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Contamination of water resources in Tarkwa mining area of Ghana
Contamination of water resources in
Tarkwa mining area of Ghana
A Minor Field Study for Master of Science Thesis
Royal Institute of Technology
Ragnar Asklund and Björn Eldvall
LTH, Ekosystemteknik
Supervisors
Professor P.W.K Yankson
Assistant Professor John Koku
Department of Geography and
Resource Development
University of Ghana
Legon-Accra, Ghana
Assoc. Prof. Prosun Bhattacharya
Department of Land and Water
Resources Engineering
Kungliga Tekniska Högskolan
Stockholm, Sweden
Examiner
Assoc. Prof. Gerhard Barmen
Department of Engineering Geology
Lunds Tekniska Högskola
Lund, Sweden
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Authors
Ragnar Asklund
Björn Eldvall
Title
Contamination of water resources in Tarkwa mining area of Ghana
Titel
Förorening av vattenresurser i Tarkwa gruvområde, Ghana
Keywords
Ghana, Tarkwa, Water quality, Gold mining, Heavy metals
Nyckelord
Ghana, Tarkwa, Vattenkvalitet, Guldgruvor, Tungmetaller
Published by
Department of Engineering Geology
Lund University
Lund 2005
Printed by
KFS AB, Lund 2005, Sweden
ISRN
LUTVDG/TVTG--5092--SE
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Contamination of water resources in Tarkwa mining area of Ghana
Preface
This study has been carried out within the framework of the Minor Field Studies
Scholarship Programme, MFS, which is funded by the Swedish International
Development Cooperation Agency, Sida.
The MFS Scholarship Programme offers Swedish university students an opportunity to
carry out two months’ field work in a Third World country. The results of the work are
presented in a report at the Master’s Degree level, usually the student’s final degree
project. Minor Field Studies are primarily conducted within subject areas that are
important from a development perspective and in a country where Swedish
international cooperation is ongoing.
The main purpose of the MFS Programme is to enhance Swedish university students’
know-ledge and understanding of these countries and their problems and opportunities.
MFS should provide the student with initial experience of conditions in such a country.
A further purpose is to widen the Swedish human resources cadre for engagement in
international development cooperation.
The International Office at the Royal Institute of Technology, KTH, Stockholm,
administers the MFS Programme for the faculties of engineering and natural sciences
in Sweden.
Sigrun Santesson
Programme Officer
MFS Programme
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Contamination of water resources in Tarkwa mining area of Ghana
Abstract
Heavy metals in groundwaters cause serious health problems in many parts of the world.
This study is carried out in an area in western Ghana that has a long history of mining
activity. There are fears that the mining activity is causing serious metal pollution of the
water resources by for example arsenic, lead, cadmium and mercury. Earlier studies have
shown that metal concentrations in groundwater exceed WHO:s guidelines. This study is
part of a bigger project linking technical, socio-economical and gender dimensions to the
contamination of water resources in Tarkwa mining area.
The aim of this study is to investigate the groundwater chemistry with special concern to
metal pollution in selected communities. The result of this Minor Field Study will help to
determine the most crucial areas for further investigation and contribute with more physical
data for the continuous work within the bigger project. Another objective is to conduct a
literature study of the area.
42 water samples, mainly from drilled wells, were collected. The groundwaters are
generally neutral to acidic and oxidizing. The dominating ions are sodium and bicarbonate.
The metal concentrations in the study area are generally lower than expected. 17 wells
show metal concentrations exceeding WHO:s guidelines of at least one metal. The main
contaminants are manganese and iron but arsenic and aluminium are also exceeding the
guidelines in some wells. The composition of the groundwater indicates influence of Acid
Mine Drainage in some wells. Sorption processes are probably crucial for determining
metal concentrations in the groundwater in the area. Hydrochemical modelling indicates
that some minerals containing aluminium- and iron-hydroxides/oxides are supersaturated.
This suggests that precipitation controls heavy metal concentrations in the groundwater.
The occurrence of arsenic in three wells is most probably natural and not considered a
major problem in the area. Manganese, iron and aluminium are all parts of common
minerals and they probably origin from dissolution of minerals. The area is very hilly
which causes a lot of water divides. The groundwater system will therefore not be strongly
affected by weathering of minerals due to short residence time. The local groundwater
systems also prevent mines to affect larger groundwater systems on a regional scale.
Seven locations have been selected as the most interesting sites for further studies. The
locations are: Simpa, Aboso, Samahu, Eyinaise, Huniso, New Atuabo and Akoon.
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Sammanfattning
Tungmetaller i grundvatten är ett allvarligt hälsoproblem i många delar av världen. Denna
studie är utförd i ett område i västra Ghana som har en lång tradition av gruvdrift. Det finns
misstankar om att områdets vattenresurser allvarligt påverkas av gruvdriften genom utsläpp
av tungmetaller, t ex arsenik, bly, kadmium och kvicksilver. Tidigare studier har påvisat
metallhalter som överstiger WHO:s gränsvärden. Denna studie är en del av ett större
projekt som sammankopplar föroreningen av vattenresurser i området med tekniska,
socioekonomiska och genusfrågor.
Syftet med studien är att undersöka grundvattenkemin i utvalda områden med särskild
hänsyn till tungmetaller. Resultatet från denna ”Minor Field Study” ska ligga till grund för
vidare undersökningar i området och även bidra med information till det större projektets
fortgående. Ytterligare ett syfte är att genomföra en litteraturstudie över området.
42 vattenprover samlades in, huvudsakligen från borrade brunnar. Grundvattnet är generellt
neutralt till surt och är oxiderande. De dominerande jonerna är natrium och bikarbonat.
Halterna av metaller är generellt lägre än väntat. 17 brunnar har metallkoncentrationer som
överstiger WHO:s gränsvärden, i huvudsak magnesium och järn. Även arsenik och
aluminium har förhöjda värden i vissa brunnar. Vattnets sammansättning tyder på påverkan
från ”Acid Mine Drainage” i vissa fall. Adsorption är troligen en mycket viktig process för
grundvattnets metallkoncentrationer. Hydrokemisk modellering tyder på att vissa mineral
innehållande aluminium- och järn- hydroxider/oxider är övermättade. Troligen är utfällning
av dessa en viktig faktor för metallkoncentrationen i grundvattnet. Förekomsten av arsenik
är antagligen naturlig och anses inte vara ett stort problem. Mangan, järn och aluminium
förekommer i vanliga mineral och härstammar troligen från vittring. Området är väldigt
kuperat vilket ger upphov till många vattendelare. Grundvattnet får då korta upphållstider
och mängden metaller som härstammar från vittring är låg. De lokala grundvattenssytemen
förhindrar att gruvindustrin har en regional påverkan.
Följande sju områden har bedömts vara mest intressanta för fortsatta studier: Simpa, Aboso,
Samahu, Eyinaise, Huniso, New Atuabo och Akoon.
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Acknowledgements
A thousand thanks to our supervisors at KTH, Assoc. professor Prosun Bhattacharya and
our supervisors at university of Ghana, professor P.Yanksson, assistant professor J.Koko
who made this thesis possible and took very good care of us during our time in Ghana.
Thanks a lot to S.Kufogbe. Lots of thanks to The Wassa West District Assembly who made
this study possible by arranging transport and guidance in Tarkwa during the field study.
Thank you, Dr J.S. Kuma at Western University Collage for valuable information
concerning the sampling plan and for allowing us to use his Hach pH-meter. We are also
very grateful to Dr B. Kortatsi at Water Research Resource Institute for valuable tips in
preparing the sampling plan. Thank you, Dr. Ondra Sracek for helping us to interpret the
hydrochemical modelling. We are very grateful to Ann Fylkner for her big support in the
laboratory. At last, we like to thank associate professor Gerhard Barmen.
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Table of contents
1 Introduction
1.1 Contamination of water resources in Tarkwa Mining Area of Ghana:
Linking Technical, Social-Economical and Gender Dimensions
1.2 Objectives
2 Mining in Ghana
2.1 Historical background
2.2 Contribution to national economy
2.3 Methods of mining
2.3.1 Large-scale mining
2.3.2 Small-scale mining
2.4 Impact of mining and environmental degradation
2.4.1 Large-scale mining
2.4.2 Small-scale mining
3 Location of the study area and geographical characteristics
3.1 The Wassa West district
3.2 Climate
3.3 Geology
3.3.1 The Birimian system
3.3.2 The Tarkwaian system
3.3.3 Granitoids
3.3.4 Basic intrusives
3.4 Hydrogeology
3.5 Soils
4 Environmental geochemistry
4.1 Oxidation of sulphide minerals and Acid Mine Drainage
4.2 Factors effecting the mobility of heavy metals in the environment
4.2.1 Sorption Processes
4.2.2 Precipitation
4.2.3 Redox potential
4.2.4 Grouping of heavy metals
4.2.5 Anions
4.3 Major metallic contaminants related to mining
4.3.1 Aluminium
4.3.2 Arsenic
4.3.3 Cadmium
4.3.4 Chromium
4.3.5 Iron
4.3.6 Lead
4.3.7 Manganese
4.3.8 Mercury
4.3.9 Nickel
4.3.10 Nitrate and nitrite
4.3.11 Sulphate
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5 Materials and methods
5.1 Sampling plan
5.2 Field methods
5.2.1 Groundwater sampling
5.2.2 Surface water sampling
5.3 Water analysis
5.4 Treatment of analytical data
6 Results and discussion
6.1 Field measured parameters
6.2 Major ions
6.3 Trace elements
6.4 Differences between deep and shallow wells
6.5 Hydro-chemical modelling
6.6 Surface water chemistry
7 Conclusion
8 Recommendation
References
Appendix 1: Field measured parameters
Appendix 2: Trace elements
Appendix 3: Major Ions, NPOC, NH4+ and water type
Appendix 4: Ion Balance
Appendix 5: Correlation matrix, all wells
Appendix 6: Correlation matrix, deep wells
Appendix 7: Correlation matrix, shallow wells
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1 Introduction
The Hungarian Nobel Prize winner Albert Szent-Gyorgyi once said, "Water is life's mater
and matrix, mother and medium. There is no life without water.” If the water resources are
contaminated, so is life. Providing clean drinking water for the growing population of the
world is one of the most pressing issues we stands against in the 21th century. Both
anthropogenic and natural processes can affect the water quality. Except from the metals
man has created through nuclear reactions the rest has been on earth since the planet was
formed. There are a few examples of local metal pollutions through natural weathering but
in most cases metals become an environmental and health issue because of anthropogenic
activity. Mainly mining and smelting plant release metals from the bed-rock (Walker &
Sibly 2001). This study is focused on an area in western Ghana that has a long history of
mining activity. There are fears that the mining activity is causing serious metal pollution
to the water resources by for example arsenic, lead, cadmium, mercury and cyanide. Earlier
studies have shown that metal levels in groundwater exceed WHO:s guidelines for drinking
water in many areas in western Ghana. That represents a serious threat to public health.
This study is part of a bigger project “Contamination of water resources in Tarkwa Mining
Area of Ghana: Linking Technical, Social-Economical and Gender Dimensions” which is a
collaboration between KTH and the University of Ghana.
1.1 Contamination of water resources in Tarkwa Mining Area of
Ghana: Linking Technical, Social-Economical and Gender Dimensions
Contamination of water resources in Tarkwa mining area of Ghana: linking technical,
social-economical and gender dimensions is an interdisciplinary study which is a
collaboration between the Department of Land and Water Resources Engineering, Royal
Institute of Technology (Sweden), and the Department of Geography and Resource
Development, University of Ghana (Ghana). The general goal of this study is to gain a
deeper understanding of the extent of the problem, by linking technical knowledge on the
subject with socio-economic and gender perspectives. From the above stated general goal,
four focus areas have been identified to constitute specific objectives of the study. These
include: i) investigate metal pollution in water resources in selected communities. ii)
examine the exposure pathways of the heavy metals and how that differs among socioeconomic and gender groups in local communities. iii) assess the influence of socioeconomic/cultural factors on people’s perception of and attitude to these metal-related
problems and the remediation measures being applied in the areas; iv) suggest measures
that are technically and socio-culturally appropriate for addressing the problem of
contaminated water in the local communities. The proposed project is planned to cover
three years starting January 2004 continuing to December 2006 (Jacks et al 2003). This
minor field study covers the first objective.
1.2 Objectives
The aim of this study is to investigate the groundwater chemistry with special concern to
metal pollution in the water resources in selected communities. The result of this MFS
study will help to determine the most crucial areas for further investigation and contribute
with more physical data for the continuous work within the bigger project “Contamination
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of water resources in Tarkwa Mining Area of Ghana: Linking Technical, SocialEconomical and Gender Dimensions”.
Another objective is to conduct a literature study of the area. As much information as
possible concerning the mining industry, geology and hydrogeology conditions has been
collected.
We also hope that this thesis can be used as a comprehensive review of metals in
groundwater by the Wassa West District Assembly.
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2 Mining in Ghana
2.1 Historical background
West Africa has for centuries been one of the world’s most important gold mining regions.
Today the most significant gold producing country in this area is Ghana. Prospective gold
regions are localized in the western part of the country. Numerous hard rock deposits can
be found and significant quantities have also been re-deposited in local water-bodies,
alluvial gold. These gold deposits enhanced the development of many successful ancient
West African civilizations, and attracted both Arabic and European merchants. The country
of Ghana has taken its name from the Ancient Kingdom of Ghana, which was located about
800 km north of present Accra. Ghana was first mentioned in an Arabic source 788-93
when the trans-Saharan trade with the western savannah started. The gold trade brought
increased wealth to West Africa but ancient Ghanaian society was already in an advanced
economical and political state (Hilson 2002a).
Pre-colonial gold mining operations were extremely simple. Alluvial mining was most
widespread and practised along rivers. Sediment was scooped from the shores, stored in
canoes or bowls and washed repeatedly to separate gold particles. Shallow-pit surface
mining and deep shaft mining also occurred. At the beginning of the Trans Saharan trade
gold was collected as dust or nuggets by rural inhabitants, but increasing demand from
Arabic traders intensified gold production. For 700 years the Islamic world was the only
external influence on West Africa (Hilson 2002a).
Because of acute gold shortage during the 15th and 16th centuries, Europe’s interest in
West Africa increased. In 1471 the Portuguese reached present day Ghana and gained
control of the West African gold trade. The arrival of the Europeans simulated a shift in
activity towards the Gulf of Guinea coastlines. The Portuguese settlement in West Africa
lasted for some 100 years. They constructed a number of forts along the coast facilitating
transcontinental gold trade and preventing other Europeans from being engaged in the
trade. In 1595 the Dutch landed in the Gold Coast and the overtaking of fort Elmina in 1642
signified the end of the Portuguese occupation. The English soon challenged the Dutch.
During the 16th, 17th and 18th centuries the Dutch West India Company and the British
African Company of Merchants were extremely active in gold mining and trade in West
Africa. Towards the end of the 16th century the slave trade began. Trading of gold
decreased but was not abandoned though no advancements and improvements of mine
design and extraction were made. Britain finally got control of Ghana in the mid-1800s,
establishing the Gold Coast Colony in 1874 (Hilson 2002a).
The earliest European attempts to extract gold on a large scale were concentrated in Tarkwa
and Prestea in the late 19th century and the first official European gold mining company
was the African Gold Coast Company, registered February 18th 1878. A number of other
companies were established in Tarkwa at this time. The majority of these companies failed
due to various reasons and it was not until 1895, when a series of gold mines opened in
Obasi, increases in gold production occurred. A gold rush in the early 20th century was
followed a mass increase in gold production. After this the gold production decreased, but
experienced a gold rush after the First World War. Because of the unwillingness of
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Ghanaians to work for Europeans in late 1920s the British passed the Mercury Ordinance,
which made it illegal for Ghanaians to own Mercury. The gold production fluctuated until
Second World War. After Ghana gained independence in 1957 the industry collapsed
(Hilson 2002a). The drastic decrease in gold production was due to the many problems
resulting of the economic, financial, institutional and legal framework within which the
sector operated (Aryee 2001).
By 1966 all but one of the Ghanaian gold mines were nationalized. The industry
experienced a continuous decrease in production and the rapid deterioration was the result
of excessive state control. In the 1960s and 1970s Ghana developed one of the most
centrally controlled economies outside Eastern Europe (Hilson 2002a). This resulted in a
rise of illegal and uncontrolled artisanal mineral production and smuggling as well as
declining mineral sector performance (Addy 1998).
In 1972 the Government endorsed a new law emphasizing that 55% of equity capital of
each company to be held by the government and payment of fair compensation, based on
55% of the company’s total assets also to be made to the government. It became extremely
difficult for companies to become profitable. By 1976, gold mine production was about
60% compared to 1960 and a 50-years low record was reached in 1982 (Hilson 2002a).
In 1981, a military coup lead by Rawlings overthrew the existing government and formed
the Provisional National Defence Council (PNDC). The PNDC government soon sought
help from IMF (international money fund) to prepare a plan for economic recovery (Hilson
2002a).
In 1983 the government started the Economic Recovery Program (ERP) under guidance of
IMF (Hilson 2002a). The objective of the program was to quickly attract investors to the
mining sector and other key sectors, which had export potential, to turn around the general
economy of the country (Aryee 2001). After the implementation of the ERP the mining
industry has seen a phenomenal growth, which mainly can be attributed to the adoption of
World Bank recommendations in a new national mineral policy through the 1986 Minerals
and Mining Law. This law basically means that the government leaves the mine operation,
management and ownership to the private sector (Addy 1998). In 1989 the Small Scale
Gold Mining Law legalized small-scale gold mining as an industry in Ghana (Hilson
2002b). Records from the Minerals Commission show that US$4 billon of private
investment capital was injected in the mining sector between 1983-1998. The gold
production increased from 8.87 MT 1983 to 74.1 MT 1998 (Aryee 2001). Both large and
small-scale projects have developed during the 1990s and a wide range of companies from
Australia, Canada, the Netherlands, South Africa, the United Kingdom and the United
States now hold controlling interest in most of the gold mines currently in operation (Addy
1998). From 1992 the mineral industry became the single largest foreign exchange earner
and gold accounts for 95 % of this. Other big key sectors in Ghana are cocoa and forestry
(Aryee 2001).
2.2 Contribution to national economy
The UN definition of a mineral economy is those economies where mining generates at
least 10 % of Gross Domestic Product (GDP) and mineral exports are at least 40 % of their
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Contamination of water resources in Tarkwa mining area of Ghana
foreign exchange earnings. Ghana is not exactly classified as a mineral economy by the UN
definition. About 40 % of gross foreign exchange earnings come from the mining sector
and it generates 5.7 % of GDP (Aryee 2001). The industry also has linkages to other sectors
and is a major employer in rural areas. Mines also contribute to the development of these
areas by engaging in community activities and adding to infrastructure by building schools,
hospitals and roads (Addy 1998).
The fairly low contribution to GDP conforms that while mining is important, the
dependence of this sector is comparable or lower than a number of other mining countries
(Aryee 2001).
The ERP was well received by for example IMF and the World Bank and Ghana has
received a mass inflow of loans and development funds. Since 1980 the gold production
has increased with 700%. Gold mining has long been an important economic activity in
Ghana and has recently become the main industry of the county (Hilson 2002a). The
mining sector is very important to the nation’s economic recovery (Addy 1998).
2.3 Methods of mining
In Ghana there are both small-scale miners and large-scale mining. The general processing
techniques are handpicking, amalgamation, cyanidation, flotation, electrowinning and
roasting of ore (Akosa et al. 2002). The technique differs between large- and small-scale
mining and also varies depending on the type of deposit and its location (Ntibery et al.
2003). The area has three main gold deposits. Placer or alluvial deposit, non-sulphidic
paleplacer or free milling ore and oxidized ore (Kortatsi 2004).
2.3.1 Large-scale mining
Large-scale mining is today conducted as surface mining. Cyanidation is the most common
technique in the study area and is used for non-sulphidic paleplacer ore (Akosa & Adimado
2002 and Kortatsi 2004). Non-sulphidic paleplacer ore occurs mainly in hard rock. It is
particularly associated with the Banket conglomerates of the Tarkwa formation. Teberebie
Goldfields Limited and the Ghana Australian Goldfields use this ore (Kortatsi 2004). This
technology is typically conducted as drilling, blasting, haulage of the ore, crushing and
screening, agglomeration, haulage and stacking. Lime (CaO) is now applied to the ore to
raise the pH to between 10.5 and 11.0. Sodium cyanide solution (NaCN) is used for
dissolution of the gold. The prepared ore is heaped into plastic lined pads but between 45450 l/day of sodium cyanide solution per hectare possibly leaks out into the environment
(Kuma & Younger 2004). Finally gold is recovered through electro winning (Akosa et al.
2002 and Kortatsi 2004).
Oxidized ore occurs in weathered rocks and is derived from sulphides, arsenopyrite, realgar
(AsS), opiment (As2S3) pyrites etc. Roasting is used (Kortatsi 2004). In the Wassa west
district there are in this moment seven large-scale mines which extract and process two
metals, gold (six mines) and manganese (one mine). A number of these are located in our
study area namely; Ghana Australian Goldfields (GAG), Teberebie Goldfields Limited
(TGL), Goldfields Ghana limited (GGL) (Kuma & Younger 2004), Ghana Manganese
Company in Nsuta (Kortatsi 2004) and also Abosso Goldfield at Damang (Nankara, T.
2004, pers. comm., 16 Sep).
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2.3.2 Small-scale mining
Small-scale mining in Ghana is defined as “mining by any method not involving substantial
expenditure by any individual or group of persons not exceeding nine in number or by a cooperative society made up of ten or more persons” (Government of Ghana 1989). They are
estimated to number over 150,000 in Ghana, of which many operate illegally on
concessions belonging to large scale operators, or in restricted areas (Ghana academy of
arts and sciences 2003). The illegal small-scale miners account for approximately 10% of
the gold production in Ghana (Ntibery, B. 2004, pers. comm., 9 Sep). These are locally
referred to as galamsey (Hilson 2002c). The technique mostly used for small-scale mining
is amalgamation (Akosa 2002). In this process mercury is mixed with gold concentrate to
form gold amalgam, which is heated to separate the gold (Ntibery et al. 2003). Both legal
and illegal small scale mining is practised in the district (Avotri et al. 2002). In the Tarkwa
area small-scale mining is found all around, both in the forest and along the rivers. It is
practised all year around and number about 20 000 in the Wassa West district. Of these
small-scale miners about 90 % are illegal. At the moment 168 small-scale mining
concessions are valid in the region (Ntibery, B. 2004, pers. comm., 9 Sep).
Figure 2-1:Blanket washing of milled ore
(to the left). The concentrate from this
gravity method is repeatedly washed and
gold amalgam is formed when mercury has
been added (to the right).
At present there are totally about 237 companies (154 Ghanaian and 83 foreign)
prospecting for gold and another 18 are operating gold mines in Ghana (Hilson 2002a).
2.4 Impact of mining and environmental degradation
Large- and small-scale mining cause somewhat different environmental concerns.
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Contamination of water resources in Tarkwa mining area of Ghana
2.4.1 Large-scale mining
The major concerns observed in this area are (the following section is mostly collected
from Akosa et al. (2002)):
• Land degradation, for example removal of vegetative cover and destruction of flora and
fauna
• Impact due to processing technique includes contamination of water bodies and soil by
release of cyanide (see below), arsenic, sulphates, and heavy metals as Pb, Cu, Zn and Fe
• Cyanide spillage. There have been a number of accidental cyanide spillages in Ghana. The
major spillages occurred in 1989, 1991, 1994, 1996, 1999 and 2001
• Roasting of ore containing pyrite gives a rise to the production of SO2 in the atmosphere
which produces acid rain. The acid water then releases high levels of toxic ions from the
rock matrix in the groundwater. This has been the main mode of extraction for the Prestea
mine during the last decade. SO2 could also been transported with north-eastern winds from
the Ashanti Goldfields in the northeast (Kortatsi 2004)
• Noise and vibrations
• Dust from blasting operations
• AMD (Acid Mine Drainage) from solid waste from sulphidic ore leaching heavy metal
and acidity into water and soil
• Siltation of surface waters
• Grease and oils from various activities in the mine
The management of waste from large scale mining is done in accordance to approved
environmental plans. The spent heap and waste rock heaps are stabilized and re-vegetated.
Tailing slurries are channelled into tailing dams that also are re-vegetated. Reagent
containers and packing materials are sold out to contractors who dispose of them. The
monitoring of these contractors is poor. Spent oil and grease are sold to end-users.
2.4.2 Small-scale mining
Illegal miners account for the most significant part of the environmental damage of the
small-scale miners. Legal small-scale miners must have environmental permits and are
monitored regularly by field officers. Amalgamation is the technique mostly used (Ntibery
et al. 2003). The main environmental problems are mainly (the following section is mostly
collected from Ntibery et al. (2003)):
• Land degradation
• Pollution of rivers and streams by mercury
• Atmospheric impacts from mercury fumes during gold recovery and dust
•Mercury in groundwater from accidental spillage during gold processing (Akosa et al.
2002)
• AMD from solid waste from sulphidic ore leaching heavy metal and acidity into water
and soil (Akosa et al. 2002)
• Siltation of surface waters (Akosa et al. 2002)
• Deforestation due to wood used for stabilizing mining shafts (Ntibery, B. 2004, pers.
comm., 9 sep)
• Damage to infrastructure due to undermining of roads and houses
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The management of waste on small-scale mines particularly illegal ones does not have a
waste management plan but simply leave the waste.
Estimated 5 tonnes mercury is released from small-scale mining operations in Ghana each
year (Hilson 2001). High concentrations of mercury have been found in sediments and fish
in the vicinity of small-scale mining activities using amalgamation as their main technique.
The concentration in most fish fillets in these areas exceeds the recommendations of the
United States Food and Drug Agency (Babut et al. 2003).
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3 Location of the study area and geographical
characteristics
3.1 The Wassa West district
The Wassa West district occupies the
mid-southern part of the Western region
of Ghana with Tarkwa as its
administrative capital. The population of
the district is approximately 236 000 and
is mainly composed of the indigenous
Wassa tribe but all tribal entities in Ghana
are well represented. Subsistence farming
is the main occupation of the people
although rubber, oil palm and cocoa are
also produced Mining is the main
industrial activity in the area (Avotri el al.
2002). The area lies within the main gold
belt of Ghana that stretches from Axin in
the southwest, to Konongo in the
northeast (Kortatsi 2004). Location of the
Wassa West district and the study area is
shown in figure 3-1.
Figure 3-1: Location of the study area
3.2 Climate
Wassa west district is situated on the border of two climatic regions. The south part belongs
to the south western equatorial climatic region and the northern part has a wet semiequatorial climate. Generally the rainfall pattern follows the northward advance and the
southward retreat of the inter-tropical convergence zone that separates dry air from Sahara
and the moisture-monsoon air from the Atlantic Ocean. The north air mass, locally called
the Harmattan, brings in hot and dry weather during December to February ( Dickson &
Benneh 1980). The area is characterized by double rainfall maxima. The first and largest
peak occurs in June, whilst the second and smaller peak occurs in October. Around 53% of
all rain in the region falls between March and July. The mean annual rainfall is
approximately 1874mm with max and min values of 1449mm and 2608, respectively. The
mean pH of the rain water in the area during 2000-2001 was 6.07 (Kortatsi 2004). The area
is very humid and warm with temperatures between 26-30C°. ( Dickson & Benneh 1980)
3.3 Geology
Ghana is underlain partly by what is known as the Basement Complex. It comprises a wide
variety of Precambrian igneous and metamorphic rock which covers about 54% of the
country, mainly the southern and western parts of the country, Figure 3-2. It consists
mainly of gneiss, phyllites, schists, migmatites, granite-gneiss and quartites. The rest of the
country is underlain by Palaeozoic consolidated sedimentary rocks referred to as the
Voltaian Formation consisting mainly of sandstones, shale, mudstone, sandy and pebbly
beds and limestones (Gyau-Boakye and Dapaah-Siakwan 1999).
9
Asklund and Eldvall
LUTVDG/TVTG--5092--SE
Figure 3-2: Simplified geological map of southwest Ghana (modified from Kuma 2004)
The Basement complex is further divided into different sub provinces including the
metamorphosed and folded rocks of the Birimian and Tarwaian system (Gyau-Boakye and
Dapaah-Siakwan 2000). In several places these systems are intruded by sills and dykes of
igneous rocks ranging from felsite and quartz porphyry to metadolerite, gabbro and norite
(Kortatsi 2004).
10
Contamination of water resources in Tarkwa mining area of Ghana
Figure 3-3: Geological map of the Tarkwa-Prestea area (modified from Kortatsi 2004)
The geomorphology of the Tarkwa-Prestea area consists of a series of ridges and valleys
parallel to each other and to the strike of the rocks. The strike of the rock are generally in
north-south direction (Kortatsi 2004). Both the Tarkwaian and Birimian systems are folded
along axes that trend northeast (Gyau-Boakye and Dapaah-Siakwan 2000). The general
11
Asklund and Eldvall
LUTVDG/TVTG--5092--SE
type of topography reflects underlying geology (Kortatsi 2004). The geology of the Tarkwa
Prestea area is shown in Figure 3-3.
The area has three main gold deposits. Placer or alluvial deposit, non-sulphidic paleplacer
or free milling ore and oxidized ore (Kortatsi 2004).
Alluvial deposits occur in streams draining areas with auriferous deposits where the
bedrock only is slightly metamorphosed and intruded by Dixcove granite particularly in
Biriminan rock areas (Kortatsi 2004).
Non-sulphidic paleplacer ore occurs mainly in hard rock. It is particularly associated with
Banket conglomerates of Tarkwa formation (Kortatsi 2004).
Oxidized ore occurs in weathered rocks and is derived from sulphides, arsenopyrite, realgar
(AsS), opiment (As2S3) pyrites etc (Kortatsi 2004).
3.3.1 The Birimian system
The Birimian system consists of a great thickness of isoclinally folded metamorphosed
sediments intercalated with metamorphosed tuff and lava. Large masses of granite have
also intruded the Birimian system (Gyau-Boakye and Dapaah-Siakwan 2000). The
Birimian system is largely folded. It is fissured to a larger extent compared to the
Tarkwaian system. This system is divided into to upper and lower Birimian Series (Kortatsi
2004).
The sediments are predominant in the lower part of the system. These sediments have been
metamorphosed to schist, slate and phyllite (Gyau-Boakye and Dapaah-Siakwan, 2000).
There are also some tuff and lava.
The upper part of the system is dominantly of volcanic and pyroclastic origin. The rocks
consist of bedded groups of green lava (Kortatsi 2004). Lava and tuff dominate this part
(Gyau-Boakye and Dapaah-Siakwan 2000). Several bands of phyllite occur in this zone and
are manganiferous in places. The thickest sequence occurs in Nsuta where manganese is
being mined (Kortatsi 2004).
In the Birimian system, gold occurs in five parallel, more than 300km long, northeasttrending volcanic belts. They are separated by basins containing pyroclastic and metasedimentary units. The gold occurrence is 2 to 30 ppm in quartz veins of laterally extensive
major ore bodies. They deeply penetrate fissures and shear zones in contact between metasedimentary and meta-volcanic rocks. The veins consist of quartz with carbonate minerals,
green sericite, carbonaceous partings and metallic sulphides and arsenides of Fe, As, Zn,
Au, Cu, Sb and Pb (Dzigbodi-Adjimah 1993).
The Birimian system has a higher content of heavy metals than the Tarkwaian system
(Kortatsi 2004).
12
Contamination of water resources in Tarkwa mining area of Ghana
3.3.2 The Tarkwaian system
The Tarkwaian system is an elongated and narrow syncline about 250 km long and 16 km
wide (Kortatsi 2004). The system consists of slightly metamorphosed, shallow-water,
30
Awudua
5 25'
16
Pa m
pam
e
Po n
Prestea
45
50
43
15
tu
Huniso
M esa
AN
KO
BRA
Hu
ni
37
Awiafu
C
37
Adja Bippo
esa
m
ak
Nt
a
as
AbosoF
Huni Sandstone and Phyllite
30
5 20'
20
L E G E N D
55
50
45
10
Akontasi
20
16 65
15 20
30
20
14
15
Tarkwa
10
INTRUSIVE
ROCKS
Nsuta
ben
Upper Birrimian
Epidiorite, Gabbro and allied
Intrusions
Dixcove Granite and Porphyry
Faults
D
46
Small Sills and Dykes
45
75
30
33
80
Achim
40
50
60
Road
80
80
5 15'
Kawere Group
BIRRIMIAN
SYSTEM
ona
50
40
Banket Series; Conglomerate
Zone shown thus
Anw
Ahum
abuni
Ab
om
pu
nu
Tarkwa
60
Tarkwa Phyllite
TARKWAIAN
SYSTEM
Railway
River
45
30
10
50
59
55
50
18
wu
iabe
Bed
59
ng 43
po
Su 46
A
15
50
ka
Tra
B
70
5 10'
Settlement
Agona
O
NS
Strike and Dip
80
Bonsaso
A
GEOLOGICAL SECTIONS
Scale:-
Horizontal... 1cm = 2.50 km
Vertical..... 1cm = 480 km
B
Wuruwuru R.
Dompim
Supong R.
Ongwan Hill
Traka R.
W
ur
uw
ur
u
25
Bediabewu R.
Enifutu R.
Anwonaben R.
Abompunu R.
Ntakasa R.
Huni R
Ankobra R.
Ahumabuni R.
Line A-B
Simpa
5 05'
Line C-D
5 Miles
8 Kms.
Railway
4
6
Mesamesa R.
3
4
Awiafutu R.
2
Huni R.
2
1
Pampam R.
0
Pone R.
0
Line E-F
2 10'
2 00'
2 05'
1 55'
Figure 3-4: Geological map of the Tarkwa area. Coordinates are given as degrees North
and West (Kortatsi 2004)
13
Asklund and Eldvall
LUTVDG/TVTG--5092--SE
sedimentary strata. It is chiefly sandstone, quartzite, shale and conglomerate and is resting
on and derived from the Birimian system (Gyau-Boakye and Dapaah-Siakwan 2000).
Intrusive igneous rocks contribute to about 20% of the total thickness of the Tarkwaian
System in the Tarkwa area. These range from hypabyssal felsic to basic igneous rocks
(Kuma 2004). Granitoids of the Dixcove Granitoids systems has also intruded the
tarkwaian system in many places (Kortatsi 2004). A detailed geological map is presented in
Figure 3-4.
The rocks of the Tarkwaian system consist of the Kawere Group, The Banket Series, the
Tarkwa Phyllite and the Huni Sandstone. Most of the rocks that resemble sandstone at the
surface are weathered equivalents of parent quartzites (Kuma & Younger 2001). See table
3-1.
Table 3-1: Division of the Tarkwaian system (from Kuma & Younger 2001)
Series
Kawere Group
Banket Series
Thickness (m)
250-700
120-160
Tarkwa Phyllite
120-400
Huni Sandstone
1370
Composite lithology
Quartzites, grits, phyllites and conglomerates.
Tarkwa phyllite transitional beds and sandstones, quartzites, grits
breccias and conglomerates.
Huni sandstone transitional beds, and greenish-grey phyllites and
schists.
Sandstones, grits and quartizes with bands of phyllite.
The Kawere Group is the oldest group and the conglomerates consist of silicified Birimian
greenstone and hornstone with minor jasper, quartz, quartz-porhyry, tourmaline-quartz
rocks with Birimian phyllites and schists in a matrix with quartz, feldspar, chlorite,
carbonate, epidote and magnetit (Kuma 2004).
The Banket conglomerate consists of 90% quartz and the rest is Birimian schist, quartzite,
hornstone, chert and gondite (Kuma 2004). It represents a fluviatile series (Kortatsi 2004).
The Tarkwa Phyllite consists of chloritoid and magnetite or hematite with sericite and
chlorite (Kuma 2004).
Huni Sandstone (a quartzite) consists of variable amounts of feldspar, sericite, chlorite,
ferriferous carbonate, magnetite or hematite and epidote.
In the Tarkwaian system the gold occurs in various auriferous and quartz-pebble
conglomerates (Ghana academy of arts and sciences 2003).
3.3.3 Granitoids
Large masses of granitoids of the Cape Coast Granitoids Complex have intruded the
Birimian system. It is well foliated and the potash rich rocks consist of muscovite and
biotite granite and granodiorite ore porphyrblastic biotitic gneiss, applite and pegmatite.
They are characterized by the presence of many enclaves of schists and gneisses. The
Dixcove Granitoids Complex has intruded both the Birimian and Tarkwaian systems in
many places. It consists of hornblende granite or granodiorite grading locally into quartz
diorite and hornblende diorite. The complex forms non-foliated discordant or semidiscordant bodies in the enclosing country rock, generally Upper Birimian meta-volcanics.
14
Contamination of water resources in Tarkwa mining area of Ghana
3.3.4 Basic intrusives
Intrusive igneous rocks contribute to about 20% of the total thickness of the Tarkwaian
System in the Tarkwa area. These range from hypabyssal felsic to basic igneous rocks
(Kuma 2004). The igneous rocks are mainly of basic composition. Their composition range
from quartz-porphyry and albite through keratophyre, granophyric albite-dolerite to fresh
olivine-gabbro and norite. Pyrite is common in many of the igneous rocks and quartz veins
that intruded the Birimian and the Tarwaian rocks that underlie the area (Kortatsi 2004).
3.4 Hydrogeology
Groundwater is the main source of water supply in the study area. Most major towns in the
area except from Tarkwa rely solely on groundwater. To match the demand for potable
water the number of boreholes and hand dug wells are increasing rapidly (Kortatsi 2004).
Surface water taken from the River Bonsa at Bonsaso is treated and distributed to Tarkwa
town. Some villages between Bonsaso and Tarkwa are also connected to the pipe (Nankara,
T. 2004, pers. comm., 16 Sep). Yield varies from 0.4-18 m3h-1 with an average of 2.4 m3h-1.
The depth varies between 18m to 75m with an average of 35.4m but has little or no effect
on borehole yields (Kortatsi 2004).
In the Tarkwa-Prestea area groundwater occurrence is associated with the development of
secondary porosity through fissuring and weathering. The rock underlying the area lack
primary porosity since they are consolidated. The weathering depth is greatest in the
Birimian system where depths between 90m and 120 m have been reached. Also in
granites, porhyrites, felsites and other intrusive rock the weathering depth is great. In the
Tarkwa system however, and especially in the Banket series quartzites, grits, conglomerates
and Tarkwa phyllite, the weathering depth rarely exceed 20 m. Clay, silts, sandy clays and
clayey sands are mostly the result of the weathering. In this area two types of aquifers
occur. The weathered aquifer occurs mainly above the transition zone between fresh and
weathered rock. Due to the soils content of clay and silt, these aquifers have high porosity
and storage but low permeability. The aquifer in the fractured/fissured zone occurs below
the transition zone. They have relatively high transmissivity but low storage (Kortatsi
2004).
The recharge of groundwater in the area occurs mainly by direct seepage or infiltration. In
some places groundwater is in hydraulic contact with rivers and recharge from them can
also take place (Kortatsi 2004).
Groundwater circulation in the Tarkwa-Prestea area is mainly localized due to the
numerous low hills that act as groundwater divides. Groundwater circulation is mainly
restricted to quartz veins and fissures-faults-brecciated zones. Groundwater velocities are
not known. It might be that pollution has not yet reached the domestic wells (Kortatsi
2004).
Low conductivity values of the groundwater in the area indicate that the water is unable to
react with the rock matrix to equilibrium which indicates short resident times (Kortatsi
2004).
15
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LUTVDG/TVTG--5092--SE
Groundwater in mining areas as the Tarkwa-Prestea area is known to be vulnerable to
pollution from mining that may have a serious effect on human health. In gold mining areas
sulphides oxidation leads to production of low pH in the groundwater that encourages the
dissolution of trace metals in the groundwater in very high concentrations (Kortatsi 2004).
3.5 Soils
There are mainly two types of soil in the Tarkwa-Prestea area, the forest oxysols in the
south and the forest ochrosol-oxysol integrates in the north (Kortatsi 2004).
The forest oxysols are porous, well drained, and generally loamy brown to orange. Due to
the heavy and plentiful rainfalls in the south, a high degree of leaching and reduction of
calcium, magnesium and other nutrients have occurred in the soil. This has made the soil
acidic. The forest ochrosol-oxysol integrated is an intermediate between the forest oxysols
and the forest orhrosol. The forest ochrosol-oxysol integrated is highly coloured as it is less
leached, as a result of reduced rainfall in the north. It contains more of its nutrients and is
therefore more alkaline then the forest oxysols in the south (Kortatsi 2004).
The soil in the Tarkwa area consists of mostly silty-sands with minor patches of laterite,
mainly on hilly areas (Kuma & Younger, 2001). The distribution of size fractions is given
in Table 3-2.
Table 3-2: Characteristics of soils in the Tarkwa area (from Kuma & Younger 2001)
Soil type
Banket
Huni
Kawere
Tarkwa Phyllite
Weathered dyke
Texture
Silty-Sand
Laterite
Silty-Sand
Silt Sand
Laterite
Silt
Percentage
Gravel
2
69
2
0
69
3
Sand
59
14
55
47
9
20
Silt
20
10
33
40
13
64
All these soil types are considered to be clayey soil-types (Svensson 1999).
16
Clay
10
7
10
13
16
13
Contamination of water resources in Tarkwa mining area of Ghana
4 Environmental geochemistry
The groundwater composition varies widely and is a combined result of the composition of
the water entering the groundwater reservoir and the reactions with minerals present in the
rock that may modify the water composition. Some minerals dissolve quickly and
significally change the water composition, like carbonates, others dissolve slowly and have
less effect on the water composition, like silicates. The retention time is also important in
determining the water chemistry. Long residence times allow reactions to take place and
these waters are likely to have higher concentrations of ions than water with short residence
times (Appelo & Postma 1999). Usually in unaffected environments the concentration of
most metals is very low and is mostly determined by the mineralogy and the weathering
(Espeby & Gustafsson 2001). There are a few examples of local metal pollution through
natural weathering but in most cases metals become an environmental and health issue
because of anthropogenic activity. Mainly mining and smelting plants release metals from
the bedrock (Walker & Sibly 2001). Soil concentration of adsorbing surfaces (oxide
surfaces, clay mineral and humic substances) and the pH are very important parameters
effecting the transportation of metals in the groundwater system (Espeby & Gustafsson
2001).
4.1 Oxidation of sulphide minerals and Acid Mine Drainage
Oxidation of pyrite and other sulphide minerals by oxygen have a large environmental
impact and play a key role in Acid Mine Drainage (AMD). It is a source of sulphate,
acidity, and iron in groundwater and is a source of heavy metals in the environment.
Sulphide mine tailings are a notorious source to contamination of both streams and
groundwater by heavy metals (Appelo & Postma 1999) see Figure 4-1.
Figure 4-1: Pyrite oxidation by oxygen supplied by purely advective flow. Groundwater
saturated with O2 is transported through a pyritic layer. Oxygen is consumed and values of
SO42- and Fe2+ are increased (Appelo & Postma 1999).
17
Asklund and Eldvall
LUTVDG/TVTG--5092--SE
Sulphide minerals can be oxidized through three different processes: chemical oxidation
with oxygen, chemical oxidation with Fe3+ and oxidation catalyzed by micro organisms. All
these reactions accelerate at low pH. (SEPA 2002)
Chemical oxidation with oxygen: The following reaction describes the overall process
FeS 2 (s ) +
15
7
O2 + H 2 O ⇒ Fe(OH )3 (s ) + 2SO42− (aq ) + 4 H +
4
2
This complete oxidation involves oxidation of both polysulphide S22- and Fe2+. Under
natural conditions the oxidation often proceeds in two steps:
7
FeS 2 (s ) + O2 + H 2 O ⇒ Fe 2+ (aq ) + 2SO42− (aq ) + 2 H +
2
1
1
Fe 2+ (aq ) + O2 + H + ⇒ Fe 3+ (aq ) + H 2 O
4
2
Fe3+ may precipitate as FeOOH depending on pH. Energy yield of polysulphide oxidation
is larger than that of Fe2+ oxidation and incomplete oxidation of pyrite, resulting in
solutions rich of Fe2+ and SO42- occurs naturally as well as complete oxidation.
The second mechanism for pyrite oxidation is by reaction with Fe3+:
FeS 2 (s ) + 14 Fe 3+ (aq ) + 8H 2 O ⇒ 15Fe 2+ (aq ) + 2SO42− (aq ) + 16 H +
This reaction is particularly important at low pH since the solubility of Fe3+ decreases as a
third function with increasing pH. This reaction is about ten times faster than the first
reaction. The process rapidly consumes all Fe3+ and oxidation would cease unless Fe3+ is
replenished by oxidation of Fe2+ by oxygen:
1
1
Fe 2+ (aq ) + O2 + H + ⇒ Fe 3+ (aq ) + H 2 O
4
2
Below pH 4.5 this reaction is considerably slower than pyrite oxidation of Fe3+ and is the
rate limiting step at these pH values (Appelo & Postma 1999). Iron oxidizing bacterias, as
Thiobacillus ferrooxidans, are able to increase the Fe2+ oxidation rate up to four orders of
magnitude which brings up this reaction to the same magnitude as pyrite oxidation by Fe2+
(Salmon 2003).
There is a close connection between sulphide minerals, especially arsenopyrite, and gold in
most parts of Ghana (Smedley 1996). AMD have been reported from a number of mines in
Ghana. Monitoring of a large spoil heap in the Tarkwa area show water quality consistent
with AMD characteristics. The pH is consistently below 4, has high concentrations of
sulphate, silica, aluminium, iron, and manganese, and shows little variation during the year
(Kuma 2003). The major minerals associated with AMD that occurs in the Tarkwa-Prestea
area shown in Table 4-1.
The problems associated with AMD can therefore be expected in gold mining areas in
Ghana. The pH levels of the groundwater in the Tarkwa Prestea indicate AMD. Acid rain
and acid geothermal waters cannot fully explain these low values (Kortatsi 2004).
18
Contamination of water resources in Tarkwa mining area of Ghana
Table 4-1: Minerals associated with AMD in the study area (Kortatsi 2004)
Mineral
Arsenopyrite
Bournonite
Chalcopyrite
Galena
Pyrite
Sphalerite
Tennalite
Composition
FeS2, FeAs, FeAsS
PbCuSbS3
CuFeS2
PbS
FeS2
ZnS
[(Cu, Fe, Zn,)As4S]
4.2 Factors effecting the mobility of heavy metals in the environment
Besides the metals man have created through nuclear reactions the rest have been on earth
since the planet was formed (Walker & Sibly 2001). The metals exist naturally in the
bedrock and are released through weathering. In water, metals exist in different forms, both
solved and suspended, depending on a number of different parameters. The solubility,
transportation and toxicity differ between different metal species.
The transportation of metals with groundwater is normally affected by sorption to solid
aquifer material (Appelo & Postma 1999). The most important chemical retention
mechanisms are sorption processes and precipitation (Espeby & Gustafsson 2001).
Other chemical processes of importance are redox reactions and complexation. An
increased aqueous complexation often makes an element more soluble, but the form is often
less toxic. The redox status decides the speciation of some redox-sensitive elements.
Different redox species have different retention capacity and the redox status is important
for transport (Espeby & Gustafsson 2001).
These mechanisms and the mobility of metals are affected by a number of different
parameters e.g. the oxidation state of the metal ion, pH and Eh (Appelo & Postma 1999).
Determining the mobility of heavy metals is a very complex matter.
4.2.1 Sorption Processes
The pH is crucial for the extent of sorption. Anions adsorb more strongly with decreasing
pH while the reverse is true for cations (Espeby & Gustafsson 2001). This is caused by the
increase in H+, which binds to charged surfaces instead of metals. Since binding sites are
limited, metals will go into solution. Sorption processes is a generic term for a number of
different mechanisms. Adsorption indicates that a chemical adheres to the surface of the
solid, absorption suggest that the chemical is taken up into the solid and exchange involves
the replacement of one chemical for another at the solid surface. The major difference
between adsorption and ion exchange is that ion exchange considers the concentration of
two chemicals and adsorption considers one. (Appelo & Postma 1999).
Ion Exchange: An ion in solution can be electrostatically attracted to a charged surface, and
is adsorbed (Fetter 2001). Only electrostical forces cause the adsorption and the ion is
situated at a certain distance from the surface and can easily be substituted by competing
ions. Soil particles are mostly negatively charged and ion exchange is most important for
cations but Fe- and Aloxides have positive charge, and adsorption of anions can occur
19
Asklund and Eldvall
LUTVDG/TVTG--5092--SE
(Espeby & Gustafsson 2001). Ion exchange sites are found primarily on clay and organic
materials however all soils and sediments have some ion-exchange capacity (Fetter 2001)
A general order of cation exchangeability in groundwater is (Fetter 2001 and Appelo &
Postma 1999):
Na > K > Fe > Mn > Mg > Ca
Adsorption and absorption: Ions have different tendencies to form complexes with different
substances. Many cations can form complexes with hydroxyl groups (OH), or carboxyl
groups (COOH) and therefore these ions easily are adsorbed to surfaces with these groups.
Many anions form complexes with surfaces containing Fe or Al and can be adsorbed by
them (Espeby & Gustafsson 2001).
In soils, only particles with large specific surface have the ability to adsorb ions
significantly. Coarser particles have much smaller surface and are therefore insignificant
for sorption processes. Examples of soils with large specific surface are clay-minerals and
oxides (Espeby & Gustafsson 2001). The charge of clay minerals varies depending on
protonation of of surface oxygen and deprotonation of surface hydroxyls. The surface
charge and capacity of sorption is therefore pH-dependent (Appelo & Postma 1999). Table
4-2 shows adsorption of two cations, Calcium (mostly ion exchange), and Copper (mostly
adsorption and absorption), and one anion, Arsenic, to different surfaces (Espeby &
Gustafsson 2001).
Table 4-2: Examples of adsorption by different soils
Type of surface
Dominating mechanism
Clay minerals
Fe/Al oxides
Mn-oxide
Ion exchange
Adsorption or absorption
Adsorption or absorption
Adsorption
of Calcium
Large
Very small
Small
Adsorption
of Copper
Large
Medium
Very large
Adsorption
of Arsenic
Small
Very large
Medium
4.2.2 Precipitation
The most important precipitations are different oxides/hydroxides and carbonates.
Concerning Manganese, Aluminium, Crom and Iron, formation of oxides/hydroxides are
important throughout the natural range of pH, but also for other elements when they occur
in high concentrations. Some elements form carbonates and hydroxycarbonates but only at
high pH>7-8. During reducing conditions sulphides can be very important for precipitation
(Espeby & Gustafsson 2001).
4.2.3 Redox potential
Redox reactions imply a electrone transfer from one atom to another. Redox processes are
generally very slow (Appelo & Postma 1999). The solubility of many substances is
governed by the redox state. Some examples of metals greatly effected by the redox state
are: Manganese, Cromium, Arsenic, Selenium and Iron. Sulfate ions can be reduced to
sufide and react with metals and form complexes which often have very low solubility.
This can considerably decrease the mobility of metals such as, Iron Copper, Lead, Zink,
Mercury, Cadmium and Nickel (Espeby & Gustafsson 2001).
20
Contamination of water resources in Tarkwa mining area of Ghana
Different species are reduced in a specific sequence. O2 is reduced first, followed by
reduction of nitrate, followed by reduction of Mn(IV) to Mn(II), followed by reduction of
Fe(III) to Fe(II), followed by reduction of organic matter, SO42- reduction etc. (Appelo &
Postma 1999)
4.2.4 Grouping of heavy metals
To enhance the clearness of this review we have separated some of the most important
metals into four groups. The following information is collected mostly from Espeby &
Gustafsson (2001):
Group 1: Metals forming hydroxides. Aluminium and Crom(III).
Both these metals easily form complexes in the form of hydroxides, which dominate when
pH> approx. 4-5. Precipitation of the hydroxides commonly determines the solubility above
this pH. At lower pH the solubility is usually determined by adsorption. These metals form
strong complexes with organic ligands and because of this, the metals are usually
transported as organic complexes at pH> 4-5. The solubility of Cr(VI) is instead governed
by adsorption to oxides and varies significally with pH.
Group 2: Strongly adsorbing cations. Copper, Lead and Mercury.
These metals are strongly bonded to variable charged surfaces, for example clay minerals,
because they easily form complexes in the form of hydroxides. The transport of these
metals is retarded mostly through adsorption to clay minerals and Fe/Al oxides. The
adsorption of these metals is very strong. At high pH (pH>7-8) precipitations like
Cu2(OH)2CO3 (malakite) can be important . The solubility of the free ions is very low and
the metals are almost always transported in different complexes (mostly organic
complexes). If conditions are reducing, the metals are strongly bonded as sulphides with
very low solubility.
Group 3: Cations which are adsorbed with average strength. Cadmium, Nickel and Zink
These metals are adsorbed fairly strongly in the soil, both through specific adsorption and
ion exchange. This group is not adsorbed as strongly as group 2. The solubility varies
strongly as a function of pH (lower pH gives higher solubility), organic content and
mineralogy. During reducing conditions these metals form sulphides with low solubility.
Group 4: Weakly adsorbed cations. Calcium, Manganese, Potassium and Sodium.
Potassium and Sodium are almost solely bonded electrostatically through ion exchange
while Calcium and Manganese also form weak surface complex with organic materials.
Arsenic
Arsenic is a metalloid, behaving more like a non-metal then a metal. It forms compound
with oxygen. That makes As mobile in both oxidizing and reducing environments and it is
mainly controlled by adsorption. That makes solid concentrations of oxides and hydroxides
of Fe, Al and Mn essential parameters for controlling As transportation (Smedley &
Kinniburgh 2001).
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LUTVDG/TVTG--5092--SE
4.2.5 Anions
Anions as phosphate can be adsorbed strongly, mostly to oxide surfaces. Sulfate is weakly
adsorbed, this mechanism is only important at low pH (pH<6).
4.3 Major metallic contaminants related to mining
Several of the metals are essential to the human body. The metals are mainly utilised in
enzymes to make them function properly. But we only need the metals in small quantities
(WHO 1996). Some of them we need as trace elements and some are non-essential for us.
Calcium, sodium and magnesium are essential metals and cobalt, molybdenum, selenium,
chromium, nickel, vanadium and silicon are added as trace metals. Mercury and cadmium
are examples of non-essential metals (Walker & Sibly 2001).
The term heavy metals is used for metals with a density more than 5 g/cm3 (Walker & Sibly
2001). Heavy metals important in environmental and health issues are for example Arsenic,
Lead, Cadmium, Copper, Chromium, Mercury, Zink, Cobalt, Nickel, Tin and Vanadium
(SEPA 2003). Those are not normally a part of the human body and are more poisonous to
us than other metals (WHO 1996).
Many metals can be stored in living tissue and remain there for a long time (SEPA 2003). If
a metal acts as a pollutant or becomes harmful to our health depends on both the properties
of the metal and the environment it is acting in. Both humans and plants exhibit a big
variation concerning both the need of essential metals and the sensitivity to non-essentials
metals and to high levels of essential metals and trace metals. Some metals are harmful
mostly to plants, for example zinc, nickel and chromium, and some mostly to animals, for
example cadmium and molybdenum (Pettersson 1994). Previous work in the Tarkwa area
has mainly been conducted by Kuma and Kortatsi. Some of their results are presented in the
section below.
4.3.1 Aluminium
Aluminium salts are widely used in water treatment as flocculants. An associated link
between the Alzheimer disease and aluminium in drinking water has lately been suspected.
The associated link is not confirmed and more studies need to be conducted. The
epidemiological and physiological evidence do not at present support a health-based
guideline value for aluminium. However aluminium concentration levels of 0.2 mg/l have
been suggested as a good compromise between practical use and caution (WHO 1996).
Previous studies show maximum levels in groundwater to be 2.51 mg/l (Kortatsi 2004) and
maximum levels in surface water to be 0.22 mg/l (Kuma 2004).
4.3.2 Arsenic
The results of available studies indicate that arsenic may be an essential element for several
animal species, but there is no evidence that it is essential for humans. The level of arsenic
in natural waters generally varies between 1 and 2 µg/l. Concentrations may be elevated,
however, in areas containing natural sources; values as high as 12 mg/l have been reported.
Inorganic arsenic compounds are classified as carcinogenic to humans. Lethal doses in
humans range from 1.5 mg/kg to 500 mg/kg of body weight depending on the compound.
22
Contamination of water resources in Tarkwa mining area of Ghana
Early clinical symptoms of acute intoxication include abdominal pain, vomiting, diarrhoea,
muscular pain, and weakness, with flushing of the skin. These symptoms are often followed
by numbness and tingling of the extremities, muscular cramping, and the appearance of a
papular erythematous rash. Within a month, symptoms may include burning paraesthesias
of the extremities, palmoplantar hyperkeratosis, Mee’s lines on fingernails, and progressive
deterioration in motor and sensory responses. Signs of chronic arsenicalism, including
dermal lesions, peripheral neuropathy, skin cancer, and peripheral vascular disease, have
been observed in populations ingesting arsenic-contaminated drinking-water.
In view of reducing the concentration of arsenic in drinking-water, a provisional guideline
value of 0.01 mg/l is recommended. The guideline value has been derived on the basis of
estimated lifetime cancer risk (WHO 1996). Previous studies show maximum levels in
groundwater to be 0.046 mg/l (Kortatsi 2004) and maximum levels in surface water to be
0.137 mg/l (Kuma 2004).
4.3.3 Cadmium
Cadmium is chemically similar to zinc and occurs naturally with zinc and lead in sulphide
ores. Cadmium concentrations in unpolluted natural waters are usually below 1 µg/l.
Median concentrations of dissolved cadmium measured at 110 stations around the world
were less then 1 µg/l. The maximum value recorded being 100 µg/l in the Rio Rimao in
Peru. Food is the main source of cadmium intake. Crops grown in polluted soil or irrigated
with polluted water may contain increased concentrations, as may meat from animals
grazing on contaminated pastures. The estimated lethal oral dose for humans is 350-3500
mg of cadmium; a dose of 3 mg of cadmium has no effects on adults. A guideline value for
cadmium is calculated to 0.003 mg/l drinking-water (WHO 1996). Previous studies show
maximum levels in groundwater to be 0.003 mg/l (Kortatsi 2004) and maximum levels in
surface water to be <0.05 mg/l (Kuma 2004).
4.3.4 Chromium
Chromium is widely distributed in the earth’s crust. In water, chromium(III) is a positive
ion that forms hydroxides and complexes, and is adsorbed at relatively high pH values. The
ratio of chromium(III) to chromium(VI) varies widely in surface water. In general,
chromium(VI) salts are more soluble than those of chromium(III), making chromium(VI)
relatively mobile. The daily chromium requirement for adults is estimated to be 0.5-2 µg of
absorbable chromium(III). That equals to approximately 2-8 µg of chromium (III) per day
since only about 25% can be absorbed. The average concentration of chromium in
rainwater is approximately 0.2-1µg/l. Natural chromium concentrations in seawater have
been measured to 0.04-0.7 µg/l. The chromium concentration in groundwater is generally
low (<1 µg/l). The natural total chromium content of surface water is approximately 0.5-2
µg/l and the dissolved chromium content 0.02-0.3 µg/l. Most surface water contain between
1 and 10 µg of chromium per litre. In general, the chromium content of surface water
reflects the extent of industrial activity.
The health effects are mostly determined by the oxidation state. Therefore two different
guidelines for chromium(III) and chromium(VI) should be derived. However, current
analytical methods and the variable speciation of chromium in water favour a guideline
value for total chromium. As a practical measure, the guideline is set to 0.05 mg/l, which is
23
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considered to be unlikely to give rise to significant risks to health (WHO 1996). Previous
studies show maximum levels in groundwater to be 0.066 mg/l (Kortatsi 2004) and
maximum levels in surface water to be 0.49 mg/l (Kuma 2004).
4.3.5 Iron
Iron is an essential element in human nutrition. Estimates of the minimum daily
requirement for iron depend on age, sex, physiological status and iron bioavailability and
range from about 10 to 50 mg/day. In drinking-water supplies, iron(II) salts are unstable
and are precipitated as insoluble iron(III)hydroxide, which settles out as a rust-coloured silt.
Anaerobic groundwater may contain iron(II) at concentrations of up to several milligrams
per litre without discolouration or turbidity in the water when directly pumped from a well.
Turbidity and discolouration may develop in piped systems at iron levels above 0.05-0.1
mg/l, whereas levels of 0.3-3 mg/l are usually found acceptable. As a precaution against
storage of excessive iron in the body a provisional maximum tolerable daily intake was
calculated to about 2 mg/l drinking water. That level does not present a hazard to health.
The taste and appearance of drinking water will usually be affected below this level,
although iron concentrations of 1-3 mg/l can be acceptable for people drinking anaerobic
well-water.
No health-based guideline value for iron is proposed (WHO 1996). Previous studies show
maximum levels in groundwater to be 18.3 mg/l and maximum levels in surface water to be
4.01 mg/l (Kuma 2004).
4.3.6 Lead
Lead is the most common of the heavy elements, accounting for 13 mg/kg of the earth’s
crust.More than 80% of the daily intake of lead is derived from the ingestion of food, dirt,
and dust. That means that an average of 5 µg/l lead intake from water forms a relatively
small proportion of the total daily intake for children and adults, but a significant one for
bottle-fed infants. Lead is possible human carcinogen (evidence inadequate in humans,
sufficient in animals) and it is also a cumulative poison so that any increase in the body
burden of lead should be avoided. A provisional tolerable daily intake is set to 3.5 µg of
lead per kg of body weight for infants lead to a calculated guideline value of 0.01 mg/l. As
infants are considered to be the most sensitive subgroup of the population, this guideline
value will also be protective for other age groups (WHO 1996). Previous studies show
maximum levels in groundwater to be 0.026 mg/l (Kortatsi 2004) and maximum levels in
surface water to be <0.05 mg/l (Kuma 2004).
4.3.7 Manganese
Manganese concentrations above 0.1 mg/l impart an undesirable taste to drinking water.
Even at about 0.02 mg/l, manganese will form coatings on piping that may later tear off as
a black precipitate. When manganese(II) compounds in solution undergo oxidation,
manganese is precipitated. Humans can consume as much as 20 mg/day without apparent ill
effects. Manganese is believed to have a neurotoxic effect; a provisional health-based
guideline value of 0.5 mg/l is proposed to protect public health (WHO 1996). Previous
studies show maximum levels in groundwater to be 1.3 mg/l (Kortatsi 2004) and maximum
levels in surface water to be 2.43 mg/l (Kuma 2004).
24
Contamination of water resources in Tarkwa mining area of Ghana
4.3.8 Mercury
Almost all mercury in uncontaminated drinking water is thought to be in the form of Hg2+.
It is only the carbon-mercury bond in organic mercury compounds that are chemically
stable. The solubility of mercury compounds in water varies. Mercury(II) chloride is readily
soluble, mercury(I) chloride much less soluble, mercury sulphide has a very low solubility
and elemental mercury vapour is insoluble. Some anaerobic bacterias are capable of
mercury methylation. Methyl mercury can then easily enter the food chain as a
consequence of rapid diffusion and tight binding to proteins. Environmental levels of
methyl mercury depend on the balance between bacterial methylation and demethylation.
Naturally occurring levels of mercury in groundwater and surface water are less than 0.5
µg/l. The WHO guideline value for total mercury is 0.001 mg/l. Previous studies show
maximum levels in groundwater to be 0.037 mg/l (Kortatsi 2004) and maximum levels in
surface water to be 0.093 mg/l (Kuma 2004).
4.3.9 Nickel
In aqueous solution, nickel occurs mostly as the green hexa-aquanickel(II) ion, Ni(H2O)62+.
The nickel ion content of groundwater may increase as a result of the oxidation of natural
nickel containing ferrosulphide deposits. Oxidation can occur if the groundwater table is
lowered or if nitrate has leached from the soil. Nickel concentrations in drinking water
around the world are normally below 20µg/l, although levels up to several hundred
micrograms per litre in groundwater and drinking water have been reported. Leaching from
nickel-chromium plated taps and fittings is also a factor. The nickel intake from food
exceeds that from drinking water, even if a health-based guideline value for drinking water
is calculated to 0.02 mg/l. That should provide sufficient protection even for nickelsensitive individuals (WHO 1996). Previous studies show maximum levels in groundwater
to be 0.076 mg/l (Kortatsi 2004).
4.3.10 Nitrate and nitrite
Nitrate and nitrite are naturally occurring ions that are part of the nitrogen cycle. The nitrate
ion (NO3-) is the stable form and it can be reduced by microbial action to a nitrite ion
(NO2-) which is a relatively unstable oxidation state for the ion. It is the nitrite ion that
constitutes the toxicity to humans. It is involved in the oxidation of normal haemoglobin to
methaemoglobin, which is unable to transport oxygen to the tissues. Therefore the health
guideline for nitrate-nitrogen is set to 10 mg/l. This value should not be expressed in terms
of nitrate-nitrogen but as nitrate itself which is the chemical entity of health concern, and
the guideline value for nitrate alone is therefore 50 mg/l (WHO 1996). Previous studies
show maximum levels in groundwater to be (NO3-) 27.0 mg/l (Kortatsi 2004) and
maximum levels in surface water to be (NO3-) 60 mg/l (Kuma 2004).
4.3.11 Sulphate
The presence of sulphate in drinking water results in a noticeable change of taste. The
lowest taste threshold concentration for sulphate is approximately 250 mg/l. The
physiological effects resulting from the intake of large quantities of sulphate are catharsis,
dehydration, and gastrointestinal irritation. Water containing magnesium sulphate at levels
above 600 mg/l acts as a purgative in humans. Sulfate may also contribute to the corrosion
of distribution systems. Drinking water should not have sulphate levels exceeding 500mg/l
25
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(WHO 1996). Previous studies show maximum levels in groundwater to be 21.0 mg/l and
maximum levels in surface water to be 490 mg/l (Kuma 2004).
WHO:s guidelines for important elements are summarized in Table 4-3
Table 4-3: Summary of WHO:s guidelines for drinking water (µg/l)
Element
WHO Guideline
Al
200
As(tot)
10
Ba
700
B
300
Cd
3
Cl
250 000
Cr
50
Cu
2000
Element
WHO Guideline
Fe
3000
Mn
500
Ni
20
NO350 000
Na
200 000
Pb
10
SO42500 000
Zn
3000
26
Contamination of water resources in Tarkwa mining area of Ghana
5 Materials and methods
5.1 Sampling plan
Information for supporting the sampling plan was collected during spring and summer of
2004 in Sweden and at the University of Ghana during the period 24/8-8/9 2004. In Ghana,
information was collected at the Department of Geography and Resource Development, the
Department of Geology and the Water Research Resource Institute, CSIR. A review was
made of previous work not to overlap our work with previous studies in the area made by
Kuma 2004 and Kortatsi 2004. Our supervisors had areas of special interest which were
also taken into account in preparing the sampling plan. In Tarkwa, the Wassa West District
Assembly and Small Scale Mining Centre were visited to finalize the sampling plan. More
information concerning water supply in different communities and borehole data were
collected. During the field study the sampling plan was revised according to local
conditions.
5.2 Field methods
The fieldwork was conducted during September 2004. Both ground and surface water
samples were collected. Wassa West Assembly supported us with a car, driver and a guide.
This enabled us to visit remote locations and expand our sampling. Our sampling positions
(marked red) and supplementary information is presented in Figure 5-1.
5.2.1 Groundwater sampling
The most important aspects according to our sampling protocol include the following:
•Clear pump before sampling to avoid any stagnant water in the pump system.
•Pumping for 7 min is usually adequate.
•Filter the sample through a 0.45 µm filter.
•Rinse the sampling bottle with groundwater before taking a sample.
•Avoid mixing water with air at sampling
•Avoid sampling during heavy raining
Each sample was collected in 100 and 50 ml polyeten bottles where the 50 ml bottle was
acidified with concentrated HNO3.
Measured field parameters are Eh, conductivity, pH and temperature using Ecoscan pH 6,
Ecoscan Con 5 and Hach Sension 2. A flowcell was used for measuring the Eh value.
Measurements were made until stable values were achieved. The sample sites were
positioned with a Cobra GPS100 to get their exact location.
A calibration of the pH-meter was conducted each morning and also during field studies if
unusual measures were made. A calibration of the Eh-meter and the conductivity-meter
were made in the mornings of the 10th, 13th and 15th of september. Field measurements
and sample collections were done during the period 10/9-15/9 2004. This period is at the
end of the rainy season and it usually rained at least once a day during the period.
27
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Information about the wells was also collected when possible in the field. Unfortunately
there was no information about the depth or screening of the wells.
Figure 5-1: Map with locations of sampling points. The numbers mark the sampling
numbers (Modified from Kortatsi 2004)
5.2.2 Surface water sampling
Treated water from the River Bonsa is distributed to Tarkwa and its surroundings. Both
treated and untreated water was collected and field measurements were taken to examine
the quality of the treatment and quality of distributed water. The flowcell was not used in
for the untreated surface water sample. Otherwise the same parameters were analysed as for
groundwater sampling.
5.3 Water analysis
Determination of carbonate alkalinity was measured by following standard SS-EN ISO
9963-2. Automatic titration equipment ABU 80 Autoburette, PHM 82 Standard pH meter
and TTT80 Titrator from Radiometer Copenhagen were used. Unacified samples were
used.
28
Contamination of water resources in Tarkwa mining area of Ghana
For organic carbon, NPOC, a TOC-5000 Shimadzu Total Organic Carbon Analyser was
used.
Cl-, NO3- and SO42- were analysed at KTH on Dionex DX-120 Ion Chromatograph.
Unacified samples were used. PO43--P (acified samples) and NH4+-N (unacified samples)
was determined using Aquatec 5400 Analyser and 5027 Sampler, Tecator following
application note ASN 140-01/90 and ASN 146-01/90. These analyses were performed at
the laboratory of the Department of Land and Water Resource Engineering at KTH.
Trace elements were analysed at the Department of Geology and Geochemistry at
Stockholm University. Acified samples were used. Element concentrations were measured
on an ICP-OES (Optical Emission Spectroscopy) made by Varian (Varian Vista Ax Pro,
equipped with a CCD camera) and an axial mounted torch. All runs were made on a system
using a small concentric spray chamber and a seaspray nebulizer (RF power 1.3kW, 5s
integration time, 0.9 L/minute nebulizer flow). The typical precision in analyses based on
measurements of certified standards was typically better than 4%. The precision was only
available if using different emission lines in different concentration ranges. This means that
several emission lines must be measured for each element. For example for Ca; the Ca
396.847 nm emission line was used up to 3 ppm and 317.933 nm from 3 ppm and up.
5.4 Treatment of analytical data
Analytical treatment of data was made in Excel, AquaChem, PHREEQC, Minteq, Surfer
and Arcview. When analyzing in AquaChem, half of the detection limit was used for values
below the detection limit. This is madein order not to exclude important minerals when
modelling. For analyzing in Surfer and Arcview values below detection level were set to
zero.
The depths of drilled wells are set to 35 m which is the average depth in the area. These
well are supposed to represent confined or semi-confined aquifers. Hand-dug well are
approximated to a depth of 5 m are supposed to represent unconfined aquifers.
Electro neutrality of anions and cations were calculated to determine the accuracy of
chemical analysis. If samples depart more than 5% from electroneutrality a comparison of
sums of anions, cations and conductivity were also made to determine where the error is
likely to be situated (Appello and Postma 1999).
A correlation matrix was made for all analysed parameters for three groups, all wells, deep
wells and shallow wells. To keep a 95% confidence level (p=0.05) samples with five or less
analyses were excluded. For the group shallow wells with six samples only, correlations
above r2= 0.66 (r= 0.81) are significant (Håkansson & Peters 1995). Marked r-values in the
correlation matrices are all statistically significant. Parameters that Aquachem can not
handle are excluded from the correlation matrices.
29
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30
Contamination of water resources in Tarkwa mining area of Ghana
6 Results and discussion
Detailed data on groundwater chemistry is presented in Appendix 1-4. Statistical summary
for groundwater chemistry is presented in Table 6-1. To be able to draw further conclusions
on groundwater chemistry and dominating processes in the area the wells were divided,
according to if they are drilled or hand dug, into two groups, deep and shallow wells.
6.1 Field measured parameters
Average groundwater temperature was measured to 26.6 C° ranging from 25.4-28.5 C°
The pH varies between 4.19 to 6.92 with an average of 5.38. The redox potential was
measured within a range of 192-523 mV with an average of 357 mV. No samples have
negative redox potentials. Electric conductivity ranges between 11.0 and 780 µS/cm with
an average of 301 µS/cm. Table 6-1 shows a statistical summary of groundwater chemistry
for all the wells. Elements with only a few samples above the detection limit are not used in
further discussion and are not included. The positions of the wells are presented graphically
in Figure 5.1 and in Appendix 1.
Table 6-1: Statistical summary of groundwater chemistry (values exceeding WHO:s
guidelines for drinking water are marked bold).
Parameter
pH
Eh
Cond
Temp
DOC
HCO3NO3SO42PO43ClNH4+
Na+
K+
Ca2+
Mg2+
Al
As(tot)
B
Ba
Cd
Cr
Cu
Fe
Li
Mn
Ni
Pb
Si
Sr
Zn
Unit
mV
µS/cm
C°
mg/l
mg/l
mg/l
mg/l
mg/l
mg/l
mg/l
mg/l
mg/l
mg/l
mg/l
µg/l
µg/l
µg/l
µg/l
µg/l
µg/l
µg/l
µg/l
µg/l
µg/l
µg/l
µg/l
µg/l
µg/l
µg/l
Min
4.19
192
11.0
25.4
0.636
bld
bdl
0.220
bdl
3.160
bdl
2.19
0.266
1.27
0.583
3.49
bdl
bdl
7.24
bdl
bdl
bdl
1.16
bdl
5.50
bdl
bdl
2940
32.5
5.95
Max
6.92
523
780
28.5
3.89
378
146
68.5
0.352
117
0.262
69.7
22.7
111.
22.4
2180
69.4
53.1
469
0.704
2.00
17.0
10800
28.2
2040
19.0
4.11
26000
3260
650
31
Average
6.00
357
301
26.6
1.09
139
9.70
10.3
0.061
20.1
0.0107
17.1
2.28
34.1
7.48
80.8
2.26
10.4
89.0
bdl
bdl
2.68
1160
6.72
432
3.39
bdl
14200
398
56.9
Median
6.07
344
276
26.5
0.940
107
0.230
2.73
0.034
8.55
bld
12.1
0.912
27.3
5.77
8.35
bdl
9.08
39.6
bdl
bdl
bdl
102
5.73
303
bdl
bdl
15800
198
19.9
St. Dev.
0.631
103
197
0.654
0.590
103
26.0
16.2
0.072
25.7
0.0422
12.6
3.98
25.9
5.91
352
11.2
10.5
101
0.163
0.415
5.25
2350
5.99
403
5.10
0.650
6350
625
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6.2 Major ions
Electro neutrality was examined and all but two samples, no. 23 and 37, showed smaller
deviations than 5%, see Appendix 4. The results from the groundwater sampling are
considered to be reliable. Comparison with conductivity showed equal deviation for sample
23 and for sample 37 it was concluded that the sum of cations were to low. Table 6-1 shows
a statistical summary of groundwater chemistry.
Results of groundwater sampling are presented in Piper diagram in Figure 6-1.
Legend
Legend
80
A
C
80
60
80
40
A
A
A
20
A
AC CA
A A C A
C AC
A AA
A A A
AAA
A AA
A AAA A
AA
AA
20
SO4
80
60
60
40
40
20
80
60
40
20
40
60
80
Na+K
A
AA A
C
A
A
A
A
A
A C
A
A C
C
A
A
A
A
A
A
C
A
A
A
A
A A
A
20
AA
A AA
AA
A
AAAA A A
AA
A A
20
A AA
A
CA
C
CA AC A
ACA
C
Ca
Shallow wells
60
40
Mg
Deep wells
HCO3
Cl
Figure 6-1: Piper diagram showing composition of groundwater.
Most of the samples, 95%, are of a bicarbonate type according to anions. Two samples have
higher levels of Cl- and for these Cl- is the dominating anion. According to cations the
classification is not as clear. 75% of the samples have Ca as dominating cation and the rest
has Na followed by Mg as dominating cation.
67% of the hand dug wells have a Ca-HCO3 water type. The rest have Ca-Na-HCO3-NO3Cl or Na-Ca-HCO3-Cl-NO3. 21% of drilled wells have a Ca-HCO3 kind of water. This
coincides with theory, since groundwater should develop following the water type sequence
with increasing age (Bhattacharya, P. pers. comm. 2004-12-02):
Ca-HCO3 → Ca-Cl-HCO3 → Na-Ca-Cl-HCO3 → Na-HCO3→Na-Cl
32
Contamination of water resources in Tarkwa mining area of Ghana
This indicates that Calcium is replacing Sodium and to some extent Magnesium through
ion exchange in the soil matrix as the groundwater increases in age (see chapter 4.2.1). For
the drilled wells about 25% of the samples has Na or Mg as dominating cation,. For the
hand dug wells there is one sample, about 17%, with Na as dominating cation, The
differences between the groups should be interpreted with caution since we have very few
wells belonging to the group shallow wells. For a complete list of water types see Appendix
3.
NO3- is the only major ion that exceeds WHO:s guidelines. This occurs at two locations, see
Table 6-2.
6.3 Trace elements
A total of 17 wells have higher metal content than WHO:s guidelines when these are
interpreted strictly regarding iron and aluminium. As(tot), Mn, Fe and Al show values
exceeding WHO: s guidelines. As(tot) exceeds the guidelines at two locations, Samahu,
15.6 µg/l and Eyinaise, 69.4 µg/l. Mn is the major contaminant and of all 17 wells with
high contents of metals, 14 has elevated Mn-levels. Fe exceeds the guideline in seven
wells. Al is exceeding the guideline at two locations, Huniso and Akoon. These wells also
display the two lowest pH-values measured. NO3- is exceeding the guidelines in the same
two wells in Huniso and Akoon. The wells with elevated metal content are presented in
Table 6-2. All the wells with metal concentrations exceeding WHO:s guidelines are
boreholes except New Atuabo which is a hand dug well.
Table 6-2: Samples exceeding WHO:s guidelines (mg/l).
Location
WHO guideline
Simpa
Dadwen
Dompim
Odumase
Nsuaem
Aboso
Samahu
Akotomu
Eyinaise
Eyinaise
Mile 8
Huniso
Huniso
New Atuabo
Bompieso
Bompieso
Akoon
No
3
4
5
8
9
10
13
17
24
25
29
32
33
35
40
41
42
As(tot)
0.01
0.0156
0.0694
Mn
0.5
0.916
0.567
0.851
0.850
0.565
0.640
0.926
0.605
0.968
0.676
1.07
Fe
3
3.41
7.29
5.20
3.37
Al
0.2
NO3
50-
2.18
146
0.594
66.4
4.33
4.69
10.8
0.713
0.602
2.04
If iron and aluminium not is interpreted strictly 14 well exceed drinking water limits mainly
because of high levels of manganese.
Maps of metal concentration in groundwater are shown in Figure 6-2 and 6-3. Figure 6-4
and 6-5 shows other parameters associated with metal distribution. Kriging is used to
33
Asklund and Eldvall
LUTVDG/TVTG--5092--SE
interpolate concentrations. Sampling positions are marked with a cross. Coordinates are
given as degrees North and West.
Damang
New Kyekyewere
5.5
Damang
New Kyekyewere
5.5
Yaryeyaw
Yaryeyaw
Huni Valley
Suwinso
Akotomu
Gordon
5.45
Kofi Gyan Camp
Kofi Gyankrom
Bompieso
5.4
Huniano no 1
Huniso
Samahu
Aboso
Kokoase
Nsuaem
Odumase
Atwerboanda
New Atuabo
Akoon Domeabra
5.35
5.3
Enyinasie
Tarkwa Banso
Teberebe
5.25
Akyem
Adieyie
5.2
5.15
2100
2000
1900
1800
1700
1600
1500
1400
1300
1200
1100
1000
900
800
700
600
500
400
300
200
100
0
-100
-200
60
55
Huniano no 1
Huniso
50
45
Samahu
Aboso
Kokoase
Nsuaem
Odumase
Atwerboanda
New Atuabo
Akoon Domeabra
5.35
5.3
40
35
30
25
Enyinasie
Tarkwa Banso
20
Teberebe
15
5.25
Akyem
Adieyie
10
5
5.2
0
-5
Dompim
Dadwen
Dadwen
-2.1
65
Kofi Gyan Camp
Kofi Gyankrom
Bompieso
5.4
5.15
Dompim
Huni Valley
Suwinso
Akotomu
Gordon
5.45
-2.05
-2
-1.95
-1.9
-2.1
-1.85
-2.05
-2
-1.95
-1.9
-1.85
Figure 6-2: Concentration of Al (µg/l) to the left and As(tot) (µg/l) to the right. Bold line
indicates WHO:s guideline.
Damang
New Kyekyewere
5.5
Damang
New Kyekyewere
5.5
Yaryeyaw
Yaryeyaw
Suwinso
Akotomu
Gordon
5.45
Huni Valley
Kofi Gyan Camp
Kofi Gyankrom
Bompieso
5.4
Huniano no 1
Huniso
Samahu
Aboso
Kokoase
Nsuaem
Odumase
Atwerboanda
New Atuabo
Akoon Domeabra
5.35
5.3
Enyinasie
Tarkwa Banso
Teberebe
5.25
Akyem
Adieyie
5.2
5.15
10000
9500
9000
8500
8000
7500
7000
6500
6000
5500
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
0
-500
Huniano no 1
Huniso
Samahu
Aboso
Kokoase
Nsuaem
Odumase
Atwerboanda
New Atuabo
Akoon Domeabra
5.35
5.3
Enyinasie
Tarkwa Banso
Teberebe
5.25
Akyem
Adieyie
5.2
Dompim
Dadwen
Dadwen
-2.1
Huni Valley
Kofi Gyan Camp
Kofi Gyankrom
Bompieso
5.4
5.15
Dompim
Suwinso
Akotomu
Gordon
5.45
-2.05
-2
-1.95
-1.9
-2.1
-1.85
-2.05
-2
-1.95
-1.9
-1.85
Figure 6-3: Concentration of Fe (µg/l) to the left and Mn (µg/l) to the right. Bold line
indicates WHO:s guideline.
34
2100
2000
1900
1800
1700
1600
1500
1400
1300
1200
1100
1000
900
800
700
600
500
400
300
200
100
0
Contamination of water resources in Tarkwa mining area of Ghana
Damang
New Kyekyewere
5.5
Damang
New Kyekyewere
5.5
Yaryeyaw
Suwinso
Akotomu
Gordon
5.45
Yaryeyaw
Huni Valley
Kofi Gyan Camp
Kofi Gyankrom
Bompieso
5.4
Huniano no 1
Huniso
Samahu
Aboso
Kokoase
Nsuaem
Odumase
Atwerboanda
New Atuabo
Akoon Domeabra
5.35
5.3
Enyinasie
Tarkwa Banso
Teberebe
5.25
Akyem
Adieyie
5.2
5.15
5.45
6.9
6.8
6.7
6.6
6.5
6.4
6.3
6.2
6.1
6
5.9
5.8
5.7
5.6
5.5
5.4
5.3
5.2
5.1
5
4.9
4.8
4.7
4.6
4.5
4.4
4.3
4.2
Huni Valley
560
540
Kofi Gyan Camp
Kofi Gyankrom
Bompieso
5.4
520
500
Huniano no 1
Huniso
480
460
Samahu
Aboso
Kokoase
Nsuaem
Odumase
Atwerboanda
New Atuabo
Akoon Domeabra
5.35
5.3
Enyinasie
Tarkwa Banso
5.25
420
400
380
360
320
300
Akyem
Adieyie
440
340
Teberebe
280
260
240
5.2
220
200
5.15
Dompim
Dompim
Dadwen
Dadwen
-2.1
Suwinso
Akotomu
Gordon
-2.05
-2
-1.95
-1.9
-2.1
-1.85
-2.05
-2
-1.95
-1.9
-1.85
Figure 6-4: pH, to the left, and Eh, to the right, in our study area
Damang
New Kyekyewere
5.5
Damang
New Kyekyewere
5.5
Yaryeyaw
Yaryeyaw
Suwinso
Akotomu
Gordon
5.45
Huni Valley
65
Kofi Gyan Camp
Kofi Gyankrom
Bompieso
5.4
Aboso
Kokoase
Nsuaem
Odumase
Atwerboanda
New Atuabo
Akoon Domeabra
5.3
Enyinasie
Tarkwa Banso
Huniso
45
Akyem
Adieyie
3.4
3.2
3
Aboso
Kokoase
Nsuaem
Odumase
Atwerboanda
New Atuabo
Akoon Domeabra
35
30
5.3
25
Enyinasie
Tarkwa Banso
2
1
5.2
0
0.8
0.6
5.15
Dompim
Dompim
Dadwen
Dadwen
-2.1
2.2
1.2
-5
5.15
2.4
1.4
Akyem
Adieyie
5
5.2
2.6
1.6
5.25
10
2.8
1.8
Teberebe
15
5.25
3.6
Samahu
5.35
40
20
Teberebe
3.8
Huniano no 1
50
Samahu
5.35
5.4
55
Huni Valley
Kofi Gyan Camp
Kofi Gyankrom
Bompieso
60
Huniano no 1
Huniso
Suwinso
Akotomu
Gordon
5.45
-2.05
-2
-1.95
-1.9
-2.1
-1.85
-2.05
-2
-1.95
-1.9
-1.85
Figure 6-5: Levels of SO42- (mg/l), to the left, and DOC (mg/l), to the right, in our study
area
Mn and Fe show similarity in distribution pattern, almost all areas with Fe-values above
WHO:s guidelines, also have high Mn-values. However, there is no visible trend between
35
Asklund and Eldvall
LUTVDG/TVTG--5092--SE
these parameters when plotted (R= 0.09). Fe and Mn distribution also show a similarity
with SO42- (R=0.18 resp. 0.63). These correlations are shown in Figure 6-6.
2.5
12
y = 0.026x + 0.8951
R2 = 0.0324
10
Mn (mg/l)
8
Fe (mg/l)
y = 0.0157x + 0.2706
R2 = 0.401
2
6
1.5
1
4
0.5
2
0
0
0
20
40
60
0
80
20
40
60
80
SO4 (m g/l)
SO4 (m g/l)
Figure 6-6: Diagrams showing correlations for Mn and SO42- and for Fe and SO42For the correlations in Figure 6-6, there are no obvious outliers that affect the correlations.
For Mn and SO42- a positive trend can be seen but for Fe and SO42- it is difficult to see any
trend. The trend between Mn and SO42- could origin from dissolution of minerals or from
AMD. For certain areas as Akoon, New Atuabo, Aboso, Enyinasie and Dadwen high levels
of both Fe and SO42- and low pH indicate acid mine drainage. For Akoon, the location that
display the highest levels of both Fe and Mn, there were small-scale mining activities just
about 100m from the well. These activities are not seen in Figure 5-1 and small-scale
mining activities are difficult to locate since the majority are illegal. Samples with high Fe
content and low SO42- can also indicate AMD since SO42- can be transformed to H2S
through redox reactions. AMD can therefore not be excluded on basis on low SO42--values.
This can also explain the low correlation between Fe and SO42-. But it is not possible to see
any trend between SO42- and Eh and the correlation is very low (R2=0.007).However it
must be kept in mind that field measured Eh is an unreliable parameter. Many samples
display both low Fe and SO42- values. This can be explained by reducing conditions. H2S
can precipitate by forming iron sulphides. This is not the case for Mn and SO42- as Mnsulphides are much more soluble.
Fe and Mn can also act as redox couples. Mn is reduced, Mn4+→Mn2+, and Fe is oxidized,
Fe2+→Fe3+. Fe3+ can precipitate as for example ferryhydrit, which is supersaturated in some
samples according to the hydrochemical modelling, see chapter 6.5. This can also act as a
sink for Fe(tot). Another reaction that can govern the concentration of Fe2+, especially in
sedimentary aquifers with reducing conditions is precipitation of siderite (FeCO3)
(Bhattacharya, P. pers.comm. 2004-12-10). Precipitation of different minerals can disturb
the correlations. Measurements of Eh show no reducing conditions in our sampling area,
and modelling shows that siderite is undersaturated. However the field measured Eh is an
unreliable measurement and the modelling is only conducted at four wells. It is not unlikely
that there are reducing conditions and that this process could be of importance.
36
Contamination of water resources in Tarkwa mining area of Ghana
Al show similarity in distribution patterns with pH and Eh (R=-0.53 resp. 0.32). This is due
to the strong depency of pH for Al mobility. At higher pH-values Al is precipitated as
hydroxides.
2 .5
12
y = - 0 .0 0 6 4 x + 1 .3 8 1 5
R 2 = 0 .0 0 5
10
y = 0 .0 0 8 7 x + 0 .13 4 7
R 2 = 0 .3 1 3 7
2
Mn (mg/l)
Fe (mg/l)
8
6
4
1 .5
1
0 .5
2
0
0
50
0
100
0
50
C a ( m g /l)
100
C a ( m g /l)
Figure 6-7: Diagrams showing the correlations for Mn and Ca and Fe and Ca
As shown in Figure 6-7, Mn show strong positive correlations with Ca, and the other major
cations. Sample 42 (Akoon) show a strongly deviant value. If this value is exluded the R2value is increased to 0.64. Fe show weak negative correlations with Ca. This could indicate
that Fe is replaced by Ca at exchange sites, but not Mn. The correlation between Mn and Ca
could origin for dissolution minerals containing both elements.
120
100
120
y = 41.31x - 216.97
R2 = 0.6262
80
Ca (mg/l)
Ca (mg/l)
80
60
40
20
0
4.50
y = 0.2436x - 0.9703
R2 = 0.8527
100
60
40
20
0
5.00
5.50
6.00
6.50
7.00
pH
0.0
100.0
200.0
300.0
400.0
HCO3 (m g/l)
Figure 6-8: Diagrams showing correlations between Ca and pH and Ca and HCO3-.
Ca shows a strong correlation with pH and HCO3- shown in Figure 6-8. Sample 32 and 42
are not included due to probable acid mine drainage. This indicates that CaCO3 is the origin
of the Ca content. The quantity Ca is considerably lower than the quantity HCO3-. Na, Mg,
Mn and Ba also show high correlation with HCO3- (R= 0.62, 0.66, 0.70 and 0.66).
Therefore it is plausible that there are some minerals containing Na, Mg, Mn, Ba and
HCO3- which are contributing with ions to the groundwater.
37
Asklund and Eldvall
LUTVDG/TVTG--5092--SE
The As(tot) kriging should be interpreted with extreme caution since it is based only on
three samples.
If the Figures 6-2 to 6-5 are compared with the geological map of the Tarkwa-Presea area,
Figure 3-3, it can be seen that Fe, Mn, As and SO42- does not show any major differences in
distribution between the Tarkwaian and Birimain system. For Al the values exceeding
WHO:s guidelines is found only in the Tarkwaian system. This is most likely a result of the
low pH in these two wells and not an effect of the difference in geology between the two
systems.
6.4 Differences between deep and shallow wells
To be able to draw further conclusions on groundwater chemistry and dominating processes
in the area the wells were divided, according to depths, into two groups. Since there is no
data on how deep the wells are, the groups are called deep wells and shallow wells
depending on if they are drilled or hand dug. The shallow wells are number 12, 20, 23, 30,
34 and 35. These groups have been investigated separately to investigate differences in
parameters between deep and shallow wells. The results are presented in Figure 6-9. Only
parameters showing differences between the groups are shown.
There are differences as can be seen from Figure 6-9 in Mn, HCO, pH and Cond. The
differences in median value between the groups can have different reasons.
• pH: There is a distinct difference in pH between the shallow and deep wells. Large scale
mines in the area uses roasting of ore as processing method. This can give raise to acidified
rain. But the low pH can not solely be explained by acid rain. The measured pH of the rain
water in the area is higher than the pH of shallow wells. It could be that AMD is causing
the low pH. However, the lowest recorded pH values are found in the deep wells.
• HCO3-: These waters also have very low alkalinity which supports the AMD theory. In the
deep wells buffering reactions with carbonates or silicates can have a more pronounced
neutralizing effect thanks to longer residence times.
• Conductivity and Mn: These parameters show higher values for deep wells. This is
probably due to higher dissolution of minerals due to longer residence times of these
groundwaters.
• Eh: The shallow wells have slightly higher Eh. This could be due to that oxygen
consuming materials in the aquifers have affected the groundwater more due to longer
residence times in the deep groundwaters.
With regard to minerals in the area associated with AMD, see Table 4-1, the following
correlations between the different groups have been investigated: Fe-SO4, Cu-SO4, Zn-SO4,
Cu-Fe, Fe-Mn, and Fe-Zn. Appendix 4-6 contains three correlation matrices one for all
samples, one for deep well and one for shallow wells. None of the groups, deep, shallow
and all well show any strong, significant correlation between these parameters.
Considerations of outliers have been taken into account.
Shallow wells are only six compared to 34 deep wells. The results should therefore be
interpreted with great caution because many correlations are determined by one dominating
value. There is generally no major difference between the correlation matrix for deep wells
38
Contamination of water resources in Tarkwa mining area of Ghana
and the one for all wells. To be able to draw conclusions about the difference between deep
and shallow wells a more even distribution between the groups should have been accounted
for in the sampling plan.
2.2
Max.
75 percentile
Mn mg/l
1.8
Median
25 percentile
1.3
Min.
0.9
0.4
0.0
Shallow wells
800
320
640
Cond uS/cm
HCO3 mg/l
Deep wells
400
240
160
80
480
320
160
0
0
Deep wells
Shallow wells
600
6.4
500
5.8
400
pH
Eh mV
7.0
5.2
4.6
Deep wells
Shallow wells
Deep wells
Shallow wells
300
200
4.0
100
Deep wells
Shallow wells
Figure 6-9: Box plots of selected parameters for deep and shallow wells.
6.5 Hydro-chemical modelling
Sample 13, 24, 32 and 42 was chosen to be investigated by hydro-chemical modelling. This
was based on levels of metals and the pH, see Table 6-2. All are drilled wells. Comparison
with map materials gives that these wells are located in the following geological
formations:
39
Asklund and Eldvall
LUTVDG/TVTG--5092--SE
• Sample 13: The Tarkwaian system, Huni Sandstone: (a quartzite) consists of variable
amounts of feldspar, sericite, chlorite, ferriferous carbonate, magnetite or hematite and
epidote.
• Sample 24: The upper Birimian system: dominantly of volcanic and pyroclastic origin.
The rocks consist of bedded group of green lava. Lava and tuff dominate this part. Several
band of phyllite occurs in this zone and are manganiferous in places.
• Sample 32: The Tarkwaian system, Huni Sandstone: (a quartzite) consists of variable
amounts of feldspar, sericite, chlorite, ferriferous carbonate, magnetite or hematite and
epidote.
• Sample 42: The Tarkwaian system, Banket Series: 90% quartz and the rest is Birimian
schist, quartzite, hornstone, chert and gondite
Table 6-3: Result of modeling with Phreeqc Interactive.
Sample 13
Phase
Al(OH)3(a)
Albite(low)
Analbite
Analcime
Annite
Aragonite
Barite
Boehmite
Calcite
Chalcedony
CO2(g)
Diaspore
Dolomite
Fe3(OH)8
Ferrihydrite
Gibbsite(C)
Goethite
Gypsum
Hercynite
Hydroxyapatite
Laumontite
Leucite
Lime
Magnesite
Microcline
MnHPO4(C)
Nsutite
Pyrolusite
Quartz
Rhodochrosite
Sanidine(H)
Siderite
SiO2(a)
SiO2(am)
Strengite
Strontianite
Vivianite
ZnSiO3
SI
-1.13
0.64
-0.27
-0.19
2.04
-0.37
-0.72
0.67
-0.23
0.23
-1.35
2.37
-0.96
-0.96
0.79
0.47
5.22
-3.15
-0.71
0.21
1.69
-1.61
-21.9
-1.23
0.89
1.31
-12.9
-11.2
0.71
-0.52
0.45
-1.05
-0.28
-0.58
-0.57
-1.21
-5.63
0.15
Sample 24
Phase
Al(OH)3(a)
Albite(low)
Alunite
Analbite
Analcime
Anhydrite
Annite
Aragonite
Barite
Boehmite
Calcite
Chalcedony
CO2(g)
Dolomite
Diaspore
Fe3(OH)8
Ferrihydrite
Gibbsite(C)
Goethite
Gypsum
Hercynite
Jarosite-K
Jarosite-Na
Laumontite
Leucite
Lime
Magnesite
Manganite
Microcline
MnHPO4(C)
Nsutite
Pyrolusite
Quartz
Rhodochrosite
Sanidine(H)
Siderite
SiO2(a)
SiO2(am)
Strengite
Vivianite
ZnSiO3
SI
-1.05
0.01
-0.64
-0.89
-0.87
-1.81
1.8
-1.62
-0.02
0.76
-1.48
0.28
-0.87
-3.07
2.45
-1.56
0.5
0.54
4.97
-1.63
-0.47
1.63
-0.51
0.37
-2.01
-23.5
-2.07
-7.75
0.53
1.97
-15.0
-13.2
0.75
-1.43
0.09
-0.57
-0.22
-0.53
1.19
-1.57
-0.93
40
Sample 32
Phase
Al(OH)3(a)
AlOHSO4
Barite
Boehmite
Chalcedony
CupricFerrite
Diaspore
Ferrihydrite
Gibbsite(C)
Goethite
Gypsum
Halloysite
Leonhardite
Lepidocrocite
Maghemite
Manganite
Microcline
MnHPO4(C)
Montmorillonite
Nsutite
Pyrolusite
Quartz
Sanidine(H)
SiO2(a)
SiO2(am)
Strengite
Vivianite
SI
-1.5
-1.11
-0.26
0.3
-0.07
0.79
1.99
-2.58
0.09
1.87
-2.77
1.55
1.3
0.94
-1.77
-8.38
-1.36
-0.56
1.79
-12.9
-11.1
0.41
-1.8
-0.57
-0.88
-1.02
-17.3
Sample 42
Phase
Al(OH)3(a)
AlOHSO4
Alunite
Anhydrite
Barite
Boehmite
Chalcedony
Diaspore
Fe(OH)2.7Cl0.3
Ferrihydrite
Gibbsite(C)
Goethite
Gypsum
Halloysite
Kaolinite
Lepidocrocite
Lime
Magnetite
Manganite
MnHPO4(C)
Muscovite
Nsutite
Pyrophyllite
Quartz
SiO2(a)
SiO2(am)
Vivianite
SI
-2.78
-1.1
0.75
-2.19
0.25
-0.97
-0.48
0.72
1.55
-4.25
-1.19
0.21
-2
-1.81
1.44
-0.73
-27.6
-1
-9.2
-0.31
-1.61
-14.5
0.64
0
-0.98
-1.29
-19.3
Contamination of water resources in Tarkwa mining area of Ghana
The modelling is conducted on four wells and the results and interpretation are to be seen as
suggestions to which processes that can be important for the area. For the wells that are not
used in the modelling, other processes can be dominating. Table 6-3 shows the saturation
indicies for these wells for selected minerals. Intervall (-2<SI<2) are chosen to get realistic
values for most minerals (Bhattacharya, P., pers. comm. 2004-11-30). For reactive
minerals, as calcite, dolomite and gypsum, SI-values are presented regardless magnitude
(Sracek, O. pers. comm. 2004-12-09).
Our principal interest is the precipitation and stability of oxides and hydroxides of Fe, Al
and Mn. Oversaturated minerals are mostly Al/Fe-oxides and hydroxides and different
silicates. Various minerals containing Al- oxides/hydroxides can precipitate, but bohemite,
diaspore and kaolinite react very slowly in groundwater environments and are not likely to
interfere with sorption processes. These minerals are generally formed by recrystallization
of precipitated Al(OH)3. But this mineral is undersaturated and not likely to interfere in
sorption processes. Some Fe oxides/hydroxides are supersaturated and can contribute to
sorption processes. For example, goethite and montmorillonite can precipitate and these
minerals can form possible sites for sorption since they have a cation exchange capacity
(CEC) at up to more than 100 meq/100g (Appelo & Postma 1999). Mn oxides and
hydroxides are under saturated and are therefore not likely to precipitate.
Minerals like siderite, vivanite and rhodochrosite are sinks for dissolved Fe and Mn and
their precipitation can disturb correlation between Fe, Mn and As. For the chosen wells
these minerals are undersaturated but there is a possibility that these minerals precipitate in
other wells.
Saturation Indices, SI, show that most possible minerals are undersaturated. This indicates
that the groundwater has short residence times and natural equilibrium with minerals never
is reached. The minerals in the investigated wells are mostly quite insoluble and this could
also be an explanation to this situation.
In large-scale mining lime is used but since lime and carbonates are undersaturated, we can
not state that these groundwaters are polluted from large scale mining according to SI.
Minerals containing sulphates are generally also under saturated. Only in well 32 and 42 we
have minerals containing sulphates that are oversaturated. These well have very low pH and
this indicates acid mine drainage in spite of that the wells have low iron content. This could
be explained by precipitation of sulphides if SO42-is transformed by redox reactions. In
wells with low pH, some silicates are oversaturated and could provide a sink for Al. Table
6-4 show modelled aqueous speciation in Phreeqc for four wells.
The dominating forms are Fe2+, Mn2+, Al3+ and H2AsO4-. There is a difference between Al
speciation in the different wells. The Al speciation is clearly correlated to pH, which can be
seen in the correlation matrix, Appendix 5. For the other wells the speciation is roughly
similar.
The field measured Eh is not a very reliable parameter. The aqueous forms can give us
additional information where our samples are situated in the redox sequence. Mn and Fe
41
Asklund and Eldvall
LUTVDG/TVTG--5092--SE
have both been reduced but SO42- seems not be in a reduced form since analysed values of
SO42- and total S correspond very well. Than the reduced form of SO42-, H2S, easily form
complexes with different metals, like iron, which precipitates. Therefore it is not stated that
reduction of SO42- does not occur because of the absence of H2S. Measurements of Eh show
no reducing conditions in our sampling area, than the field measured Eh is, as mentioned,
an unreliable parameter. It is not unlikely that there are reducing conditions and this could
explain some of the low Fe-values.
Table 6-4: Modelled speciation for four wells.
Specie
Fe+2
FeSO4
FeH2PO4+
FeHPO4
Fe(OH)2+
FeOH+2
13
92.1%
0.2%
0.1%
0.1%
7.0%
0.6%
24
93.8%
5.0%
0.4%
0.2%
0.6%
0.0%
32
91.2%
0.7%
0.1%
0.0%
6.9%
1.2%
42
94.5%
4.6%
0.1%
0.0%
0.7%
0.2%
Mn+2
MnSO4
MnCl+
Mn(NO3)2
Mn(III), (Sum all sp.)
93.2%
6.6%
0.1%
0.0%
0.00%
91.6%
4.9%
3.1%
0.4%
0.00%
98.4%
0.9%
0.8%
0.0%
0.00%
94.9%
4.7%
0.4%
0.0%
0.00%
Al
48.8%
45.7%
5.4%
0.1%
0.0%
0.0%
47.4%
35.1%
10.8%
5.9%
0.6%
0.1%
78.9%
16.3%
2.5%
2.2%
0.1%
0.0%
73.6%
15.9%
9.3%
0.8%
0.5%
0.0%
As(V)
H2AsO4HAsO4-2
As(III), (sum all sp.)
63.1%
36.9%
0.0%
78.5%
21.5%
0.0%
-
-
pH
6.9
6.1
4.4
4.2
Fe(II)
Fe(III)
Mn(II)
Al+3
AlSO4+
AlOH+2
Al(OH)2+
Al(SO4)2Al(OH)3
Fe and Mn can, as mentioned, act as redox couples. This is supported by the aqueous
speciation where all Mn is present as Mn2+ and Fe is present as mostly Fe2+ but also as Fe3+.
The low values of Fe3+ can be explained by precipitation as Ferryhydrit, which is
supersaturated in some samples, see Table 6-3 and act as a sink for Fe(tot).
Arsenic is not reduced and is solely in As(V), which move slower then As(III) under
current pH conditions. Elevated As concentrations is not unusual in nature. The relative low
As(tot) values in well 24 and well 13 most likely origins from local geological formations.
6.6 Surface water chemistry
Electro neutrality was examined and the sample in River Bonsa, no. 2 showed a deviation
of 65%, Appendix 4. Comparison with conductivity showed that the sum of anions is far to
high and it is HCO3- that is causing this high value.
42
Contamination of water resources in Tarkwa mining area of Ghana
7 Conclusion
The metal concentrations in the study area are generally lower than expected. The intensive
mining industry and Kortatsi´s study (2004) in a nearby area indicated higher values
(values above WHO:s guidelines for Al, As, Cd, Cr, Fe, Mn, Ni, Pb and Zn). The
groundwaters in some zones of the study area have values of Mn, Fe, As and Al exceeding
WHO:s guidelines. Mn and Fe account for almost all the elevated values. These metals do
not have the same serious health effect as heavy metals like Cd, Cr, Hg and Pb and the
groundwater quality is therefore better than expected. The groundwaters are generally
neutral to acidic and oxidizing. The dominating ions are sodium and bicarbonate.
Mining activities probably affect the groundwater through Acid Mine Drainage in areas
where high correlation of Fe and SO42- and low pH coincide. Areas with high Fe and low
SO42- can also indicate AMD since SO42- can be transformed by redox reactions and
precipitate as sulphides. Probable areas affected by AMD are New Atuabo, Akoon,
Teberebe, Huniano no 1, Dadwen and Aboso. The occurrence of As at three sample sites is
most probably natural and not considered a major problem in the area. The rest of the
metals exceeding the guidelines, are all parts of common minerals and they probably origin
from dissolution of minerals. For Al there is a strong correlation with pH since it is
precipitated as different hydroxides at pH-values higher than 4-5. Mn and Fe are quite
strongly correlated to Eh, which determines the oxidation state of the metals. This strongly
affects the mobility of the metals. There are also reasons to believe that Fe and Mn act as a
redox couple and affect the oxidation state of each other.
There are a number of reasons that can explain the low metal values:
• Sorption processes are probably very important for metal concentrations of the
groundwater in the area. Sorption can considerably lower the metal concentration in the
groundwater. All soil-types in the Tarkwaian system are clayey and the soils of the
Birimian system most likely have the same composition. The soils have a lot of adsorption
and absorption sites due to their content of clay and abundance of Al/Fe oxides/hydroxides
like goethite and montmorillonite. Heavy metals as Cu, Pb, Hg and Cd are strongly
bounded to these sites and this probably explains why many of the heavy metals display
very low values. Ion exchange in clays can remove heavy metal cations and provide some
protection to groundwater supplies. In the investigated area tendencies have been seen for
ion change between Fe and Ca, but no other ion exchange reactions including heavy metals
have been discovered. Protection by ion exchange is not stable over a long time period but
will end when all exchange sites are taken. The pH is one of the most important parameters
concerning metal mobility. However, of all metals investigated, only Al and Ca are found
to be strongly correlated to pH. This can be due to the low level of metals, and the
correlations can also be disturbed by precipitation of minerals like ferrihydrite, siderite,
vivianite and rhodochrosite. There is also the possibility that minerals containing Al- and
Fe hydroxides/oxides precipitate and withdraw metals from the groundwater and also
continuously form new adsorption sites.
• The area is very hilly and there are a lot of water divides. This gives rise to local
groundwater systems with short residence times. This is also supported by the negative
saturation indices. The Piper diagram, Figure 6.1, also shows that the composition of most
43
Asklund and Eldvall
LUTVDG/TVTG--5092--SE
samples corresponds to the characteristics of relatively young groundwaters. The
groundwater system will not be strongly affected by dissolution of minerals due to the short
contact time. The local groundwater systems also prevent mines to affect larger
groundwater systems on a regional scale. However, there is a possibility that local mining
pollutants not yet have reached the wells and that the groundwater quality will deteriorate
in some of the wells in the future.
• The samples for this study were collected between two rainfall maxima during the rainy
season. This might have a diluting effect on the concentrations of contaminants in the
groundwater. It could explain the low values compared to Kortatsi´s results (2004) who
sampled during the whole year.
No major conclusion can be drawn from the comparison between the shallow wells and the
deep wells as the classification not is based on very detailed information. The differences in
HCO3, conductivity and Eh between the two groups are probably explained by the longer
residence time of the water in the deep wells. The low pH-value in the shallow wells could
be caused by AMD.
Mercury was analysed but the method used has a high detection limit, 30 µg/l, while
WHO:s guideline is 1 µg/l. Therefore we can not draw any conclusions concerning mercury
in groundwater, we can just conclude that there are no samples with a content above 30
µg/l. Nevertheless, a literature study shows that there are elevated levels of mercury in the
area but the most important path of exposure is probably not by water but by mercury
vapour and by ingestion. Mercury is often adsorbed very strongly to for example clay
minerals and organic material.
44
Contamination of water resources in Tarkwa mining area of Ghana
8 Recommendation
Although this study generally shows quite good drinking water quality, contamination of
groundwater from mining activities have been stated at some locations and further
contamination is possible.
The metals exceeding WHO:s guidelines are Al, As, Fe, and Mn. As is only detected in 3
wells and exceed the guideline in two wells. Elevated levels of Mn, Fe and Al are all quite
easy to treat. In the most affected areas aeration and adjustment of pH should be able to
improve the drinking water quality.
Seven locations have been determined as the most interesting sites to for further studies
based of the results from this field study. The locations are: Simpa(2), Aboso(10),
Samahu(13), Eyinaise(24), Huniso(32), New Atuabo(35) and Akoon(42).
For further investigations in the area information about the depth of the wells, the
groundwater flow patterns, the location of small-scale mining activities and more detailed
geological information of the sampling positions would give a better understanding of the
processes governing the groundwater quality in the area. This would make a more
comprehensive assessment of the groundwaters vulnerability concerning metal pollution
possible.
45
Asklund and Eldvall
LUTVDG/TVTG--5092--SE
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48
Contamination of water resources in Tarkwa mining area of Ghana
Appendices
Appendix 1: Field measured parameters
Appendix 2: Trace elements
Appendix 3: Major Ions, DOC, NH4+ and water type
Appendix 4: Ion balance
Appendix 4: Correlation matrix, all wells
Appendix 6: Correlation matrix, deep wells
Appendix 7: Correlation matrix, shallow wells
49
Asklund and Eldvall
LUTVDG/TVTG--5092--SE
Appendix 1: Field measured parameters
No
Location
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
Bonsaso
River Bonsa
Simpa
Dadwen
Dompim
Dompim
Atwerboanda
Odumase
Nsuaem
Aboso
Kokoase
Samahu
Samahu
Yaryeyaw
Suwinso
Gordon
Akotomu
Kofi GyanCamp
Kofi Gyankrom.
Huniano n 1
Tarkwa Banso
T. Banso
Domeabra
Enyinasie
Enyinasie
Akyem
Akyem
Adieyie
Mile 8
Teberebe
Teberebe
Huniso
Huniso
Abekoase
New Atuabo
Koduakrom
Damang
NewKyekyewere
Huni Valley
Bompieso
Bompieso
Akoon
Date
10-sep
10-sep
10-sep
10-sep
10-sep
10-sep
11-sep
11-sep
11-sep
11-sep
11-sep
11-sep
11-sep
12-sep
12-sep
12-sep
12-sep
12-sep
12-sep
12-sep
13-sep
13-sep
13-sep
13-sep
13-sep
13-sep
13-sep
13-sep
13-sep
14-sep
14-sep
14-sep
14-sep
14-sep
14-sep
15-sep
15-sep
15-sep
15-sep
15-sep
15-sep
15-sep
Lat.
(dd)
5.179
5.179
5.108
5.131
5.157
5.153
5.330
5.341
5.350
5.364
5.356
5.373
5.369
5.488
5.470
5.452
5.457
5.433
5.423
5.396
5.284
5.288
5.309
5.295
5.296
5.251
5.251
5.248
5.244
5.271
5.268
5.382
5.382
5.374
5.323
5.523
5.517
5.506
5.474
5.418
5.421
5.314
Long.
(dd)
2.044
2.043
2.111
2.097
2.076
2.077
1.885
1.913
1.916
1.944
1.953
2.000
1.999
2.002
2.007
2.030
1.988
1.994
1.996
1.997
1.959
1.959
1.942
1.930
1.929
1.979
1.980
2.091
2.079
2.032
2.031
2.064
2.063
2.016
1.977
1.815
1.863
1.878
1.914
1.940
1.936
1.991
50
Temp
(C)
28.0
25.2
27.3
26.6
26.7
27.8
27.4
26.8
26.4
27.1
26.6
26.0
26.1
26.4
26.3
26.4
26.7
27.0
25.4
26.3
27.2
26.1
28.5
27.3
26.2
26.5
26.0
28.5
26.0
26.5
25.9
26.9
26.2
26.4
27.0
26.6
26.1
25.9
26.2
26.2
26.5
26.9
pH
10.06
6.62
6.07
5.98
6.06
6.30
6.20
6.85
6.85
6.31
6.75
5.97
6.87
6.06
6.01
5.91
6.42
5.39
5.90
6.23
5.71
6.30
5.30
6.07
6.23
6.13
6.06
6.31
6.92
5.18
5.33
4.44
6.11
5.56
5.82
6.87
4.75
5.85
5.71
6.67
6.51
4.19
Eh
(mV)
286
435
308
268
272
328
337
192
433
215
211
440
233
498
372
467
276
519
416
376
517
297
455
263
262
307
337
350
223
440
399
523
444
421
285
239
474
439
490
234
228
489
Cond
(µS/cm)
145.7
46.0
545.0
126.0
169.0
261.0
483.0
465.0
780.0
457.0
426.0
123.0
510.0
114.1
123.8
127.2
429.0
107.2
163.6
324.0
96.3
266.0
74.4
460.0
276.0
375.0
275.0
355.0
468.0
119.5
83.2
681.0
638.0
109.5
130.1
454.0
32.9
11.0
113.4
471.0
400.0
423.0
Contamination of water resources in Tarkwa mining area of Ghana
No
Remarks
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
Treated surface water from River Bonsa distributed with pipes
Surface water from River Bonsa. Occasionally used, bathing and drinking
Borehole with hand pump, drilled 1983
Borehole with hand pump, drilled 2001, designation WW 33
Borehole with hand pump
Mechanized borehole distributed to 10 places in village, drilled 2002
Borehole with hand pump, a lot of air in the sample water. Also another well.
Borehole with hand pump, complaints iron, drilled 2001, designation WW 90
Borehole with hand pump, complaints salty, also drinks surface water
Borehole, hand pump not as drinking water, also surface water from River Bonsa, 2001, des. WW 42
Borehole with hand pump, drilled 2001, designation WW 1
Hand dug well with hand pump, main drinking water supply, drilled 2002, designation SMHU-22
Borehole with hand pump ; not used for drinking water
Borehole with hand pump, hand dug well with fitted hand pump also used, , drilled 2001, Des. WW 78
Borehole with hand pump, there is also a hand dug well with out pump, drilled 2001, des. WW 83
Borehole with hand pump, another one is used and surface water is used, designation no.94
Borehole with hand pump, designation no.63
Borehole with fitted hand pump, drilled 2001, designation WW 103
Borehole with hand pump, particles. Also surface water is used, drilled 2001, designation WW 62
Hand dug well with hand pump, very much particles, filter clogged after 5 s.
Borehole with hand pump, the mostly used in village. Another 3 but one broken, designation no.209
Borehole with hand pump, complaints, used because distance to next well, designation no.271
Hand dug well with hand pump, two very close this mostly used, bigger flow
Borehole with hand pump, drilled 1997, designation no.37
Borehole with hand pump, also hand dug well but out of order, drilled 2001, designation WW 38
Borehole with hand pump, designation 21A032
Borehole with hand pump, drilled 2002, designation no.7702
Borehole with hand pump, external treatment fitted to pump, installed 9 sep, drilled 2001, des. WW 92
Borehole with hand pump, drilled 2001, designation WW 93
Hand dug well with hand pump. Very many particles, 2002, designation HIPC
Borehole with hand pump. Very many particles, designation no.388
Borehole with hand pump, another borehole and also a hand dug well used, drilled 2001, des. WW 44
Borehole with hand pump, drilled 2000, designation no.7710
Hand dug well with hand pump, upset feelings conc. water quality. At least other 2 hand dug wells used
Hand dug well with hand pump just at city clinic, another 4 hand dug wells used.
Borehole with hand pump, designation WW 39
Borehole with hand pump, Village supplied by mining company but this mostly used for drinking water
Borehole with hand pump, designation no.92
Borehole- hand pump, mechanized system broken, whole community depends on this well, des. 3653
Borehole with hand pump, designation no.127
Borehole with hand pump, total 3 boreholes but one not fitted with pump, designation no.20
Borehole with hand pump, drilled 2001, designation WW 72
51
Asklund and Eldvall
LUTVDG/TVTG--5092--SE
Appendix 2: Trace elements
No
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
Al
ug/
975.38
111.95
9.54
4.98
6.89
5.64
6.25
8.21
9.28
7.84
8.22
7.60
8.80
5.03
4.56
5.31
8.76
12.88
7.82
18.24
3.49
7.71
61.93
8.64
9.45
7.01
7.39
8.73
26.12
30.97
36.66
2175.33
11.32
12.31
5.54
8.47
44.35
3.95
5.46
9.32
8.01
593.81
As
ug/
<5.200
<5.200
<5.200
5.50
<5.200
<5.200
<5.200
<5.200
<5.200
<5.200
<5.200
<5.200
15.56
<5.200
<5.200
<5.200
<5.200
<5.200
<5.200
<5.200
<5.200
<5.200
<5.200
69.36
<5.200
<5.200
<5.200
<5.200
<5.200
<5.200
<5.200
<5.200
<5.200
<5.200
<5.200
<5.200
<5.200
<5.200
<5.200
<5.200
<5.200
<5.200
B
ug/
27.16
26.40
<5.700
<5.700
43.27
13.47
11.11
<5.700
14.91
<5.700
<5.700
10.01
10.97
9.11
9.64
7.60
8.82
<5.700
10.09
16.42
10.09
10.64
10.63
<5.700
13.41
7.09
6.70
11.20
9.04
13.65
14.53
28.78
13.68
7.32
<5.700
7.73
6.56
7.06
6.52
7.69
7.07
53.08
Ba
ug/
17.12
23.81
228.98
28.81
18.20
20.22
166.63
39.08
370.82
136.60
149.55
27.25
468.58
8.52
20.49
23.10
23.92
35.53
16.16
40.17
10.97
48.68
58.46
52.78
170.10
34.07
34.80
210.74
99.51
51.57
23.20
209.76
184.95
32.30
29.00
101.54
25.17
10.11
7.24
135.47
101.82
104.07
Be
ug/
<0.093
<0.093
<0.093
<0.093
<0.093
<0.093
<0.093
<0.093
<0.093
<0.093
<0.093
<0.093
<0.093
<0.093
<0.093
<0.093
<0.093
0.11
<0.093
<0.093
<0.093
<0.093
<0.093
<0.093
<0.093
<0.093
<0.093
<0.093
<0.093
<0.093
<0.093
0.23
<0.093
<0.093
<0.093
<0.093
<0.093
<0.093
<0.093
<0.093
<0.093
0.19
52
Cd
ug/
<0.459
<0.459
0.70
0.51
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
0.60
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
<0.459
Co
ug/
<1.190
<1.190
9.57
4.03
1.85
1.73
5.27
<1.190
1.45
3.78
<1.190
<1.190
<1.190
<1.190
<1.190
<1.190
3.42
1.58
<1.190
<1.190
<1.190
1.96
<1.190
16.25
<1.190
<1.190
4.38
2.00
<1.190
<1.190
<1.190
9.03
<1.190
1.37
1.33
<1.190
1.64
<1.190
<1.190
<1.190
<1.190
21.01
Cr
ug/
<0.481
1.17
<0.481
<0.481
<0.481
<0.481
<0.481
<0.481
<0.481
<0.481
<0.481
<0.481
<0.481
0.67
<0.481
<0.481
<0.481
1.08
<0.481
<0.481
2.00
<0.481
<0.481
<0.481
<0.481
<0.481
<0.481
<0.481
<0.481
<0.481
<0.481
0.60
<0.481
<0.481
<0.481
<0.481
0.91
0.95
<0.481
<0.481
<0.481
<0.481
Cu
ug/
0.59
1.36
2.23
<0.341
<0.341
0.62
<0.341
<0.341
0.97
<0.341
<0.341
<0.341
<0.341
13.58
<0.341
0.48
<0.341
13.98
4.51
<0.341
16.97
<0.341
<0.341
<0.341
<0.341
<0.341
0.87
<0.341
<0.341
0.36
0.77
10.09
9.93
<0.341
<0.341
<0.341
<0.341
1.37
13.73
<0.341
<0.341
16.57
Contamination of water resources in Tarkwa mining area of Ghana
Fe
ug/
13.17
516.13
3407.26
7290.58
5196.48
455.00
501.40
3369.70
2.78
4325.32
1135.70
4.12
122.58
5.11
102.76
4.18
464.92
8.67
25.53
16.91
4.61
711.94
10.59
4685.11
2172.40
245.86
184.71
68.80
75.94
29.08
156.50
14.19
3.70
11.85
10836.02
101.40
4.38
1.16
4.01
281.59
479.40
5.71
Hg
ug/
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
<30
Li
ug/
<1.776
<1.776
12.39
11.58
2.47
9.98
16.78
<1.776
6.07
6.70
12.37
2.14
9.58
5.17
10.83
7.16
7.74
3.53
7.24
7.02
2.32
4.34
<1.776
3.01
2.73
7.00
4.37
5.40
12.97
<1.776
<1.776
11.46
28.19
<1.776
<1.776
19.24
<1.776
4.75
2.66
9.97
9.56
1.94
Mn
ug/
25.86
70.36
915.93
312.37
291.80
243.42
406.13
567.46
850.67
850.21
248.83
92.95
564.92
8.42
271.75
235.06
640.18
92.57
249.81
62.19
5.50
454.35
62.88
926.39
604.92
421.05
499.85
292.75
967.76
74.68
91.48
676.13
1068.99
93.04
288.58
391.51
40.08
38.30
35.61
712.84
602.13
2040.56
Mo
ug/
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
2.24
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
<1.761
Ni
ug/
<2.485
<2.485
9.32
5.27
<2.485
<2.485
14.02
<2.485
8.86
3.12
<2.485
<2.485
<2.485
<2.485
<2.485
2.96
4.51
5.50
<2.485
5.12
2.94
<2.485
<2.485
18.98
<2.485
4.66
12.44
4.37
<2.485
<2.485
<2.485
13.75
<2.485
2.77
2.75
<2.485
<2.485
<2.485
<2.485
<2.485
<2.485
14.16
53
P
ug/
<15.872
17.37
106.95
384.77
249.16
162.75
<15.872
19.41
<15.872
154.38
103.93
<15.872
<15.872
55.37
76.93
96.33
74.91
74.06
28.28
31.35
57.42
115.03
<15.872
117.25
100.27
31.45
58.00
28.71
<15.872
<15.872
<15.872
21.18
<15.872
<15.872
<15.872
<15.872
<15.872
76.06
76.92
34.29
21.84
<15.872
Pb
ug/
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
4.11
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
<2.301
Rb
ug/
8.72
9.54
<6.731
<6.731
<6.731
<6.731
16.57
<6.731
16.36
<6.731
<6.731
<6.731
<6.731
<6.731
<6.731
7.19
<6.731
8.92
<6.731
10.39
<6.731
<6.731
<6.731
7.97
8.87
11.99
7.37
<6.731
<6.731
10.76
8.16
16.30
<6.731
9.27
11.65
<6.731
7.60
7.47
<6.731
<6.731
<6.731
17.41
S
ug/
7891.77
1140.24
11893.99
2061.49
2184.73
3205.16
14752.70
453.49
9100.88
5714.44
3424.41
468.66
487.97
193.63
292.87
183.43
628.34
326.61
236.68
2561.27
409.01
195.99
399.91
19029.82
879.07
10894.33
2426.53
491.98
463.07
739.42
1543.23
2719.81
4721.42
381.84
2143.04
527.53
29.51
96.63
372.47
1948.91
1871.82
17818.21
Asklund and Eldvall
No
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
Sc
ug/
<0.166
<0.166
<0.166
<0.166
<0.166
<0.166
<0.166
<0.166
<0.166
<0.166
<0.166
<0.166
0.21
<0.166
<0.166
<0.166
<0.166
<0.166
<0.166
<0.166
<0.166
<0.166
<0.166
<0.166
<0.166
<0.166
<0.166
<0.166
<0.166
<0.166
<0.166
0.88
<0.166
<0.166
<0.166
0.35
<0.166
<0.166
<0.166
<0.166
<0.166
0.25
Si
ug/
4266.28
3748.38
18524.80
14926.45
21227.82
25988.95
16714.75
13446.24
10915.53
20679.44
20099.44
6404.64
14604.74
12022.95
17025.06
12577.17
22127.74
9764.51
18130.19
9084.08
10610.18
20906.09
3286.77
16972.02
19790.14
14251.36
19429.44
23245.37
17866.51
3450.62
3351.68
7541.71
14846.02
4596.28
4080.25
17700.57
6564.92
19699.98
18305.31
17277.31
16718.86
2941.12
LUTVDG/TVTG--5092--SE
Sn
ug/
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
<7.072
Sr
ug/
53.46
34.71
455.89
53.48
157.28
154.06
484.11
246.70
310.25
311.80
925.10
98.24
2490.73
125.12
140.90
106.87
332.34
95.70
163.77
403.00
51.83
157.06
70.91
489.95
169.13
227.69
277.22
285.19
533.77
58.06
36.78
360.25
894.09
71.06
38.57
3258.73
32.48
228.57
168.76
794.45
560.37
95.99
Ti
ug/
<0.117
0.44
<0.117
0.13
<0.117
<0.117
<0.117
<0.117
<0.117
<0.117
<0.117
<0.117
0.12
<0.117
<0.117
0.12
<0.117
0.14
0.17
<0.117
<0.117
<0.117
<0.117
<0.117
<0.117
<0.117
<0.117
<0.117
<0.117
<0.117
0.28
<0.117
<0.117
<0.117
<0.117
<0.117
0.12
<0.117
<0.117
<0.117
<0.117
<0.117
54
V
ug/
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
1.46
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
2.10
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
<0.987
Y
ug/
<0.127
0.43
<0.127
<0.127
<0.127
<0.127
<0.127
0.58
<0.127
<0.127
<0.127
<0.127
0.39
<0.127
<0.127
<0.127
<0.127
0.92
<0.127
0.15
<0.127
<0.127
0.21
<0.127
<0.127
<0.127
<0.127
<0.127
0.34
0.48
1.00
43.63
1.48
0.34
0.15
1.58
0.69
<0.127
<0.127
<0.127
<0.127
33.10
Zn
ug/
5.15
7.63
49.94
33.42
18.12
650.35
16.11
13.34
15.88
13.83
8.96
25.77
5.95
16.58
12.79
17.36
27.20
32.94
81.14
138.50
40.29
11.82
29.71
11.38
14.87
19.60
21.50
495.36
11.53
39.09
21.70
47.00
86.58
70.03
44.57
7.16
15.80
18.81
20.27
7.38
11.42
53.24
Zr
ug/
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
<0.336
Contamination of water resources in Tarkwa mining area of Ghana
55
Asklund and Eldvall
LUTVDG/TVTG--5092--SE
Appendix 3: Major Ions, NPOC, NH4+ and water type
No
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
HCO3mg/l
43.7
126.9
119.3
57.1
82.5
87.1
150.2
294.9
377.6
230.3
262.4
81.5
328.7
79.3
81.9
65.4
258.8
36.6
95.0
150.5
59.3
162.1
18.9
152.3
156.8
145.2
158.8
230.9
312.9
26.0
20.3
0.0
221.3
69.2
46.4
292.3
12.0
82.7
75.3
258.2
228.3
0.0
Clmg/l
7.45
4.29
84.2
6.9
10.85
27
49.3
5.05
58.35
26.18
8.92
3.65
5.08
5.14
5.07
7.87
12.57
10.2
12.17
17.83
4.8
3.43
5.91
47.36
6.47
32.5
8.18
4.59
3.89
13.85
14.6
117.4
76.55
3.18
5.72
4.2
3.16
3.48
5.39
23.94
14.12
46.15
NO3mg/l
0.17
0.53
2.35
0
0.18
1.74
4.7
0.18
35.9
0
0
2.01
0.19
3.31
0
11.60
0.19
18.7
0
0.27
2.45
0
17.89
1.42
0
7.3
0
0.22
0.22
25.4
0.16
146.4
25.35
0.78
0.23
0.22
2.41
0
0
0
SO42mg/l
27.05
3.01
36.75
6.56
6.86
9.74
45.9
1.43
30.4
18.62
10.28
1.43
1.47
0.54
0.88
0.55
1.89
1.16
0.73
7.46
1.18
0.65
1.22
68.52
2.75
33.45
7.74
1.45
1.35
2.7
4.62
10.4
15.1
1.24
6.35
1.63
0.22
0.39
1.13
6.16
5.82
55.35
66.35
56
PO43Mg/l
0
0.0530
0.298
1.08
0.751
0.481
0.0521
0.0553
0
0.478
0.312
0
0.0389
0.163
0.217
0.296
0.244
0.217
0.0881
0.0894
0.166
0.334
0
0.412
0.303
0.0976
0.199
0.0797
0
0.0374
0.0419
0.605
0.0430
0
0
0
0.125
0.214
0.250
0.110
0.0902
0.0587
Ca2+
mg/l
21.90
3.55
47.58
6.72
18.38
20.27
45.10
66.82
111.18
47.36
43.29
21.03
67.29
13.92
14.12
9.68
55.42
7.20
16.50
42.01
7.18
34.82
8.60
50.42
34.03
41.97
30.91
40.65
77.32
9.93
4.15
23.69
86.28
19.91
14.11
58.02
1.26
12.16
9.13
68.65
55.88
22.51
Contamination of water resources in Tarkwa mining area of Ghana
Mg2+
mg/l
1.68
1.68
9.16
5.39
3.97
6.71
20.73
14.02
22.45
18.31
17.79
2.40
11.12
4.16
2.61
5.95
11.74
4.22
5.54
4.54
5.16
5.16
0.91
20.83
4.83
9.50
8.89
12.99
5.73
1.28
1.10
10.63
9.10
0.58
0.64
5.82
0.89
2.20
3.44
6.67
6.13
5.80
Na+
mg/l
3.19
2.60
44.58
10.32
11.43
20.31
21.51
10.91
29.69
25.39
23.82
3.02
24.52
8.65
11.03
10.30
18.16
9.63
12.39
11.32
7.28
11.71
3.70
20.26
10.53
24.78
13.07
16.01
16.83
11.50
9.51
69.70
29.37
2.19
3.44
30.26
2.56
11.57
11.68
21.17
18.34
31.55
K+
mg/l
1.91
1.48
1.00
0.37
0.54
0.53
5.66
0.76
7.85
0.59
0.62
0.82
0.82
0.27
0.41
0.38
0.59
1.04
0.34
5.16
0.38
1.60
0.53
1.28
3.67
2.24
1.11
0.73
0.68
2.56
2.81
22.69
1.12
1.05
2.00
2.42
0.55
1.73
1.78
0.82
0.60
11.24
NPOC
mg/l
5.32
14.91
1.40
1.00
0.87
1.08
0.87
0.93
0.76
1.06
0.93
0.73
0.86
0.71
0.94
0.81
0.87
0.99
0.96
2.68
1.05
0.94
1.03
0.90
0.96
0.97
0.83
1.31
1.23
2.10
3.89
1.09
0.89
0.94
1.53
0.65
0.70
0.73
0.64
0.86
0.83
1.30
NH4+-N
µg/l
0
0
84.02
0
0
0
0
0
0
26.24
0
0
0
1.20
0
1.70
0
0
0
0
0
0
0
0
0
0
24.05
0
0
0
0
0
0
0
261.82
0
0
0
0
0
0
28.31
57
Water Type
Ca-Na-Cl-HCO3
Na-Mg-Ca-HCO3
Ca-Na-HCO3
Ca-Na-Mg-HCO3-Cl
Ca-Mg-HCO3-Cl
Ca-Mg-HCO3
Ca-Mg-HCO3
Ca-Mg-Na-HCO3
Ca-Mg-Na-HCO3
Ca-HCO3
Ca-HCO3
Ca-Na-Mg-HCO3
Ca-Na-HCO3
Mg-Ca-Na-HCO3
Ca-Mg-HCO3
Na-Ca-Mg-HCO3-Cl-NO3
Ca-Na-Mg-HCO3
Ca-HCO3
Mg-Ca-Na-HCO3
Ca-HCO3
Ca-Na-HCO3-NO3-Cl
Ca-Mg-HCO3-SO4-Cl
Ca-HCO3
Ca-Na-HCO3-Cl
Ca-Mg-HCO3
Ca-Mg-HCO3
Ca-HCO3
Na-Ca-HCO3-Cl-NO3
Na-Ca-Cl-HCO3
Na-Ca-Cl-NO3
Ca-HCO3-Cl
Ca-HCO3
Ca-HCO3
Ca-Na-HCO3
Na-Mg-Ca-HCO3-Cl
Ca-Na-HCO3
Na-Ca-Mg-HCO3
Ca-HCO3
Ca-HCO3
Na-Ca-Cl-SO4-NO3
Asklund and Eldvall
LUTVDG/TVTG--5092--SE
Appendix 4: Ion Balance
No
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
HCO3mek/l
0.716
2.080
1.956
0.936
1.352
1.428
2.461
4.833
6.189
3.775
4.301
1.336
5.387
1.300
1.342
1.072
4.241
0.600
1.558
2.467
0.972
2.657
0.310
2.495
2.569
2.380
2.603
3.785
5.129
0.426
0.332
0.000
3.627
1.134
0.760
4.791
0.196
1.356
1.234
4.231
3.741
0.000
Clmek/l
0.210
0.121
2.375
0.195
0.306
0.762
1.391
0.142
1.646
0.739
0.252
0.103
0.143
0.145
0.143
0.222
0.355
0.288
0.343
0.503
0.135
0.097
0.167
1.336
0.183
0.917
0.231
0.129
0.110
0.391
0.412
3.312
2.159
0.090
0.161
0.118
0.089
0.098
0.152
0.675
0.398
1.302
NO3mek/l
0.0027
0.0085
0.0379
0.0000
0.0029
0.0281
0.0758
0.0029
0.5791
0.0000
0.0000
0.0324
0.0031
0.0534
0.0000
0.1871
0.0031
0.3016
0.0000
0.0044
0.0395
0.0000
0.2886
0.0229
0.0000
0.1177
0.0000
0.0035
0.0035
0.4097
0.0026
2.3614
0.4089
0.0126
0.0037
0.0035
0.0389
0.0000
0.0000
0.0000
0.0000
1.0702
58
SO42mek/l
0.5632
0.0627
0.7651
0.1366
0.1428
0.2028
0.9556
0.0298
0.6329
0.3877
0.2140
0.0298
0.0306
0.0112
0.0183
0.0115
0.0393
0.0242
0.0152
0.1553
0.0246
0.0135
0.0254
1.4266
0.0573
0.6964
0.1611
0.0302
0.0281
0.0562
0.0962
0.2165
0.3144
0.0258
0.1322
0.0339
0.0046
0.0081
0.0235
0.1283
0.1212
1.1524
PO43mek/l
0.0000
0.0017
0.0094
0.0341
0.0237
0.0152
0.0016
0.0017
0.0000
0.0151
0.0098
0.0000
0.0012
0.0051
0.0068
0.0093
0.0077
0.0068
0.0028
0.0028
0.0052
0.0105
0.0000
0.0130
0.0096
0.0031
0.0063
0.0025
0.0000
0.0012
0.0013
0.0019
0.0014
0.0000
0.0000
0.0000
0.0040
0.0067
0.0079
0.0035
0.0028
0.0019
Anions
mek/l
1.492
2.273
5.143
1.301
1.827
2.435
4.885
5.010
9.047
4.916
4.777
1.501
5.565
1.514
1.510
1.502
4.646
1.220
1.919
3.133
1.177
2.778
0.791
5.294
2.819
4.114
3.002
3.951
5.270
1.284
0.844
5.892
6.511
1.262
1.057
4.947
0.333
1.469
1.417
5.038
4.264
3.526
Contamination of water resources in Tarkwa mining area of Ghana
Na+
mek/l
0.139
0.113
1.939
0.449
0.497
0.884
0.936
0.475
1.291
1.105
1.036
0.132
1.067
0.376
0.480
0.448
0.790
0.419
0.539
0.492
0.317
0.509
0.161
0.881
0.458
1.078
0.568
0.696
0.732
0.500
0.414
3.032
1.278
0.095
0.150
1.316
0.111
0.503
0.508
0.921
0.798
1.372
K+
mek/l
0.049
0.038
0.026
0.010
0.014
0.014
0.145
0.020
0.201
0.015
0.016
0.021
0.021
0.007
0.011
0.010
0.015
0.027
0.009
0.132
0.010
0.041
0.014
0.033
0.094
0.057
0.028
0.019
0.018
0.066
0.072
0.580
0.029
0.027
0.051
0.062
0.014
0.044
0.046
0.021
0.015
0.287
Mg2+
mek/l
0.138
0.138
0.754
0.443
0.327
0.552
1.705
1.153
1.846
1.506
1.464
0.197
0.915
0.342
0.215
0.489
0.966
0.347
0.455
0.373
0.425
0.424
0.075
1.713
0.397
0.782
0.731
1.068
0.471
0.105
0.090
0.875
0.749
0.048
0.053
0.478
0.073
0.181
0.283
0.549
0.504
0.477
Ca2+
mek/l
1.093
0.177
2.374
0.335
0.917
1.012
2.251
3.334
5.548
2.363
2.160
1.049
3.358
0.694
0.704
0.483
2.766
0.359
0.824
2.096
0.358
1.738
0.429
2.516
1.698
2.095
1.542
2.028
3.858
0.496
0.207
1.182
4.306
0.994
0.704
2.895
0.063
0.607
0.456
3.426
2.788
1.123
59
Cations
mek/l
1.419
0.466
5.093
1.237
1.755
2.461
5.036
4.981
8.886
4.989
4.676
1.399
5.361
1.419
1.409
1.430
4.537
1.152
1.827
3.094
1.109
2.712
0.679
5.143
2.648
4.011
2.870
3.812
5.079
1.166
0.783
5.669
6.361
1.164
0.958
4.752
0.262
1.335
1.292
4.917
4.106
3.260
Balans
%
-2.51
-65.97
-0.49
-2.53
-2.01
0.52
1.52
-0.29
-0.90
0.73
-1.07
-3.51
-1.87
-3.25
-3.44
-2.44
-1.19
-2.88
-2.46
-0.63
-2.93
-1.21
-7.61
-1.45
-3.13
-1.26
-2.24
-1.79
-1.85
-4.79
-3.77
-1.93
-1.17
-4.05
-4.93
-2.01
-11.85
-4.75
-4.62
-1.22
-1.88
-3.93
Eh
Cond Temp
pH
-0.72 0.38 -0.09
Eh
-0.37 -0.02
Cond
0.12
Temp
TOC
Li
Na
K
Ca
Mg
Fe
Mn
Ba
Cu
Ni
Zn
Al
Cr
Rb
NH4
Sr
Cl
B
HCO3
NO3
SO4
TOC Li
Na
K
Ca
Mg
Fe
Mn
-0.23 0.29 -0.05 -0.47 0.68 0.43 0.09 0.01
0.06 -0.23 -0.06 0.33 -0.50 -0.37 -0.45 -0.25
-0.14 0.50 0.78 0.44 0.87 0.75 -0.04 0.70
-0.03 0.00 0.15 0.06 -0.05 0.27 0.20 0.05
-0.02 -0.07 0.13 -0.19 -0.23 0.03 -0.11
0.42 0.03 0.48 0.19 -0.07 0.19
0.70 0.43 0.49 -0.09 0.57
0.06 0.19 -0.13 0.36
0.65 -0.07 0.56
0.08 0.43
0.09
Ba
0.30
-0.21
0.73
0.10
-0.10
0.30
0.57
0.31
0.65
0.54
-0.10
0.43
Cu
-0.43
0.73
-0.03
0.23
-0.29
-0.18
0.05
0.20
-0.18
-0.15
-0.24
0.19
-0.13
Ni
-0.30
0.03
0.56
0.08
-0.17
0.09
0.52
0.49
0.26
0.50
-0.10
0.59
0.30
-0.01
Zn
0.04
0.03
0.00
0.50
0.11
0.03
0.03
-0.05
-0.06
0.03
-0.09
-0.10
0.01
-0.21
-0.20
Al
-0.53
0.32
0.32
0.09
0.03
0.05
0.70
0.90
-0.09
0.06
-0.11
0.26
0.19
0.24
0.39
-0.02
Cr
0.32
0.20
-0.39
0.50
0.38
-0.70
-0.43
-0.43
-0.48
-0.15
-0.34
-0.45
-0.43
0.46
-0.89
0.30
-0.43
Rb
-0.17
0.18
0.76
0.53
-0.09
0.47
0.69
0.75
0.48
0.48
-0.04
0.60
0.70
0.52
0.33
0.13
0.50
-0.87
NH4
0.03
-0.47
-0.16
0.44
0.78
0.67
-0.22
-0.02
-0.11
-0.46
0.94
-0.13
-0.01
-0.24
-0.35
0.52
-0.16
0.00
0.16
Sr
0.47
-0.40
0.44
-0.09
-0.18
0.51
0.35
-0.02
0.48
0.22
-0.13
0.16
0.51
-0.04
0.57
-0.11
-0.03
-0.55
0.31
-0.24
Cl
-0.25
0.17
0.68
0.19
0.03
0.38
0.86
0.69
0.34
0.44
-0.01
0.54
0.44
0.03
0.63
0.03
0.64
-0.42
0.74
-0.03
0.02
B
-0.47
0.15
0.21
0.17
0.16
-0.19
0.39
0.54
-0.08
0.03
0.47
0.56
0.10
0.41
0.59
0.03
0.46
-0.33
0.66
0.65
-0.13
0.42
HCO3
0.90
-0.61
0.87
-0.06
-0.25
0.37
0.62
0.23
0.92
0.66
-0.11
0.70
0.66
-0.18
0.22
-0.04
-0.28
-0.04
0.59
-0.41
0.58
0.22
-0.07
NO3
-0.22
0.06
0.29
-0.04
-0.10
0.06
0.43
0.48
0.09
0.03
-0.15
0.28
0.17
0.25
0.44
-0.11
0.52
-0.46
0.61
-0.34
-0.03
0.36
0.19
0.12
SO4
-0.12
-0.08
0.49
0.28
-0.01
0.02
0.44
0.31
0.29
0.59
0.18
0.63
0.24
0.11
0.76
-0.05
0.11
-0.36
0.51
-0.06
-0.02
0.60
0.51
0.16
0.05
Si
0.63
-0.54
0.25
0.04
-0.40
0.13
0.15
-0.32
0.32
0.39
0.04
0.09
0.10
-0.28
0.03
0.31
-0.26
0.04
-0.26
-0.44
0.22
-0.06
-0.22
0.44
-0.08
-0.01
Asklund and Eldvall
LUTVDG/TVTG--5092--SE
Appendix 5: Correlation matrix, all wells, 40 samples
Correlations stronger than 0.5 is marked with lightly marked and correlations
stronger than 0.8 are darker marked.
60
Eh
Cond Temp TOC Li
Na
K
Ca
Mg
Fe
Mn
Ba
Cu
Ni
Zn
Al
Cr
Rb
Sr
Cl
B
Si
NH4 HCO3 NO3 SO4
pH
-0.73 0.32 -0.02 -0.25 0.32 -0.16 -0.53 0.65 0.38 0.13 -0.08 0.28 -0.57 -0.36 0.03 -0.57 -0.36 -0.50 0.45 -0.31 -0.50 0.63 0.02 0.89 -0.24 -0.18
Eh
-0.34 -0.06 0.05 -0.18 0.01 0.36 -0.49 -0.34 -0.48 -0.19 -0.19 0.74 0.16 0.04 0.34 0.55 0.40 -0.39 0.23 0.27 -0.56 -0.51 -0.60 0.08 -0.04
Cond
0.23 -0.12 0.49 0.76 0.44 0.86 0.71 0.01 0.68 0.73 -0.15 0.40 -0.02 0.32 -0.41 0.29 0.40 0.67 0.11 0.05 0.85 0.85 0.28 0.46
Temp
-0.02 0.03 0.25 0.10 0.02 0.43 0.23 0.11 0.13 0.06 0.38 0.61 0.10 0.08 0.08 -0.07 0.26 0.06 0.21 0.69 0.04 -0.08 0.36
TOC
-0.19 -0.01 0.08 -0.20 -0.18 -0.02 -0.01 -0.06 -0.04 -0.02 0.08 0.06 -0.08 0.09 -0.19 0.07 0.13 -0.37 0.84 -0.23 -0.09 0.03
Li
0.45 0.05 0.46 0.19 -0.09 0.21 0.33 -0.10 -0.03 0.06 0.06 -0.28 -0.12 0.52 0.40 -0.13 0.22 0.67 0.36 0.10 0.06
Na
0.74 0.37 0.39 -0.04 0.52 0.55 0.11 0.48 0.03 0.73 -0.21 0.40 0.30 0.86 0.29 -0.11 0.92 0.55 0.42 0.41
K
0.04 0.18 -0.18 0.38 0.32 0.31 0.55 -0.07 0.92 0.00 0.75 -0.03 0.70 0.55 -0.45 0.07 0.27 0.48 0.31
Ca
0.62 -0.03 0.54 0.65 -0.32 0.08 -0.09 -0.11 -0.41 0.03 0.45 0.30 -0.11 0.20 0.78 0.92 0.07 0.25
Mg
0.22 0.34 0.51 -0.26 0.52 0.02 0.04 -0.29 0.29 0.15 0.40 -0.16 0.19 0.26 0.60 -0.04 0.57
Fe
0.12 -0.08 -0.30 0.16 -0.11 -0.13 -0.22 -0.21 -0.13 0.03 -0.05 0.23 0.62 -0.07 -0.17 0.25
Mn
0.40 0.08 0.48 -0.10 0.25 -0.39 0.35 0.11 0.52 0.46 -0.15 0.41 0.67 0.26 0.62
Ba
-0.14 0.18 0.01 0.18 -0.24 0.24 0.49 0.42 0.08 -0.03 0.93 0.66 0.13 0.21
Cu
0.14 -0.05 0.32 0.55 0.19 -0.20 0.21 0.33 -0.48 -0.21 -0.36 0.15 0.04
Ni
-0.07 0.42 -0.07 0.67 -0.13 0.61 0.24 -0.24 0.46 0.02 0.16 0.80
Zn
-0.02 -0.08 -0.15 -0.12 0.03 0.07 0.38 0.69 -0.07 -0.12 -0.06
Al
0.09 0.50 -0.05 0.64 0.46 -0.38 0.02 -0.15 0.51 0.10
Cr
0.02 -0.20 -0.12 -0.07 -0.34 -0.43 -0.41 -0.05 -0.23
Rb
-0.19 0.53 0.44 -0.54 -0.13 -0.02 0.24 0.57
Sr
-0.02 -0.11 0.13 0.84 0.56 -0.06 -0.06
Cl
0.29 -0.24 0.94 0.16 0.34 0.59
B
-0.30 -0.09 -0.08 0.22 0.24
Si
0.31 0.27 -0.20 -0.16
NH4
0.24 -0.09 0.61
HCO3
0.19 0.11
NO3
0.02
Contamination of water resources in Tarkwa mining area of Ghana
Appendix 6: Correlation matrix, deep wells, 34 samples
61
Eh
Cond Temp TOC Li
Na
K
Ca
Mg
Fe
Mn
Ba
Cu
Ni
Zn
Al
Rb
Sr
Cl
B
Si
HCO3 NO3 SO4
pH
-0.47 0.75 -0.51 0.25 0.84 0.03 0.52 0.85 0.73 0.17 0.18 -0.66 -0.60 0.71 0.61 -0.63 0.09 0.69 0.19 0.33 0.89 0.88 -0.84 0.62
Eh
-0.32 0.11 -0.32 -0.90 0.05 -0.37 -0.21 0.00 -0.90 -0.88 0.48 0.29 -0.52 -0.28 0.55 -0.65 -0.11 -0.07 -0.56 -0.15 -0.21 0.55 -0.79
Cond
-0.43 0.79 0.98 0.63 0.93 0.93 0.90 -0.09 -0.16 -0.15 -0.15 0.89 0.92 -0.30 0.41 0.96 0.77 0.77 0.89 0.90 -0.41 0.78
Temp
-0.21 -0.34 -0.25 -0.38 -0.53 -0.43 0.12 0.03 0.69 -0.15 -0.31 -0.34 0.83 -0.35 -0.30 -0.19 -0.12 -0.56 -0.59 0.43 -0.20
TOC
0.70 0.91 0.95 0.53 0.61 0.02 -0.08 0.26 0.38 0.65 0.71 -0.02 0.67 0.69 0.96 0.92 0.45 0.46 0.12 0.78
Li
0.53 0.87 0.94 0.97 0.02 -0.44 -0.21 -0.31 0.88 0.87 -0.31 0.33 0.99 0.69 0.74 0.94 0.92 -0.52 0.94
Na
0.82 0.38 0.59 -0.27 -0.36 0.42 0.64 0.36 0.52 0.13 0.47 0.58 0.97 0.92 0.35 0.32 0.41 0.49
K
0.76 0.77 0.00 -0.09 0.02 0.16 0.81 0.86 -0.22 0.62 0.85 0.91 0.88 0.69 0.71 -0.16 0.84
Ca
0.89 -0.21 -0.23 -0.36 -0.37 0.87 0.90 -0.42 0.22 0.93 0.52 0.51 0.97 0.99 -0.62 0.60
Mg
-0.35 -0.40 -0.09 -0.14 0.64 0.73 -0.17 0.03 0.94 0.69 0.77 0.94 0.87 -0.30 0.54
Fe
0.99 -0.39 -0.20 0.11 -0.15 -0.40 0.46 -0.30 -0.21 0.56 -0.24 -0.19 -0.33 0.52
Mn
-0.49 -0.21 0.06 -0.21 -0.49 0.43 -0.37 -0.32 -0.73 -0.25 -0.20 -0.39 0.44
Ba
0.45 -0.27 -0.10 0.94 -0.17 -0.01 0.38 0.33 -0.39 -0.44 0.85 -0.20
Cu
-0.37 -0.22 0.19 0.34 -0.23 0.45 0.33 -0.37 -0.40 0.78 -0.12
Ni
0.96 -0.37 0.55 0.83 0.54 0.51 0.75 0.84 -0.61 0.79
Zn
-0.23 0.48 0.91 0.67 0.57 0.78 0.85 -0.44 0.68
Al
-0.44 -0.10 0.12 0.08 -0.42 -0.49 0.74 -0.36
Rb
0.18 0.48 0.45 0.07 0.19 -0.12 0.67
Sr
0.73 0.74 0.91 0.90 -0.35 0.63
Cl
0.96 0.48 0.45 0.25 0.63
B
0.52 0.45 0.18 0.87
Si
0.98 -0.60 0.54
HCO3
-0.67 0.56
NO3
-0.39
Asklund and Eldvall
LUTVDG/TVTG--5092--SE
Appendix 7: Correlation matrix, shallow wells, 6
samples
62