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Heavy Metal Contamination of soils, stream-sediments,
natural waters and agricultural plants in Pb-Zn Mining
localities in Enyigba, southeastern Nigeria.
Smart C. Obiora
Department of Geology, University of Nigeria, Nsukka, Nigeria ([email protected]; [email protected])
Anthony Chukwu
Department of Geology and Exploration Geophysics, Ebonyi State University, Abakaliki, Nigeria ([email protected])
Theophilus C. Davies
Faculty of Natural Sciences, Mangosuthu University of Technology, Umlazi/Durban, South Africa
([email protected])
Abstract. Field and geochemical studies on the soils,
stream-sediments, agricultural plants and natural waters
in the Pb-Zn mining localities in Enyigba community in
southeastern Nigeria reveal that the immediate
environment close to the mines have been adversely
contaminated by the heavy metals associated with the
minerals recovered from the mines. Comparison of the
data on the soils and the stream-sediments with the
Canadian Environmental quality guideline for agricultural
soils and the computation of geoaccumulation indices
and enrichment factors indicate they have been
contaminated and enriched in various degrees by Pb, Zn,
Cd and Fe while the stream-sediments are contaminated
and enriched by Pb, Cd, Cu and Mn. All the agricultural
plants (food crops and grasses), with the exception of a
yam tuber, contain Pb above the WHO guideline. The
lemon grasses appear to be excellent accumulators of
Pb as they contain 242 to 464 times the guideline of 0.4
mg/kg. Abnormal concentrations of either Zn or Cd were
also recorded in a good number of the food crops and
grasses, namely: cocoyam tuber, pumpkin leaf (locally,
‘ugu’ leaf, cassava leaves, lemon grasses, and potato
tubers. The concentrations of Pb in all the water
samples, except one from a borehole, are abnormal.
These findings call for immediate remediation plan, as
well as enlightenment of the miners and the local
community on the dangers of exposure to these
contaminants.
Keywords. Heavy metals, contamination, Pb-Zn mining,
soils/stream-sediments, agricultural plants, natural
waters, Enyigba, Nigeria.
1 Introduction
Pb-Zn mineralization is prominent in the Enyigba
community which is geologically located within the
Lower Benue rift, southeastern Nigeria (politically, in
Ebonyi State of Nigeria). The Pb-Zn ores occur in veins
within slightly metamorphosed Albian sedimentary
rocks which are mainly black slates (Obiora and Umeji,
2004) in an essentially NE-SW trending belt within the
Benue rift. The ores are often associated with saline
groundwater mineralization and igneous rocks in this
belt. The mines in Enyigba community are located
within Lat. 6° 10´ 40´´N to 6° 11´55´´N and Long. 8°
08´ E to 8° 09´E. The terrain is generally flat-lying with
occasional small hills on which the mines are commonly
located. The first recorded production of Pb-Zn ore was
in 1925. Mining ceased in some of the mines during the
civil war period (1966 to 1970) (Umeji, 2000). Local
mining had, however, continued intermittently within the
‘’abandoned mines’’ after the civil war. The ore minerals
are commonly galena (PbS), sphalerite (ZnS) and rarely,
chalcopyrite (CuFeS2) and bornite (Cu5FeS4). Azurite
[2CuCO3Cu (OH)2], smithsonite(ZnCO3) and cerussite
(PbCO3) are some of the products of supergene
enrichment. The gangues include siderite (FeCO3),
calcite (CaCO3), pyrite (FeS2), marcasite (FeS2) and
quartz (SiO2) (Umeji, 2000; Obiora and Umeji, 2004).
Mine wastes had accumulated around the mines since
the onset of mining; these are dumped in the nearby soils
on which farmlands are located. Mine waters are
pumped into these nearby farmlands and natural waters
such as streams and rivers occur in the vicinity of the
mines. Pb- Zn mines are known to be potential sources
of harmful trace-elements such as Pb, Zn, As, Cd, Mn,
Fe, Se, Sb, Cu, Bi, as well as Acid Mine Drainage,
which contaminate the immediate environment of the
mines, namely: soils and surface waters (Nikolaidis et
al., 2010; Obiora, 2012).These elements can be toxic to
plants, animals, and human beings when they are
absorbed in abnormal concentrations. There are scarcely
records of detailed studies carried out to ascertain the
extent of contamination of the environment by the
mining activities in Enyigba community. The purpose of
this study was to determine the concentrations of tracemetals, including those that are essential in plant and
animal nutrition and the potentially harmful traceelements common around Pb-Zn mines, in
soils/tailings, agricultural plants (food crops/grasses),
natural waters, and stream-sediments around the
major abandoned Pb-Zn mines in the Enyigba
community in order to assess the level of contamination
of the environment surrounding the mines by the
potentially harmful elements/heavy metals.
2 Methods
2.1 Field Survey, Sampling media, methods and
procedure
The field survey was carried out in a scale of 1: 15,000
around three major mines, namely: Ndinwanu Ishiagu
Enyigba/Ikwo, Ishiagu Enyigba and Alibaruhu mines.
The first consists of three elongate pits which trend N - S
to 20° NNE – 200° SSW, with length of 250 m, width
between 3 and 6 m and depth up to 10 m. The second
consists of four elongate pits, three of which trend
essentially 345° NNW – 165° SSE to 325° NNW – 145°
SSE, with the fourth trending 30° NNE–210° SSW; the
lengths are between 30 and 90 m, with widths of 5 to 8
m and depths up to 15 m. The third mine consists two
elongate pits which trend essentially 310° WNW – 130°
ESE and 334° NNW – 154° SSE, with lengths of 20 and
50 m, respectively, with width varying from 3 to 7 m,
and depth> 10 m. The sampling area around the mines
varied from 500 square metres to 500 m x 300 m.
The method and procedure adopted in the sampling
were according to the ones recommended by Kribek
(2013) for the UNESCO-SIDA-sponsored Abandoned
Mines Project in Sub-Saharan African countries. The
samples collected include soils, stream-sediments,
tailings, agricultural plants (food crops and grasses,
namely: sweet potatoes- tuber and leaf; cassava- tuber
and leaf; cocoyam- tuber; yam -tuber; pumpkin leaf and
lemon grass), and natural/mine waters. The samples of
soils and tailings were collected from farmlands in the
vicinity of the mines (i.e. cultivated land). The sampling
points were located at every one second (approximately
16 m) along the Latitudes and Longitudes. The soil
samples were mainly collected from the top soils (within
0 – 25 cm depth). A few samples from the sub-soils
(within a depth of 50 to 100 cm) at some locations were
collected in order to obtain information about
enrichment or depletion processes between soil layers
The agricultural plants (food crops and grasses) were
collected wherever they were encountered adjacent the
sampling points for the soils and tailings. The natural
water samples were collected along the stream/river
channels around the three major mines at distances of, at
least, 50 m as long as the channel was not dried up.
Three streams sampled include the Ina stream, very close
to the Ndinwanu Ishiagu Enyigba/Ikwo mines, ‘Ngele
Odicha’, close to Alibaruhu mines and ‘Nwangele
Akpara’ stream, close to the Ishiagu Enyigba mines.The
only river samples was the Akpara River which is also
close to the Ishiagu Enyigba mines. Also, water samples
were collected from a borehole located nearby one of the
mines, Alibaruhu. The sample bottles (trace-elements
free) were thoroughly cleaned and filled with distilled
water acidified with 1.0 ml of concentrated HNO3 for
some days before the sampling. Two sets of water
samples, namely: unfiltered (for anion analyses) and
filtered (for cation analyses) water samples were
collected. The filtration of the water samples was done in
the field by the use of disposable filters. The filtered
samples were acidified right in the field by the addition
of 1.0 ml of super pure concentrated HNO3 using a
syringe. The water samples were poured in 50 -100 ml
sample bottles by the use of beakers (100 to 200 ml). To
avoid sample contamination, the beakers were cleaned
with distilled water and dilute HNO3. Also at every
location, the sample bottle and its lid were rinsed several
times with water from the sampling site. Water
temperature and pH were also measured; Electrical
Conductivity (EC) could not be measured in the field
All the water samples were kept in a portable cooler
filled with ice right in the field and transferred to the
refrigerator at the end of each day’s survey. In order to
ascertain the level of awareness of the local community
about the likely impacts of mining activities on their
environment and health, interviews were conducted
using standard questionnaire designed for the purpose.
Also, data on health status of the miners was obtained
through tests using a Resonance magnetic quantum
Analyser (produced by a Chinese Company and adopted
by Kedi Healthcare Nigeria Ltd.
2.2 Sample preparation and analyses
All the samples were air-dried over several days and
subsequently homogenized in agate ball mill to
analytical fineness (< 0.063 mm). Altogether, 49 samples
of soils, 9 of stream-sediments, 7 of tailings, 14 of
agricultural plants, including potato,yam, cocoyam and
cassava tubers, 12 of agricultural plants, including
potato, cassava, pumpkin and lemon grass leaves and 14
samples each of unfiltered and filtered natural/mine
waters were analysed geochemically. The analyses were
carried out by the Acme Analytical Laboratories
(Vancouver) Ltd, Canada, using the Inductively Coupled
Plasma, Mass Spectrometer (ICP-MS). Thirty- six (36)
trace-metals, including those essential in plant and
animal nutrition and the potentially harmful traceelements common around Pb- Zn mines, were analyzed
in the samples of soils, stream-sediments and tailings
while seventy (70) trace-metals were analyzed in the
water samples. Twelve (12) anions, as well as
conductivity were also analyzed in the water samples
3 Results and discussion
Comparison of the data on the soils, stream-sediments
and tailings with international standards such as the
Canadian Environmental quality guideline(1999) for
agricultural soils indicates that 30 out of 37 soil samples
close to the mines have values of Pb (100 to 7075.4
mg/kg) which exceed the guideline of 70 mg/kg while
10 samples have values of Zn (202 to 7379 mg/kg)
which is above the guideline of 200 mg/kg; 6 samples
contain Cd (1.6 to 20.7 mg/kg) and Cr(104 to 122
mg/kg) exceeding the guidelines of 1.4 mg/kg and 64
mg/kg, respectively; 5 other samples contain S (900 to
19900 mg/kg) and Se (1.2 to 1.3 mg/kg) which are
above the guidelines of 500 mg/kg and 1 mg/kg,
respectively. For the soil samples away from the mines
(12 in number), all, with the exception of one sample
contain Pb (88.2 to 4077.4 mg/kg) which is above the
guildeline; 5 samples contain Cr (68 to 118 mg/kg)
which exceeds the guideline; only one sample each
contain Cd (6.1 mg/kg) and Se (1.4 mg/kg) exceeding
the guideline. All the 9 samples of the stream-sediments
contain Pb (156.2 to > 10,000 mg/kg) far exceeding the
8000
7000
6000
5000
4000
Pb
Zn
3000
Cd
2000
1000
Standard
(20) 42 TS
(19) 41 TS
(18) 40 TS
(17) 39 TS
(16) 38 TS
(15) 37 TS
(9B) 32 TS
(8B) 30 SS
(9A) 31 TS
(7B) 28 SS
(8A) 29 TS
(6B) 26 SS
(7A) 27 TS
(5C) 24 SS
(6A) 25 TS
(4F) 22 SS
(5B) 23 TS
(4E) 21 TS
(3D) 20 SS
(2C) 18 SS
(3C) 19 TS
(2B) 17 TS
(1C) 15 TS
(1D) 16 SS
0
Figure 1a. Histogram plot of contaminants in soils close to
the Ndinwanu Ishiagu Enyigba/Ikwo mines.
8000
7000
6000
5000
Pb
4000
Zn
Cd
3000
2000
1000
0
(12A) 33TS (12B) 34 SS (13) 35 TS
(14) 36 TS
Standard
Figure 1b. Histogram plot of contaminants in soils away from
the Ndinwanu Ishiagu Enyigba/Ikwo mines. Note reduction in
the concentration of the contaminants when compared with
Fig.1a.
guideline; 6 contain Zn (326 to 685 mg/kg) which is
above the guideline; 4 contain Cd (1.7 to 2.3 mg/kg) also
above the guideline; 2 samples contain Cr (110 to 149
mg/kg) while three others have values of Se (1.1 to 1.6
mg/kg), all above the guidelines. For the 7 tailings
samples, all contain Pb (188.3 to >10,000 mg/kg), 6
contain Zn (394 to 891 mg/kg), 4 contain Cd (1.8 to 2.8
mg/kg), 3 contain S (800, 800, 4700 mg/kg), all
exceeding the guidelines for the respective elements.
Geoaccumulation indices computed for the soils closer
to the mines indicate they are moderately to extremely
contaminated by Pb, Zn, and Cd, and moderately by Fe.
Enrichment factor shows significant to extremely high
enrichment in Pb and significant enrichment in Zn.
Conversely, the geoaccumulation indices for soil
samples away from the mines indicate they are
moderately to extremely contaminated by only Pb and
Cd; there is significant to very high enrichment in Pb.
The geoaccumulation indices for the stream-sediments
also indicate they are heavily contaminated by Pb,
heavily to moderately contaminated by Cu and Mn, and
moderately to heavily contaminated by Cd. The
enrichment factors extremely high enrichment in Pb and
moderate enrichment in Cd. All, except one, of the 26
samples of the agricultural plants (food crops and
grasses) contain values of Pb (0.45 to 185.74 mg/kg)
which exceed the WHO (2002) guideline of 0.4 mg/kg
for agricultural plants. That one sample is a yam tuber
containing Pb of 0.24 mg/kg. Of all these samples, the
lemon grasses were the most enriched (242 to 464
times). 10 of the agricultural plants, namely: cocoyam
tuber, pumpkin leaf (locally,’ugu’ leaf), cassava leaves
and lemon grasses contain Zn (52.5 to 162.4 mg/kg)
which exceed the guideline of 50 mg/kg, with highest
value of 162.4 mg/kg recorded in the cassava leaves; 6
samples, including one potato tuber, one pumpkin leaf
and 3 lemon grasses contain Cd (0.32 to 0.61 mg/kg)
which is above the guideline of 0.2 mg/kg. Values of Pb
(0.03 to < 4 ppm) in all the water samples, except the
borehole sample (0.003 ppm or 2.9 ppb), exceeded the
EU /Canadian guidelines of 0.01ppm or 10µg/l (See
Nikolaidis et al, 2010) while four samples from the Ina
stream with Pb ( < 4 ppm) exceeded the WHO (2011)
guideline of 1.5 ppm or 1.5 mg/l. EC was up to 21900 to
35,600 µS/cm which exceed the EU guideline of 2,500
µS/cm. The water temperature measured in the field was
in the range of 28.5 to 39°C while the pH was generally
in the range of 4.1 to 6.7. This range of pH is below the
EU guideline of 6.5 to 9.5 in Nikolaidis et al (2010) and
Canadian guideline (2006) of 6.5 to 8.5. The low pH
implies that Acid Mine Drainage has been produced as
would be expected.
According to the oldest man interviewed (age: 95 years),
mining started before he was born. Generally, they said
that mining had affected their life style. Government
started rehabilitation (filling the pits) in some of the
mines when there was quarrel between the local miners.
The communities are not involved in rehabilitation
activities. Generally, the people have low knowledge of
the contamination/pollution of the ecosystem by mining
activities. They however, said that crops do not do well
close to the mines. They are not aware of human/animal
diseases that may be attributed to mining.
Figure 2. Histogram plot of contaminants in the streamsediments
The test of elemental concentrations in the miners using
the Resonance magnetic quantum Analyser revealed
elevated concentrations of Pb, Cd and As in the miners.
Pb (Food crops)
45
40
35
30
25
20
15
10
5
0
Pb
effects of the contamination decrease away from the
mines. The stream-sediments have been contaminated
and/or enriched by Pb, Cd, Cu and Mn. All the
agricultural plants, with the exception of a yam tuber,
contain Pb above the WHO guideline. The lemon grasses
appear to be excellent accumulators of Pb as they
contain 242 to 464 times the guideline of 0.4 mg/kg. A
good number of the agricultural plants including
cocoyam tuber, pumpkin leaf (locally,’ugu’ leaf),
cassava leaves, potato tubers, and lemon grasses contain
either Zn or Cd above the recommended limit. Values of
Pb (0.03 to < 4 ppm) in all the water samples, except the
borehole sample (0.003 ppm or 2.9 ppb), exceeded the
EU /Canadian guidelines of 10µg/l or 0.01ppm. Acid
Mine Drainage has been produced in the immediate
surroundings of the mines. Unfortunately, the miners and
the local community are ignorant of the extent of
damage to the environment, as well as the health risks
posed by the mining activities. The findings in this study
call for immediate remediation plan, as well as
enlightenment of the miners and the local community on
the dangers of exposure to these contaminants.
Acknowledgements
Figure 3a. Histogram plot of Pb in the agricultural plants
References
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200
180
160
140
120
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0
This study, including the field survey and geochemical
analyses, was carried out under the UNESCO-SIDA-sponsored
Project on abandoned mines in Sub-Saharan African Countries.
Pb
Figure 3b. Histogram plot of Pb in agricultural plants
showing very high abnormal concentrations in the lemon
grasses
4 Conclusions
Mining activities which had lasted for over a period of
95 years in Enyigba community has adversely affected
the immediate environment surrounding the mines,
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(food crops and grasses) and natural waters.The
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