Hydrogeochemical study on the contamination of water resources in

Journal of African Earth Sciences 66–67 (2012) 72–84
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Journal of African Earth Sciences
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Hydrogeochemical study on the contamination of water resources in a part
of Tarkwa mining area, Western Ghana
Prosun Bhattacharya a,⇑, Ondra Sracek b,c, Björn Eldvall a,d, Ragnar Asklund a,d, Gerhard Barmen d,
Gunnar Jacks a, John Koku f, Jan-Erik Gustafsson g, Nandita Singh g, Berit Brokking Balfors g
a
KTH-International Groundwater Arsenic Research Group, Department of Land and Water Resources Engineering, Royal Institute of Technology (KTH), SE, 100 44 Stockholm, Sweden
Department of Geology, Faculty of Science, Palacký University, 17 listopadu 12, 771 46 Olomouc, Czech Republic
lohorská 31, 169 00 Praha 6, Czech Republic
OPV s.r.o. (Protection of Groundwater Ltd.), Be
d
Department of Engineering Geology, Technical University of Lund (LTH), Box 118, SE, 221 00 Lund, Sweden
f
Department of Geography and Development, University of Ghana at Legon, Accra, Ghana
g
Water Management Research Group, Department of Land and Water Resources Engineering, Royal Institute of Technology (KTH), SE, 100 44 Stockholm, Sweden
b
c
a r t i c l e
i n f o
Article history:
Received 27 May 2011
Received in revised form 12 March 2012
Accepted 15 March 2012
Available online 24 March 2012
Keywords:
Groundwater
Mining
Hydrogeochemistry
Metal pollution
Arsenic
Tarkwa
a b s t r a c t
The aim of this study was to investigate the groundwater chemistry with special concern to metal pollution in selected communities in the Wassa West district, Ghana. In this mining area, 40 ground water
samples, mainly from drilled wells, were collected. The groundwaters have generally from neutral to
acidic pH values and their Eh values indicate oxidising conditions. The dominating ions are calcium,
sodium, and bicarbonate. The metal concentrations in the study area are generally lower than those typically found in mining regions. Only 17 wells show metal concentrations exceeding WHO guidelines for at
least one metal. The main contaminants are manganese and iron, but arsenic and aluminium also exceed
the guidelines in some wells probably affected by acid mine drainage (AMD). Metal concentrations in the
groundwater seem to be controlled by the adsorption processes. Hydrogeochemical modelling indicates
supersaturation of groundwater with respect to several mineral phases including iron-hydroxides/oxides,
suggesting that adsorption on these minerals may control heavy metal and arsenic concentrations in
groundwater. The area is hilly, with many groundwater flow divides that result in several local flow systems. The aquifers therefore are not strongly affected by weathering of minerals due to short groundwater residence times and intense flushing. The local character of groundwater flow systems also prevents a
strong impact of acid mine drainage on groundwater systems in a regional scale.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
West Africa has been one of the world’s most important gold
mining regions for centuries. Today the most significant gold producing country in the area is Ghana (Hilson, 2002a). The earliest
European attempts to extract gold on a large scale were concentrated in Tarkwa and Prestea regions in the late 19th century. A
gold rush in the early 20th century was followed by a mass increase in gold production. After Ghana gained independence
1957 the industry collapsed and reached a 50-year low in 1982.
In 1983 the government started the Economic Recovery
Programme (ERP) under guidance of WHO. After this the mining
industry has seen a phenomenal growth and the gold production
has increased by 700% (Hilson, 2002a).
⇑ Corresponding author. Tel.: +46 8 790 7399; fax: +46 8 790 6857.
E-mail address: [email protected] (P. Bhattacharya).
1464-343X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jafrearsci.2012.03.005
Both small-scale miners and large-scale mining are currently
operating in Ghana and about 237 (154 Ghanaian and 83 foreign)
enterprises are prospecting for gold and another 18 are operating
gold mines (Hilson, 2002a,b). Large-scale mining in Tarkwa region
is conducted as surface mining. Cyanidation is the most common
technique in the region and is used for treatment of non-sulphidic
palaeoplacer ore (Akosa et al., 2002; Kortatsi, 2004). The management of waste from large scale mining is done in accordance to approved environmental plans. The waste rock heaps are stabilised
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 for further disposal, however, the
monitoring of these activities is poor. 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 co-operative society made up of ten or
more persons’’ (Government of Ghana, 1989). In the Tarkwa area,
small-scale mining is found all around, both in the forest and along
P. Bhattacharya et al. / Journal of African Earth Sciences 66–67 (2012) 72–84
the rivers. It is practised in about 20,000 small-scale mines in the
Wassa West district through out the year. Among these small-scale
miners about 90% are illegal. Currently, 168 small-scale mining
concessions are valid in the region (Asklund and Eldvall, 2005;
Balfors et al., 2007).
The general ore processing techniques are handpicking, amalgamation, cyanidation, flotation, electroextraction, 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 paleoplacer or free
milling ore and oxidised ore (Kortatsi, 2004).
This study is focused on an area in southwestern Ghana that has
a long history of mining activities where groundwater serves as the
main source of drinking water supply for local population. Most
major towns in the area except Tarkwa rely solely on groundwater.
To match the demand for potable water the number of boreholes
and hand dug wells is increasing rapidly (Kortatsi, 2004). There
are apprehensions that the mining activity is causing serious metal
pollution to the water resources by contaminants such as arsenic,
lead, cadmium, mercury, and cyanide. Earlier studies have shown
that metal levels in groundwater exceed WHO guidelines for drinking water in many areas in western Ghana (Kortatsi, 2004; Kuma,
2004).
Estimated 5 tonnes of mercury (Hg) are released from smallscale mining operations in Ghana each year (Hilson, 2001). High
concentrations of Hg have been found in sediments and fish in
the vicinity of small-scale mining activities using amalgamation
as the 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). In general, the management of waste in small-scale mines, particularly the illegal ones,
lacks waste management plan and simply leave the waste. Additionally, mining has led to conflicts among communities, displaced
by mining operations, and health and social problems, pollution of
the community water sources, and depletion of groundwater resources (Fonseca, 2004).
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 low pH in the groundwater that encourages
the mobility of trace metals which are found in the groundwater
in very high concentrations (Kortatsi, 2004). In study of Asante
et al. (2007) groundwater As was compared with urinary As levels
of local residents in Tarkwa and no difference was found compared
with a control group from Accra. Nevertheless, urine levels were
high and the authors suggested a presence of undetected sources
of As in Ghana.
The aim of this study was to investigate the salient hydrogeochemical characteristics with special emphasis on metal pollution
in the water resources that are used by the local communities in
the Tarkwa mining area. The outcome of the study will be used
to assess the vulnerability of shallow groundwater quality due to
natural geochemical environment and to distinguish it from mining pollution of the groundwater resources specifically in the region around the Tarkwa mining area.
2. Geology and hydrogeology
2.1. The study area
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. Mining is
the main industrial activity in the area (Avotri et al., 2002). The area
73
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 Fig. 1.
2.2. Climatic characteristics
The climate of the area is tropical and is characterised by seasonal weather patterns. The Wassa West district is situated at
the border of two climatic regions. The south part belongs to the
south western equatorial climatic region and the northern part
has a wet semi-equatorial climate (Dickson and Benneh, 1980).
The area is characterised by double wet season during the months
of April–June and October–November. 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 1874 mm with
max and min values of 1449 mm and 2608 mm, respectively. The
area is very humid and warm with temperatures between 28–
30 °C during the wet season and 31–33 °C during the dry season
(Dickson and Benneh, 1980; GSR, 2004). The mean pH of the rain
water in the area during 2000–2001 was around 6.1 (Kortatsi,
2004), and temperature was between 26 and 30 °C.
2.3. Geological and geomorphologic characteristics
The regional geology of Ghana is represented by a wide variety
of Precambrian igneous and metamorphic rock comprising the
Basement Complex and covers about 54% of the country, mainly
the southern and western parts (Fig. 2).
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 Dapahh-Siakwan, 2000). The general type of topography reflects underlying geology (Kortatsi, 2004). The soil in the Tarkwa
area consists of mostly silty-sands with minor patches of laterite,
mainly in hilly areas (Kuma and Younger, 2001).
The Basement complex is divided into different sub provinces
including the metamorphosed and folded rocks of the Birimian
and Tarwaian system (Gyau-Boakye and Dapahh-Siakwan, 2000)
with gneiss, phyllites, schists, migmatites, granite-gneiss and quartites as the predominant lithology (Fig. 2). The lithology of the Tarkwaian System is characterised by a sequence of metasediments
comprising quartzites, grits, phyllites and conglomerates of the Kawere Group, a predominant quartzite, grit, conglomerate sequence
of the Banket Series, Tarkwa phyllites and Huni Sandstones, grits
and quartizes with bands of phyllites (Table 1). 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). The rest of the country is underlain by Palaeozoic 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).
Sulphide minerals, like arsenopyrite are widely reported in
Ghana. There is a close association between sulphide minerals,
especially arsenopyrite, and gold in most parts of Ghana
(Dzigbodi-Adjimah, 1993; Smedley, 1996). The problems associated with AMD can therefore be expected in many gold mining
areas in Ghana. Acid mine drainage (AMD) has been reported from
a number of mines in the Tarkwa–Prestea area of southwestern
Ghana (Kortatsi, 2004). Monitoring of a large spoil dumps in the
Tarkwa area show water quality consistent with AMD characteristics. The pH is consistently below 4, the outflow from the waste
dumps has high concentrations of sulphate, silica, aluminium, iron,
and manganese, and shows little variation during year (Kuma,
74
P. Bhattacharya et al. / Journal of African Earth Sciences 66–67 (2012) 72–84
Fig. 1. Location of the study area and simplified regional geological map of southwest Ghana (modified from Kuma, 2004).
2003). The major minerals associated with AMD that occurs in the
Tarkwa–Prestea area are shown in Table 2.
2.4. Hydrogeology
In the Tarkwa–Prestea area groundwater occurrence is associated with the development of secondary porosity through fissuring
and weathering. The weathering depth is maximum in the Birimian
System in granites, porhyrites, felsites and other intrusive rocks,
where it reaches from 90 m to 120 m. Groundwater flow in the region is mainly localised due to numerous low hills that act as
groundwater divides. The rocks underlying the area lack primary
porosity and the groundwater flow is mainly restricted to preferential flow zones along the fissures and joints, quartz veins, and other
intrusives (Kortatsi, 2004).
Clay, silts, sandy clays, and clayey sands are mostly formed as
the result of weathering. In this area two types of aquifers occur.
The aquifer in weathered regolith occurs above the transition zone
between fresh and weathered rock. Due to the soils content of clay
and silt, these aquifers have relatively 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. Yield of shallow wells varies from 0.4 to
18 m3 h1 with an average of 2.4 m3 h1. The depth of wells varies
between 18 m and 75 m with an average of 35.4 m but has little or
no effect on borehole yields. The recharge of groundwater in the
area occurs mainly by direct infiltration. In some places groundwater is in hydraulic contact with rivers and recharge from them can
also take place (Kortatsi, 2004).
The discrete nature of aquifers within the Tarkwa–Prestea area
coupled with the general physiography has given rise to many local
flow systems. The numerous low hill crests form natural groundwater divides (Fig. 3). Groundwater circulation is therefore mainly
restricted within quartz veins and fissured–fault–brecciated zones.
Within the local system, flow is from the highlands towards valleys
and low order streams that drain the basin. Groundwater within
these local systems is likely to be lost by evapotranspiration in discharge zones or by baseflow in surface water drainage.
3. Materials and methods
3.1. Field investigations and groundwater sampling
Forty groundwater samples were collected during the month of
September 2004 which corresponds to the end of rainy season.
Each sampling site was located using a hand-held global positioning system Cobra GPS100. Sampling positions and supplementary
information is shown in Fig. 3. Measured field parameters were redox potential (Eh), electrical conductivity (EC), pH, and temperature using the equipments Ecoscan pH 6, Ecoscan Con 5 and
Hach Sension 2, respectively. The Eh values were corrected with respect to the standard hydrogen electrode (SHE) (Appelo and Postma, 1999). A flow-through cell was used for measurement of field
parameters. Measurements were made until stable readings were
achieved.
Water samples collected for analyses included: (i) filtered
(using Sartorius 0.20 lm online filters) for major anion analyses;
(ii) filtered and acidified with suprapure HNO3 (14 M) for the
P. Bhattacharya et al. / Journal of African Earth Sciences 66–67 (2012) 72–84
75
Fig. 2. Geological map of the Tarkwa–Prestea area (modified from Kortatsi, 2004).
Table 1
Lithological characteristics of the Tarkwaian system (from Kuma and Younger, 2001).
Series
Thickness (m)
Composite lithology
Kawere group
Banket series
Tarkwa phyllite
Huni sandstone
250–700
120–160
120–400
1370
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
analyses of cations and other trace elements including As (Bhattacharya et al., 2002).
3.2. Laboratory analyses
Groundwater alkalinity was determined using an automatic
titration equipment, ABU 80 Autoburette, PHM 82 Standard pH
metre and TTT80 Titrator from Radiometer Copenhagen. The deter2
mination of Cl, NO
3 and SO4 were carried out on the Dionex DX120 Ion Chromatograph equipped with an IonPac As14 column.
þ
The analyses of PO3
4 –P and NH4 –N were determined using Tecator
Aquatec 5400 Analyser and 5027 Auto-sampler, following application notes 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 Royal Institute of Technology, Stockholm,
Sweden. The cations (Ca2+, Mg2+, Na+ and K+) and trace elements
were analysed by inductively coupled plasma (ICP) emission spectrometry using the Varian Vista-PRO Simultaneous ICP-OES
(equipped with SPS-5 autosampler) at the Department of Geology
and Geochemistry at Stockholm University, Sweden. Charge
76
P. Bhattacharya et al. / Journal of African Earth Sciences 66–67 (2012) 72–84
Table 2
Minerals associated with AMD in the study area
(Kortatsi, 2004).
Minerals
Composition
Arsenopyrite
Bournonite
Chalcopyrite
Galena
Pyrite
Sphalerite
Tennalite
FeS2, FeAs, FeAsS
PbCuSbS3
CuFeS2
PbS
FeS2
ZnS
[(Cu, Fe, Zn,)As4S]
balance error was calculated for groundwater samples to
determine the accuracy of chemical analysis. Certified standards,
SLRS-4 (National Research Council, Canada) and GRUMO 3A (VKI,
Denmak) and synthetic multi-element chemical standards were
run following a run of every 10 samples, and background correction was done based on Y and Sc. The precision of analyses based
on measurements of certified standards was typically better than
4%. Dissolved organic carbon (DOC) in the water samples were
analysed in the non-purgeable organic carbon (NPOC) mode on
the Shimadzu 5000 TOC analyser with a detection limit and
precision of 0.5 mg/L and ±10%, respectively.
4. Results and discussion
Results of field parameter measurements and major ions together with the selected contaminants, DOC and NHþ
4 are presented in Tables 3 and 4.
4.1. Field parameters
Average groundwater temperature was 26.6 °C, ranging from
25.4 °C to 28.5 °C. The pH varies from 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. Electric conductivity (EC) ranged from 11.0 lS/cm to 780 lS/cm with an average of 301 lS/cm
(Table 3).
4.2. Major ion characteristics
Table 4 shows concentrations of major ions. In general, the
charge balance errors were less than 5%, except for samples 23
(7.6%) and 37 (11.9%). The major ion composition of the
groundwater samples are presented in Piper diagram in Fig. 4.
Statistical comparison of major ion compositions in groundwater samples from the hand dugwells and drilled wells is shown in
Fig. 5. About 67% of the hand dug wells have groundwater of
Ca–HCO3 type. The rest has groundwater of Ca–Na–HCO3–NO3–Cl
or Na–Ca–HCO3–Cl–NO3 type. For the hand dug wells there is only
one sample with Na as dominating cation. On the other hand about
21% of the drilled wells revealed a Ca–HCO3 type of groundwater
and in general about 25% of samples indicated Na or Mg as dominating cations. This may be caused by calcium replacing sodium
and to some extent magnesium on exchange sites when the
groundwater age increases. These subtle differences in major ion
chemistry are perhaps caused by the differential lithological characteristics of the wells and the groundwater flow pattern (Fig. 3).
Among the major ions, NO
3 exceeding the WHO’s guidelines
Small scale mining sites
Illegal mining (Galamsey) sites
Large scale gold mines
Groundwater divide
Groundwater f low direction
Fig. 3. Location of sampling points and conceptual flow pattern.
77
P. Bhattacharya et al. / Journal of African Earth Sciences 66–67 (2012) 72–84
Table 3
Field parameters and concentrations of selected contaminants.
⁄
No.
Location
Well type*
Temp, °C
pH
Eh, mV
EC, lS/cm
Al, lg/l
As(tot), lg/l
Fe, mg/l
Mn, mg/l
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
Simpa
Dadwen
Dompim
Dompim
Atwerboanda
Odumase
Nsuaem
Aboso
Kokoase
Samahu
Samahu
Yaryeyaw
Suwinso
Gordon
Akotomu
Kofi GyanCamp
Kofi Gyankrom.
Huniano n 1
Tarkwa Banso
Tarkwa Banso
Domeabra
Enyinasie
Enyinasie
Akyem
Akyem
Adieyie
Mile 8
Teberebe
Teberebe
Huniso
Huniso
Abekoase
New Atuabo
Koduakrom
Damang
NewKyekyewere
Huni Valley
Bompieso
Bompieso
Akoon
BH/HP
BH/HP
BH/HP
BH/HP
BH/HP
BH/HP
BH/HP
BH/HP
BH/HP
DW/HP
BH/HP
BH/HP
BH/HP
BH/HP
BH/HP
BH/HP
BH/HP
DW/HP
BH/HP
BH/HP
DW/HP
BH/HP
BH/HP
BH/HP
BH/HP
BH/HP
BH/HP
DW/HP
BH/HP
BH/HP
BH/HP
DW/HP
DW/HP
BH/HP
BH/HP
BH/HP
BH/HP
BH/HP
BH/HP
BH/HP
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
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
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
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
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
<5.2
5.5
<5.2
<5.2
<5.2
<5.2
<5.2
<5.2
<5.2
<5.2
15.6
<5.2
<5.2
<5.2
<5.2
<5.2
<5.2
<5.2
<5.2
<5.2
<5.2
69.4
<5.2
<5.2
<5.2
<5.2
<5.2
<5.2
<5.2
<5.2
<5.2
<5.2
<5.2
<5.2
<5.2
<5.2
<5.2
<5.2
<5.2
<5.2
3.41
7.29
5.20
0.46
0.50
3.37
2.78
4.33
1.14
0.004
0.12
0.005
0.10
0.004
0.47
0.009
0.026
0.017
0.005
0.71
10.59
4.69
2.17
0.25
0.19
0.069
0.076
0.029
0.16
0.014
0.004
0.012
10.84
0.10
0.004
0.001
0.004
0.28
0.48
0.006
0.92
0.31
0.29
0.24
0.41
0.57
0.85
0.85
0.25
0.09
0.57
0.008
0.27
0.24
0.64
0.093
0.25
0.062
0.006
0.45
0.063
0.93
0.61
0.42
0.50
0.29
0.97
0.075
0.92
0.68
1.07
0.093
0.29
0.39
0.04
0.04
0.04
0.71
0.60
2.04
Abbreviations: BH: Borehole; DW: Dugwell; HP: Handpump.
(Table 4), occurs at two locations and might be of principal health
concern for drinking water supplies.
Statistical comparison of the major ion chemistry of the groundwater samples from shallow and deep wells is in Fig. 5. There are
higher pH values (Fig. 6a) and lower Eh values (Fig. 6b) for groundwater from deep wells. This is consistent with longer residence
times of groundwater in deep wells with resulting advanced neutralisation in reactions with carbonates and silicates and consumption of electron acceptors such as oxygen. Longer residence time in
deep wells also results in higher EC values (Fig. 6c) and higher
bicarbonate concentrations (Fig. 6d). Concentrations of dissolved
Fe and Mn (Fig. 7) are also higher in deep wells as can be expected
due to their increased mobility under more reducing conditions.
4.3. Trace element characteristics
Statistical characteristics for dissolved Al and As(tot) are shown
in Fig. 7. A total of 17 wells have higher metal content than WHO
guidelines concerning As(tot), Mn, Fe and Al. Total As [As(tot)] exceeds the drinking water guidelines at two locations, Samahu,
15.6 lg/l and Eyinaise, 69.4 lg/l (Table 5). Manganese is the major
contaminant and among the 17 wells with high concentrations of
metals, 14 has elevated Mn-levels. Iron exceeds the guideline in seven wells and Al is exceeding the guideline at two locations, Huniso and Akoon. These wells also display the two lowest measured
pH values, 4.44 and 4.19, respectively. Nitrate is exceeding the
guidelines in the same two wells in Huniso and Akoon. The wells
with elevated metal content are presented in Table 5. All the wells
with metal concentrations exceeding WHO guidelines are boreholes except New Atuabo which is a hand dug well.
Both Mn and Fe show similarity in distribution pattern, almost
all areas with Fe concentrations (Fig. 9c) above WHO guidelines
also have high Mn concentrations (Fig. 9d). However, there is no
visible trend between these parameters when plotted (R2 = 0.09)
(not shown). For Fe and SO2
it is difficult to see any trend
4
(Fig. 8a), but there is a positive trend for Mn and SO2
(Fig. 8b).
4
The trend between Mn and SO2
4 could originate from dissolution
of carbonate minerals like kutnohorite, Ca(Fe, Mn)(CO3)2, during
neutralisation of AMD. For certain areas such as Akoon, New Atuabo, Enyinasie, and Dadwen high concentrations of both Fe and
SO2
4 and low pH values indicate the impact of 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
100 m from sampled well. In Aboso, high concentration of NHþ
4
and relatively low concentration of SO2
may indicate presence
4
of other sources of pollution than acid mine drainage. Precipitation
of Fe-oxyhydroxides can explain low correlation between Fe and
SO2
4 . Oxidation of Fe(II) and precipitation of Fe-oxyhydroxides occurs at lower redox level than oxidation of Mn(II) and precipitation
of Mn-oxyhydroxides and, thus, Mn remains dissolved even under
relatively oxidising conditions, when most of Fe has already precipitated (Drever, 1997). However, many samples display both
low Fe and SO2
values, and thus, they are not affected by acid
4
mine drainage.
78
Table 4
Concentrations of major ions, the water types, dissolved organic carbon (DOC) and ammonium.
No.
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
HCO
3
Cl
NO
3
mg/l
mg/l
mg/l
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
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
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
66.35
SO2
4
mg/l
PO3
4
mg/l
Ca2+
Mg2+
Na+
K+
mg/l
mg/l
mg/l
mg/l
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
0.30
1.08
0.75
0.48
0.05
0.06
0
0.48
0.31
0
0.04
0.16
0.22
0.30
0.24
0.22
0.09
0.09
0.17
0.33
0
0.41
0.30
0.10
0.20
0.08
0
0.04
0.04
0.61
0.04
0
0
0
0.13
0.21
0.25
0.11
0.09
0.06
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
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
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
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
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
mg/l
NHþ
4
lg/l
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
84.02
0
0
0
0
0
0
26.2
0
0
0
1.2
0
1.7
0
0
0
0
0
0
0
0
0
0
24.1
0
0
0
0
0
0
0
261.8
0
0
0
0
0
0
28.3
DOC
P. Bhattacharya et al. / Journal of African Earth Sciences 66–67 (2012) 72–84
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
Location
79
P. Bhattacharya et al. / Journal of African Earth Sciences 66–67 (2012) 72–84
metals and pH values. All selected samples are from drilled wells
located in the following geological formations:
Legend
80
80
Deep wells
Shallow wells
60
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
60
40
40
20
20
Mg
SO4
80
80
60
60
40
40
20
Ca
Na+K
80
60
40
20
40
60
80
20
20
Cl
HCO3
Fig. 4. Piper diagram showing composition of groundwater in the deep (bore) and
the shallow (dug) wells.
Iron shows only a weak negative correlation with Ca (Fig. 8c).
This is consistent with expected removal of Fe during neutralisation of AMD by carbonates, which are the source of Ca. Manganese
shows strong positive correlations with Ca (Fig. 8d). Sample 42
(Akoon) shows a strongly deviant value. If this value is excluded
the R2-value increases to 0.64. This supports the hypothesis about
Mn-carbonate as a source of Mn.
Ca shows a strong correlation with pH and HCO
3 shown in
Fig. 8e and f, respectively. Samples 32 and 42 were not included
due to probable acid mine drainage impact. This indicates that
CaCO3 is the source of Ca.
Spatial distribution based on kriging of selected species concentrations and pH is shown in Fig. 9a–e. Based on the comparison
with geological map (Fig. 2), it can be seen that Fe, Mn, As and
SO2
concentrations do not exhibit any major differences in the
4
pattern of distribution in the Tarkwaian and Birimain system of
rocks. Concentrations of Al exceeding WHO guidelines are found
locally in the Tarkwaian system (Fig. 9a). This is most likely a result
of the low pH in two wells (Fig. 9d) and is not related to the differences in geology between the two systems.
4.4. Geochemical modelling
Samples 13, 24, 32 and 42 were selected for geochemical speciation modelling. This choice was based on their concentrations of
10,00
10,00
a
Concentrations (meq/l)
Table 6 shows the saturation indices (SI) for selected mineral
phases. For most minerals only the results in the interval
(2 < SI < 2) are shown. For reactive minerals, as calcite, dolomite and gypsum, SI values are presented regardless of their
magnitude.
The issue of principal interest was the precipitation and stability of oxides and hydroxides of Fe, Al, and Mn. Groundwater was
generally supersaturated with respect to Al/Fe-oxides and hydroxides and different silicate minerals. Various minerals containing Al
such as oxides/hydroxides can precipitate, but precipitation of
bohemite, diaspore, and kaolinite is generally kinetically constrained (Appelo and Postma, 1999). Bohemite and diaspore are
rather formed by re-crystallization of precipitated amorphous
Al(OH)3, but ground water is undersaturated with respect to this
mineral phase. However, ground water is supersaturated with respect to some Fe oxides/hydroxides. For example, goethite can precipitate, providing sites for adsorption. Groundwater is
undersaturated with respect to Mn oxides and hydroxides and
their precipitation is therefore unlikely.
Minerals like siderite, vivanite, and rhodochrosite are sinks for
dissolved Fe and Mn and their precipitation can disturb correlation
between Fe, Mn, and As (Sracek et al., 2004; Hasan et al., 2007,
2009; von Brömssen et al., 2008; Bhattacharya et al., 2009). However, groundwater from selected wells is undersaturated with respect to these minerals.
Saturation indices (SIs) show that groundwater is undersaturated with respect to most potential secondary minerals. This suggests that the groundwater has short residence time and natural
equilibrium with these minerals is not reached. In large-scale mining lime is used, but groundwater is undersaturated with respect to
calcite.
b
Legend
Max.
75 percentile
Median
1,00
1,00
25 percentile
Min.
0,10
0,10
0,01
0,01
Ca Mg Na K HCO3 Cl SO4 NO3
Ca Mg Na K HCO3 Cl SO4 NO3
Fig. 5. Major ion compositions in groundwater samples from (a) deep bore wells and (b) shallow dug wells. Note the concentration units are in meq/l.
80
P. Bhattacharya et al. / Journal of African Earth Sciences 66–67 (2012) 72–84
7.0
600
a
b
500
5.8
400
pH
Eh (mV)
6.4
5.2
4.6
300
200
4.0
100
Deep wells
Shallow wells
Deep wells
Shallow wells
400
800
c
d
Max.
75 percentile
320
640
HCO3- (mg/l)
SEC (µS/cm)
Median
480
320
25 percentile
240
160
160
80
0
0
Deep wells
Shallow wells
Deep wells
Fig. 6. Box plots of selected parameters (a) pH, (b) Eh, (c) SEC, and (d)
Concentration (µg/l)
1000
100
10
Shallow wells
for deep and shallow wells.
5. Discussion and conclusions
a
10000
Legend
Max.
75 percentile
Concentration (µg/l)
HCO
3
Groundwater is generally undersaturated with respect to the
minerals containing sulphates. Only in well 32 and 42 water samples were found to be supersaturated with respect to Al-sulphate
mineral (alunite). These wells have very low pH values and this
indicates an impact of acid mine drainage. In wells with low pH,
ground water is supersaturated with respect to some silicate minerals, which could provide a sink for Al. The dominant aqueous
species are Fe2+, Mn2+, Al3+ and H2 AsO
4.
Arsenic is present as oxidised anionic species as As(V), which is
more adsorbed than As(III) under the observed pH conditions
(Bhattacharya et al., 2002; Smedley and Kinniburgh, 2002).
10000
1
Min.
1000
Median
25 percentile
Min.
100
10
b
1
Fe
Mn
Al
As(tot)
Fig. 7. Distribution of Fe, Mn, Al and As(tot) in (a) deep (bore) wells and (b) shallow
(dug) wells.
The metal concentrations in the study area are generally lower
than expected on the basis of large scale mining activities. The
intensive mining industry and the study by Kortatsi (2004) in a
nearby area indicated higher values (values above WHO guidelines
for Al, As, Cd, Cr, Fe, Mn, Ni, Pb and Zn). The groundwaters in some
wells in the study area have values of Mn, Fe, As, and Al exceeding
the WHO 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. The groundwater quality is
therefore better than expected. The groundwaters generally have
neutral to acidic pH and are oxidising. The dominant major ions
are calcium, sodium, and bicarbonate.
Mining activities probably affect the groundwater through acid
mine drainage (AMD) in areas where high concentrations of Fe and
SO2
4 and low pH coincide. Principal areas affected by AMD are New
Atuabo, Akoon, Teberebe, Huniano no 1, Dadwen and Aboso. The
occurrence of As at three sampled sites is most probably of natural
origin and is not considered as a major problem. The rest of the
metals exceeding the guidelines are all components of common
81
P. Bhattacharya et al. / Journal of African Earth Sciences 66–67 (2012) 72–84
Table 5
Samples exceeding WHO guideline values for safe drinking purposes (WHO, 1996, 2011a).
No.
Location
WHO guideline
value
As(tot)
10.0
(lg/l)
Fe
3.0
(mg/l)
3
4
5
8
9
10
13
17
24
25
29
32
33
35
40
41
42
Simpa
Dadwen
Dompim
Odumase
Nsuaem
Aboso
Samahu
Akotomu
Eyinaise
Eyinaise
Mile 8
Huniso
Huniso
New Atuabo
Bompieso
Bompieso
Akoon
bdl
bdl
bdl
bdl
bdl
bdl
15.6
bdl
69.4
bdl
bdl
bdl
bdl
bdl
bdl
bdl
bdl
3.41
7.29
5.20
3.37
Mn
0.4
(mg/l)*
Al
0.2
(mg/l)
NO
3
50
(mg/l)
0.916
0.567
0.851
0.850
0.565
0.640
0.926
0.605
0.968
0.676
1.07
4.33
4.69
2.18
146
10.8
0.713
0.602
2.04
0.594
66.4
*
WHO (2011a) revised the guideline value. There is no specific drinking water guideline for Mn with a motivation that Mn levels found in groundwater sources is not of
health concern (WHO, 2011b).
2.5
12
a
2
8
Mn (mg/l)
Fe (mg/l)
b
y = 0.026x + 0.8951
R2 = 0.0324
10
6
y = 0.0157x + 0.2706
R2 = 0.401
1.5
1
4
0.5
2
0
0
0
20
40
SO4
60
0
80
60
80
2- (mg/l)
2.5
c
d
y = -0.0064x + 1.3815
R 2 = 0.005
2
8
Mn (mg/l)
Fe (mg/l)
40
SO4
12
10
20
2- (mg/l)
6
y = 0.0087x + 0.1347
R 2 = 0.3137
1.5
1
4
0.5
2
0
0
50
0
100
0
50
Ca (mg/l)
120
120
e
y = 41.31x - 216.97
R2 = 0.6262
80
60
y = 0.2436x - 0.9703
R2 = 0.8527
80
60
40
40
20
20
0
4.50
f
100
Ca (mg/l)
Ca (mg/l)
100
100
Ca (mg/l)
0
5.00
5.50
pH
6.00
6.50
7.00
0.0
100.0
200.0
300.0
400.0
HCO 3- (mg/l)
2
Fig. 8. Bivariate plots showing correlations of: (a) Fe vs SO2
4 , (b) Mn vs SO4 , (c) Fe vs Ca, (d) Mn vs Ca, (e) Ca vs pH, and (f) Ca vs HCO3 in groundwater from the investigated wells.
82
P. Bhattacharya et al. / Journal of African Earth Sciences 66–67 (2012) 72–84
Damang
New Kyekyewere
5.5
Al
Damang
New Kyekyewere
5.5
Yaryeyaw
Huni Valley
Suwinso
Akotomu
Gordon
5.45
2100
2000
1900
1800
1700
1600
1500
1400
1300
1200
1100
1000
900
800
700
600
500
400
300
200
100
0
-100
-200
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
5.25
Teberebe
Akyem
Adieyie
5.2
5.15
Dompim
Damang
New Kyekyewere
Huniano no 1
Huniso
Samahu
Aboso
Kokoase
Nsuaem
Odumase
Atwerboanda
New Atuabo
Akoon Domeabra
5.35
5.3
Enyinasie
Tarkwa Banso
5.25
Teberebe
Akyem
Adieyie
5.2
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
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
5.25
Teberebe
Adieyie
Akyem
5.2
5.15
d
Dompim
Dadwen
-2.1 -2.05 -2
2100
2000
1900
1800
1700
1600
1500
1400
1300
1200
1100
1000
900
800
700
600
500
400
300
200
100
0
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
5.25
Teberebe
Adieyie
Akyem
5.2
5.15
b
Dompim
c
Dompim
Dadwen
Damang
New Kyekyewere
5.5
Yaryeyaw
Gordon
SO4265
Kofi Gyan Camp
Kofi Gyankrom
Bompieso
5.4
-1.95 -1.9 -1.85
Huni Valley
Suwinso
Akotomu
5.45
-2.1 -2.05 -2
-1.95 -1.9 -1.85
-2.1 -2.05 -2
pH
Mn
Huni Valley
Suwinso
Akotomu
Gordon
5.45
Dadwen
Huni Valley
Suwinso
Akotomu
10000
9500
9000
8500
8000
7500
7000
6500
6000
5500
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
0
-500
Kofi Gyan Camp
Kofi Gyankrom
Bompieso
Yaryeyaw
Gordon
Huni Valley
Gordon
5.4
-1.95 -1.9 -1.85
5.5
5.45
Damang
New Kyekyewere
5.5
Yaryeyaw
Suwinso
Akotomu
5.45
5.15
a
Dadwen
-2.1 -2.05 -2
Fe
Yaryeyaw
60
55
Huniano no 1
50
Huniso
Samahu
45
Aboso
Kokoase
Nsuaem
Odumase
Atwerboanda
New Atuabo
Akoon Domeabra
5.35
5.3
35
30
25
Enyinasie
Tarkwa Banso
5.25
40
20
Teberebe
15
Akyem
Adieyie
10
5
5.2
0
-5
5.15
-1.95 -1.9 -1.85
e
Dompim
Dadwen
-2.1 -2.05 -2
Fig. 9. Concentration of (a) Al (lg/l), (b) Fe (lg/l), (c) Mn (lg/l), (d) pH and (e)
values (WHO, 1996).
SO2
4
-1.95 -1.9 -1.85
(mg/l) in the deep wells within the study area. Bold lines indicate drinking water guideline
Table 6
Result of speciation modelling with PHREEQC.
Sample 13
Sample 24
Sample 32
Sample 42
Phase
SI
Phase
SI
Phase
SI
Phase
SI
Al(OH)3(a)
Barite
Barite
Boehmite
Calcite
Chalcedony
CO2(g)
Diaspore
Dolomite
Fe3(OH)8
Ferrihydrite
Gibbsite(C)
Goethite
Gypsum
Hydroxyapatite
Magnesite
MnHPO4(C)
Pyrolusite
Quartz
Rhodochrosite
Siderite
SiO2(am)
Strengite
Strontianite
Vivianite
1.13
0.72
0.72
0.67
0.23
0.23
1.35
2.37
0.96
0.96
0.79
0.47
5.22
3.15
0.21
1.23
1.31
11.2
0.71
0.52
1.05
0.58
0.57
1.21
5.63
Al(OH)3(a)
Alunite
Anhydrite
Barite
Boehmite
Calcite
Chalcedony
CO2(g)
Dolomite
Diaspore
Fe3(OH)8
Ferrihydrite
Gibbsite(C)
Goethite
Gypsum
Magnesite
Manganite
MnHPO4(C)
Pyrolusite
Quartz
Rhodochrosite
Siderite
SiO2(am)
Strengite
Vivianite
1.05
0.64
1.81
0.02
0.76
1.48
0.28
0.87
3.07
2.45
1.56
0.5
0.54
4.97
1.63
2.07
7.75
1.97
13.2
0.75
1.43
0.57
0.53
1.19
1.57
Al(OH)3(a)
AlOHSO4
Barite
Boehmite
Chalcedony
Diaspore
Ferrihydrite
Gibbsite(C)
Goethite
Gypsum
Halloysite
Lepidocrocite
Manganite
MnHPO4(C)
Montmorillonite
Pyrolusite
Quartz
SiO2(am)
Strengite
Vivianite
1.5
1.11
0.26
0.3
0.07
1.99
2.58
0.09
1.87
2.77
1.55
0.94
8.38
0.56
1.79
11.1
0.41
0.88
1.02
17.3
Al(OH)3(a)
AlOHSO4
Alunite
Anhydrite
Barite
Boehmite
Chalcedony
Diaspore
Ferrihydrite
Gibbsite(C)
Goethite
Gypsum
Halloysite
Kaolinite
Lepidocrocite
Manganite
MnHPO4(C)
Quartz
SiO2(am)
Vivianite
2.78
1.1
0.75
2.19
0.25
0.97
0.48
0.72
4.25
1.19
0.21
2
1.81
1.44
0.73
9.2
0.31
0
1.29
19.3
P. Bhattacharya et al. / Journal of African Earth Sciences 66–67 (2012) 72–84
minerals and they probably origin from natural processes. For Al
there is a strong correlation with pH since it precipitates as amorphous Al(OH)3 at pH higher than 4.0 (Appelo and Postma, 1999). In
strongly alkaline environment Al becomes mobile again because is
present as AlðOHÞ
4 , but such high pH values are not found in the
study area. Concentrations of Mn and Fe are inversely correlated
to Eh, which determines their oxidation state and, thus, their
mobility.
There are a number of reasons that may explain relatively low
observed dissolved metal concentrations:
Adsorption processes are probably very important and can considerably lower the metal concentrations in groundwater. All
soil types in the Tarkwaian system are clayey and the soils of
the Birimian system most likely have the same composition.
The presence of clay minerals and abundance of Al/Fe oxides/
hydroxides like goethite and montmorillonite in the soils provide significant sites for sorption. Heavy metals as Cu, Pb, Hg,
and Cd are strongly bounded to these sites and this explains
their low dissolved concentrations. The pH is the most important parameters concerning metal mobility. Value of pHZPC for
goethite is 6.0–7.0 compared to much lower value about 2.5
for montmorillonite (Drever, 1997). However, ferric minerals
are more efficient adsorbents of heavy metals than clays. In this
study, among all investigated species, only Al and Ca are found
to be strongly correlated with pH.
The area is very hilly and there are several groundwater divides
(Fig. 3). This gives rise to multiple local groundwater systems
with short groundwater residence times. This is consistent with
negative saturation indices for many potential secondary minerals. The local groundwater systems also prevent mining to
affect larger groundwater systems on a regional scale. However,
there is a possibility that at some sites local mining pollutants
have not reached yet the wells and the groundwater quality
in some wells might deteriorate in near 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 groundwater.
It could explain lower values compared to the results of Kortatsi
(2004) who sampled groundwater during the whole year.
The differences in HCO3 concentrations, electrical conductivity
and Eh values between the shallow and deep wells may be explained by longer residence time of groundwater in the deep wells.
The metals and metalloids exceeding WHO guidelines are Al, As,
Fe, and Mn. Arsenic was detected in only three wells and exceeded
the guideline in two wells. Elevated levels of Mn, Fe and Al are relatively easy to treat. In the most affected areas aeration and adjustment of pH should improve the drinking water quality.
For further investigations in the study area more information
about the depth of the wells, groundwater flow pattern, location
of small-scale mining activities and detailed geological information
of the sampling positions would provide a better understanding of
the processes governing the groundwater quality. This would make
a more comprehensive assessment of the groundwater vulnerability concerning mining pollution possible. Although results of this
study generally indicate good drinking water quality, contamination of groundwater from mining activities have been found at
some locations and further contamination is possible.
Acknowledgements
We acknowledge the financial support provided by the Swedish
International Development Cooperation Agency (Sida-SAREC) for
the research project on Contamination of Water Resources in
Tarkwa Mining Area of Ghana (SWE-2003-245). We thank Ann
83
Fylkner and Monica Löwen of the Department of Land and Water
Resources Engineering (KTH) for chemical analyses. BE and RA
acknowledge the financial support provided by the International
Programmes Office (IPK), Stockholm and the Swedish International
Development Cooperation Agency (Sida) in the form of Minor Field
Study grant during 2004. The help received from E. Kumo during
the field work carried out in the Tarkwa mining area during
August–October, 2004 is deeply appreciated. We are thankful to
the anonymous reviewer and the editors of the journal for their
thoughtful comments on an earlier version of this manuscript.
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