
water
m3/acre
98.7
14.98
nil
127.8
19.4*
5440
56.1
8.5
nil
225.96 34.3*
8200
96.6
14.65* 5600
256.2
38.9*
3500
10.6
2
nil
55.13 8.4
107.25 26.8* (Effective
irrigated
recharge 18.4%)
(26.8-8.4)
two existing tanks) for all drinking and irrigation purposes and is categorized as over exploited basin as on
2004 (stage of ground water development is129%).
Methodology
Tritium a radioactive isotope of hydrogen with a half-life of 12.43 years is the most suitable tracer in any
hydrogeological study, as it doesn’t behave differently from the normal ground water and is free from
health hazards (weak energy emission of 18kev).The tracer technique (tritium tagging method) is based on
the assumption that the soil moisture moves downward in discrete layers through unsaturated zone under
the gravity i.e. any fresh layer of water added to the surface either due to rain or due to applied irrigation
pushes an equal amount of water beneath it further down and so on (known as piston flow model)
(Zimmermann et al., 1967, Munich, 1968). The tritium is tagged below the root zone before the onset of
monsoon rains and collected after the monsoon season. The displaced position of the tritium will show how
far the water has been displaced and to locate the tritium core samples is collected up to a
144
■ Key monitoring well. ▼Tracer collected from irrigated field * Trace collected from rain fed field
Fig1. Location of key wells and tracer collection points- Mdharam watershed
maximum depth of 3 m. To know the displacement of tracer, the moisture content (%) and tritium activity
of samples vs. depth is plotted.
The recharge to ground water is calculated based on the following equation (after CGWB, 1983).
Wh = md/(100+md) x rwh
Where
Wh
md
rw
h
=
=
=
=
Recharge or the amount of water added per sq. cm of the soil.
Dry weight percent of soil moisture.
Net bulk density of the soil in-situ.
Displacement of tracer i.e. the distance between the injection depth and the
centre of gravity of the profile.
In field, as the soil cores are removed, the weight of each soil core with moisture can easily be measured
and the corresponding volume is determined either by sand logging or any other method and then the in situ
(wet bulk) densities are calculated. The samples are collected in small sections (10 or 20 cm each). Finally,
the recharge is calculated by summing up the moisture content of every individual section for a
corresponding displacement of tracer. As per piston flow model this water is added to the soil layers
subsequent to injection, an equal amount of water will be added to the ground water and by comparing this
with annual rainfall received over the area, the recharge to ground water as a percentage of rainfall is
calculated.
Tracer Studies in the Watershed area
For carrying out the studies, tritium tracer of 15 micro curie/liter was injected (@ 3 ml at five holes of
60 cm depth which are made by means of drive end rod, one hole in the center and the other
symmetrically placed on the circumference of the circle of 5 cm radius) in the first week of July 2009,
before start of khariff season at 11 sites (3 in rain fed and 8 in irrigation fields) for evaluating moisture
145
influx / recharge from rainfed and irrigated areas (Fig.1). The tracer was injected using a syringe through
copper tubes, the end of which almost touched the bottom of the hole and the opening was closed by filling
up with the local soil to prevent the loss of the tracer due to evaporation. The sites were selected based on
soil type, drainage pattern, recharge/transition/discharge zone, slope etc. Care has been taken that all sites
are in shallow fallow patch of even land, agricultural land or rain-fed agricultural field away from trees,
proper reference points (for relocating at the time of collection).
Vertical soil samples at 20 cm interval (depth wise) is collected using recovery pipes (Hoffer type augers of
varying length) of 45 cm diameter up to a depth of 3 m after post-monsoon season and post harvest season
(November) from 7 sites separately (3 from rain fed and 4 from irrigated fields) (Fig.1 and table-3).
Collections of samples from other 4 sites were not possible either because of not planting the paddy crops
or destroy of relocating features. All 7 samples collected were weighed and sealed in a polythin bag and
brought to the laboratory (NGRI, Hyderabad) for analysis.
Laboratory Determination
Soil samples brought to the laboratory are reweighed and moisture is extracted by using vacuum
distillation. Moisture content in about 25 g of soil was determined with the help of torsion balance using
infrared oven and heated at 105 0C. 4 ml distillate mixed with 10 ml of insta-gel (manufactured by Packard
instrument company, USA) in low potash glass vials and tritium activity was counted using liquid
scintillation counter having background of 50 counts per minute. The tritium activity of each section (20
cm interval) was plotted against the sample depth for all profiles.
Results
Study indicates that the natural recharge caused by rainfall is about 98.7 mm (15%) in recharge area (upper
reaches) and about 10.6 mm (2%) in discharge area (lower reaches) with a mean of 55.1 mm (8.4 %). The
recharge from irrigation return and effective rainfall together is about 127.8 mm (19.4 %) in recharge area
and 256.2 mm (39 %) in transient area with a mean of 107.25 mm (26.8 %.) The net average recharge by
groundwater irrigation return is about 18.4% of applied 5750 m3 of ground water/acre.
Conclusions
The data generated in this study are expected to improve/strengthen existing database for precise estimation
of ground water resource estimation in similar granitic aquifers. Further studies are planned to evaluate
recharge due to surface water storage structures such as tanks and to carry out depth moisture
measurements using neutron moisture probe for better understanding recharge process in rainfed and
irrigated fields in Madharam watershed. The average recharge values arrived in this study using injected
tracers is marginally lower (18.4%) than the recommended values as per GEC methodology (20-35 %)
(Table-2). The large variation in the recharge percentage from the recharge to discharge area could be due
to textural variations in soils and gradient. Continued research work on recharge affected by rainfall,
irrigation and seepage from tanks helps in formulating plans for proper scientific development and
management of ground water resource particularly in overstressed granitic aquifers of India.
Acknowledgement
The authors from CGWB thank Chairman, Central Ground Water Board for according permission to
present this paper. Sincere thanks are due to the Director, NGRI for according the permission to carry out
the collaborative study with CGWB and deputed his team of experts in carrying out this work.
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147