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 partof 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, Swedenb Department of Geology, Faculty of Science, Palacky University, 17 listopadu 12, 771 46 Olomouc, Czech Republicc OPV s.r.o. (Protection of Groundwater Ltd.), B�elohorská 31, 169 00 Praha 6, Czech Republicd Department of Engineering Geology, Technical University of Lund (LTH), Box 118, SE, 221 00 Lund, Swedenf Department of Geography and Development, University of Ghana at Legon, Accra, Ghanag Water Management Research Group, Department of Land and Water Resources Engineering, Royal Institute of Technology (KTH), SE, 100 44 Stockholm, Sweden

a r t i c l e i n f o a b s t r a c t

Article history:Received 27 May 2011Received in revised form 12 March 2012Accepted 15 March 2012Available online 24 March 2012

Keywords:GroundwaterMiningHydrogeochemistryMetal pollutionArsenicTarkwa

1464-343X/$ - see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.jafrearsci.2012.03.005

⇑ Corresponding author. Tel.: +46 8 790 7399; fax:E-mail address: prosun@kth.se (P. Bhattacharya).

The aim of this study was to investigate the groundwater chemistry with special concern to metal pol-lution in selected communities in the Wassa West district, Ghana. In this mining area, 40 ground watersamples, mainly from drilled wells, were collected. The groundwaters have generally from neutral toacidic 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 typ-ically found in mining regions. Only 17 wells show metal concentrations exceeding WHO guidelines for atleast one metal. The main contaminants are manganese and iron, but arsenic and aluminium also exceedthe guidelines in some wells probably affected by acid mine drainage (AMD). Metal concentrations in thegroundwater seem to be controlled by the adsorption processes. Hydrogeochemical modelling indicatessupersaturation 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 ingroundwater. The area is hilly, with many groundwater flow divides that result in several local flow sys-tems. The aquifers therefore are not strongly affected by weathering of minerals due to short groundwa-ter residence times and intense flushing. The local character of groundwater flow systems also prevents astrong impact of acid mine drainage on groundwater systems in a regional scale.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

West Africa has been one of the world’s most important goldmining regions for centuries. Today the most significant gold pro-ducing country in the area is Ghana (Hilson, 2002a). The earliestEuropean attempts to extract gold on a large scale were concen-trated in Tarkwa and Prestea regions in the late 19th century. Agold rush in the early 20th century was followed by a mass in-crease in gold production. After Ghana gained independence1957 the industry collapsed and reached a 50-year low in 1982.In 1983 the government started the Economic RecoveryProgramme (ERP) under guidance of WHO. After this the miningindustry has seen a phenomenal growth and the gold productionhas increased by 700% (Hilson, 2002a).

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+46 8 790 6857.

Both small-scale miners and large-scale mining are currentlyoperating in Ghana and about 237 (154 Ghanaian and 83 foreign)enterprises are prospecting for gold and another 18 are operatinggold mines (Hilson, 2002a,b). Large-scale mining in Tarkwa regionis conducted as surface mining. Cyanidation is the most commontechnique in the region and is used for treatment of non-sulphidicpalaeoplacer ore (Akosa et al., 2002; Kortatsi, 2004). The manage-ment of waste from large scale mining is done in accordance to ap-proved environmental plans. The waste rock heaps are stabilisedand re-vegetated. Tailing slurries are channelled into tailing damsthat also are re-vegetated. Reagent containers and packing materi-als are sold out to contractors for further disposal, however, themonitoring of these activities is poor. Small-scale mining in Ghanais defined as ‘‘mining by any method not involving substantialexpenditure by any individual or group of persons not exceedingnine in number or by a co-operative society made up of ten ormore 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 73

the rivers. It is practised in about 20,000 small-scale mines in theWassa West district through out the year. Among these small-scaleminers about 90% are illegal. Currently, 168 small-scale miningconcessions are valid in the region (Asklund and Eldvall, 2005;Balfors et al., 2007).

The general ore processing techniques are handpicking, amal-gamation, cyanidation, flotation, electroextraction, and roasting ofore (Akosa et al., 2002). The technique differs between large- andsmall-scale mining and also varies depending on the type of depositand its location (Ntibery et al., 2003). The area has three main golddeposits. Placer or alluvial deposit, non-sulphidic paleoplacer or freemilling ore and oxidised ore (Kortatsi, 2004).

This study is focused on an area in southwestern Ghana that hasa long history of mining activities where groundwater serves as themain source of drinking water supply for local population. Mostmajor towns in the area except Tarkwa rely solely on groundwater.To match the demand for potable water the number of boreholesand hand dug wells is increasing rapidly (Kortatsi, 2004). Thereare apprehensions that the mining activity is causing serious metalpollution to the water resources by contaminants such as arsenic,lead, cadmium, mercury, and cyanide. Earlier studies have shownthat metal levels in groundwater exceed WHO guidelines for drink-ing water in many areas in western Ghana (Kortatsi, 2004; Kuma,2004).

Estimated 5 tonnes of mercury (Hg) are released from small-scale mining operations in Ghana each year (Hilson, 2001). Highconcentrations of Hg have been found in sediments and fish inthe vicinity of small-scale mining activities using amalgamationas the main technique. The concentration in most fish fillets inthese areas exceeds the recommendations of the United StatesFood and Drug Agency (Babut et al., 2003). In general, the manage-ment of waste in small-scale mines, particularly the illegal ones,lacks waste management plan and simply leave the waste. Addi-tionally, mining has led to conflicts among communities, displacedby mining operations, and health and social problems, pollution ofthe community water sources, and depletion of groundwater re-sources (Fonseca, 2004).

Groundwater in mining areas as the Tarkwa–Prestea area isknown to be vulnerable to pollution from mining that may havea serious effect on human health. In gold mining areas sulphidesoxidation leads to low pH in the groundwater that encouragesthe mobility of trace metals which are found in the groundwaterin very high concentrations (Kortatsi, 2004). In study of Asanteet al. (2007) groundwater As was compared with urinary As levelsof local residents in Tarkwa and no difference was found comparedwith a control group from Accra. Nevertheless, urine levels werehigh and the authors suggested a presence of undetected sourcesof As in Ghana.

The aim of this study was to investigate the salient hydrogeo-chemical characteristics with special emphasis on metal pollutionin the water resources that are used by the local communities inthe Tarkwa mining area. The outcome of the study will be usedto assess the vulnerability of shallow groundwater quality due tonatural geochemical environment and to distinguish it from min-ing pollution of the groundwater resources specifically in the re-gion around the Tarkwa mining area.

2. Geology and hydrogeology

2.1. The study area

The Wassa West district occupies the mid-southern part of theWestern region of Ghana with Tarkwa as its administrative capital.The population of the district is approximately 236,000. Mining isthe main industrial activity in the area (Avotri et al., 2002). The area

lies within the main gold belt of Ghana that stretches from Axin inthe southwest, to Konongo in the northeast (Kortatsi, 2004). Loca-tion 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 sea-sonal weather patterns. The Wassa West district is situated atthe border of two climatic regions. The south part belongs to thesouth western equatorial climatic region and the northern parthas a wet semi-equatorial climate (Dickson and Benneh, 1980).The area is characterised by double wet season during the monthsof April–June and October–November. The first and largest peakoccurs in June, whilst the second and smaller peak occurs in Octo-ber. Around 53% of all rain in the region falls between March andJuly. The mean annual rainfall is approximately 1874 mm withmax and min values of 1449 mm and 2608 mm, respectively. Thearea 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 rainwater 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 varietyof Precambrian igneous and metamorphic rock comprising theBasement Complex and covers about 54% of the country, mainlythe southern and western parts (Fig. 2).

The geomorphology of the Tarkwa–Prestea area consists of aseries of ridges and valleys parallel to each other and to the strikeof the rocks. The strike of the rock are generally in north–southdirection (Kortatsi, 2004). Both the Tarkwaian and Birimian sys-tems are folded along axes that trend northeast (Gyau-Boakyeand Dapahh-Siakwan, 2000). The general type of topography re-flects underlying geology (Kortatsi, 2004). The soil in the Tarkwaarea 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 provincesincluding the metamorphosed and folded rocks of the Birimianand Tarwaian system (Gyau-Boakye and Dapahh-Siakwan, 2000)with gneiss, phyllites, schists, migmatites, granite-gneiss and quar-tites as the predominant lithology (Fig. 2). The lithology of the Tar-kwaian System is characterised by a sequence of metasedimentscomprising quartzites, grits, phyllites and conglomerates of the Ka-were Group, a predominant quartzite, grit, conglomerate sequenceof the Banket Series, Tarkwa phyllites and Huni Sandstones, gritsand quartizes with bands of phyllites (Table 1). In several placesthese systems are intruded by sills and dykes of igneous rocks rang-ing from felsite and quartz porphyry to metadolerite, gabbro andnorite (Kortatsi, 2004). The rest of the country is underlain by Palae-ozoic sedimentary rocks referred to as the Voltaian Formation con-sisting mainly of sandstones, shale, mudstone, sandy and pebblybeds and limestones (Gyau-Boakye and Dapaah-Siakwan, 1999).

Sulphide minerals, like arsenopyrite are widely reported inGhana. There is a close association between sulphide minerals,especially arsenopyrite, and gold in most parts of Ghana(Dzigbodi-Adjimah, 1993; Smedley, 1996). The problems associ-ated with AMD can therefore be expected in many gold miningareas in Ghana. Acid mine drainage (AMD) has been reported froma number of mines in the Tarkwa–Prestea area of southwesternGhana (Kortatsi, 2004). Monitoring of a large spoil dumps in theTarkwa area show water quality consistent with AMD characteris-tics. The pH is consistently below 4, the outflow from the wastedumps has high concentrations of sulphate, silica, aluminium, iron,and manganese, and shows little variation during year (Kuma,

Fig. 1. Location of the study area and simplified regional geological map of southwest Ghana (modified from Kuma, 2004).

74 P. Bhattacharya et al. / Journal of African Earth Sciences 66–67 (2012) 72–84

2003). The major minerals associated with AMD that occurs in theTarkwa–Prestea area are shown in Table 2.

2.4. Hydrogeology

In the Tarkwa–Prestea area groundwater occurrence is associ-ated with the development of secondary porosity through fissuringand weathering. The weathering depth is maximum in the BirimianSystem in granites, porhyrites, felsites and other intrusive rocks,where it reaches from 90 m to 120 m. Groundwater flow in the re-gion is mainly localised due to numerous low hills that act asgroundwater divides. The rocks underlying the area lack primaryporosity and the groundwater flow is mainly restricted to preferen-tial flow zones along the fissures and joints, quartz veins, and otherintrusives (Kortatsi, 2004).

Clay, silts, sandy clays, and clayey sands are mostly formed asthe result of weathering. In this area two types of aquifers occur.The aquifer in weathered regolith occurs above the transition zonebetween fresh and weathered rock. Due to the soils content of clayand silt, these aquifers have relatively high porosity and storage,but low permeability. The aquifer in the fractured/fissured zone oc-curs below the transition zone. They have relatively high transmis-sivity, but low storage. Yield of shallow wells varies from 0.4 to18 m3 h�1 with an average of 2.4 m3 h�1. The depth of wells variesbetween 18 m and 75 m with an average of 35.4 m but has little orno effect on borehole yields. The recharge of groundwater in thearea occurs mainly by direct infiltration. In some places groundwa-ter is in hydraulic contact with rivers and recharge from them canalso take place (Kortatsi, 2004).

The discrete nature of aquifers within the Tarkwa–Prestea areacoupled with the general physiography has given rise to many localflow systems. The numerous low hill crests form natural ground-water divides (Fig. 3). Groundwater circulation is therefore mainlyrestricted within quartz veins and fissured–fault–brecciated zones.Within the local system, flow is from the highlands towards valleysand low order streams that drain the basin. Groundwater withinthese local systems is likely to be lost by evapotranspiration in dis-charge 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 ofSeptember 2004 which corresponds to the end of rainy season.Each sampling site was located using a hand-held global position-ing system Cobra GPS100. Sampling positions and supplementaryinformation is shown in Fig. 3. Measured field parameters were re-dox potential (Eh), electrical conductivity (EC), pH, and tempera-ture using the equipments Ecoscan pH 6, Ecoscan Con 5 andHach Sension 2, respectively. The Eh values were corrected with re-spect to the standard hydrogen electrode (SHE) (Appelo and Post-ma, 1999). A flow-through cell was used for measurement of fieldparameters. Measurements were made until stable readings wereachieved.

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

Fig. 2. Geological map of the Tarkwa–Prestea area (modified from Kortatsi, 2004).

Table 1Lithological characteristics of the Tarkwaian system (from Kuma and Younger, 2001).

Series Thickness (m) Composite lithology

Kawere group 250–700 Quartzites, grits, phyllites and conglomeratesBanket series 120–160 Tarkwa phyllite transitional beds and sandstones, quartzites, grits breccias and conglomeratesTarkwa phyllite 120–400 Huni sandstone transitional beds, and greenish-grey phyllites and schistsHuni sandstone 1370 Sandstones, grits and quartizes with bands of phyllite

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analyses of cations and other trace elements including As (Bhat-tacharya et al., 2002).

3.2. Laboratory analyses

Groundwater alkalinity was determined using an automatictitration equipment, ABU 80 Autoburette, PHM 82 Standard pHmetre and TTT80 Titrator from Radiometer Copenhagen. The deter-mination of Cl�, NO�3 and SO2�

4 were carried out on the Dionex DX-120 Ion Chromatograph equipped with an IonPac As14 column.

The analyses of PO3�4 –P and NHþ4 –N were determined using Tecator

Aquatec 5400 Analyser and 5027 Auto-sampler, following applica-tion notes ASN 140-01/90 and ASN 146-01/90. These analyses wereperformed at the laboratory of the Department of Land and WaterResource Engineering at Royal Institute of Technology, Stockholm,Sweden. The cations (Ca2+, Mg2+, Na+ and K+) and trace elementswere analysed by inductively coupled plasma (ICP) emission spec-trometry using the Varian Vista-PRO Simultaneous ICP-OES(equipped with SPS-5 autosampler) at the Department of Geologyand Geochemistry at Stockholm University, Sweden. Charge

Table 2Minerals associated with AMD in the study area(Kortatsi, 2004).

Minerals Composition

Arsenopyrite FeS2, FeAs, FeAsSBournonite PbCuSbS3

Chalcopyrite CuFeS2

Galena PbSPyrite FeS2

Sphalerite ZnSTennalite [(Cu, Fe, Zn,)As4S]

76 P. Bhattacharya et al. / Journal of African Earth Sciences 66–67 (2012) 72–84

balance error was calculated for groundwater samples todetermine the accuracy of chemical analysis. Certified standards,SLRS-4 (National Research Council, Canada) and GRUMO 3A (VKI,Denmak) and synthetic multi-element chemical standards wererun following a run of every 10 samples, and background correc-tion was done based on Y and Sc. The precision of analyses basedon measurements of certified standards was typically better than4%. Dissolved organic carbon (DOC) in the water samples wereanalysed in the non-purgeable organic carbon (NPOC) mode onthe Shimadzu 5000 TOC analyser with a detection limit andprecision of 0.5 mg/L and ±10%, respectively.

4. Results and discussion

Results of field parameter measurements and major ions to-gether with the selected contaminants, DOC and NHþ4 are pre-sented in Tables 3 and 4.

Fig. 3. Location of sampling points

4.1. Field parameters

Average groundwater temperature was 26.6 �C, ranging from25.4 �C to 28.5 �C. The pH varies from 4.19 to 6.92 with an averageof 5.38. The redox potential was measured within a range of 192–523 mV with an average of 357 mV. Electric conductivity (EC) ran-ged 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, thecharge balance errors were less than 5%, except for samples 23(�7.6%) and 37 (�11.9%). The major ion composition of thegroundwater samples are presented in Piper diagram in Fig. 4.

Statistical comparison of major ion compositions in groundwa-ter samples from the hand dugwells and drilled wells is shown inFig. 5. About 67% of the hand dug wells have groundwater ofCa–HCO3 type. The rest has groundwater of Ca–Na–HCO3–NO3–Clor Na–Ca–HCO3–Cl–NO3 type. For the hand dug wells there is onlyone sample with Na as dominating cation. On the other hand about21% of the drilled wells revealed a Ca–HCO3 type of groundwaterand in general about 25% of samples indicated Na or Mg as domi-nating cations. This may be caused by calcium replacing sodiumand to some extent magnesium on exchange sites when thegroundwater age increases. These subtle differences in major ionchemistry are perhaps caused by the differential lithological char-acteristics of the wells and the groundwater flow pattern (Fig. 3).Among the major ions, NO�3 exceeding the WHO’s guidelines

Small scale mining sitesIllegal mining (Galamsey) sitesLarge scale gold minesGroundwater divideGroundwater f low direction

and conceptual flow pattern.

Table 3Field 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 Simpa BH/HP 27.3 6.07 308 545.0 9.54 <5.2 3.41 0.924 Dadwen BH/HP 26.6 5.98 268 126.0 4.98 5.5 7.29 0.315 Dompim BH/HP 26.7 6.06 272 169.0 6.89 <5.2 5.20 0.296 Dompim BH/HP 27.8 6.30 328 261.0 5.64 <5.2 0.46 0.247 Atwerboanda BH/HP 27.4 6.20 337 483.0 6.25 <5.2 0.50 0.418 Odumase BH/HP 26.8 6.85 192 465.0 8.21 <5.2 3.37 0.579 Nsuaem BH/HP 26.4 6.85 433 780.0 9.28 <5.2 2.78 0.85

10 Aboso BH/HP 27.1 6.31 215 457.0 7.84 <5.2 4.33 0.8511 Kokoase BH/HP 26.6 6.75 211 426.0 8.22 <5.2 1.14 0.2512 Samahu DW/HP 26.0 5.97 440 123.0 7.60 <5.2 0.004 0.0913 Samahu BH/HP 26.1 6.87 233 510.0 8.80 15.6 0.12 0.5714 Yaryeyaw BH/HP 26.4 6.06 498 114.1 5.03 <5.2 0.005 0.00815 Suwinso BH/HP 26.3 6.01 372 123.8 4.56 <5.2 0.10 0.2716 Gordon BH/HP 26.4 5.91 467 127.2 5.31 <5.2 0.004 0.2417 Akotomu BH/HP 26.7 6.42 276 429.0 8.76 <5.2 0.47 0.6418 Kofi GyanCamp BH/HP 27.0 5.39 519 107.2 12.88 <5.2 0.009 0.09319 Kofi Gyankrom. BH/HP 25.4 5.90 416 163.6 7.82 <5.2 0.026 0.2520 Huniano n 1 DW/HP 26.3 6.23 376 324.0 18.24 <5.2 0.017 0.06221 Tarkwa Banso BH/HP 27.2 5.71 517 96.3 3.49 <5.2 0.005 0.00622 Tarkwa Banso BH/HP 26.1 6.30 297 266.0 7.71 <5.2 0.71 0.4523 Domeabra DW/HP 28.5 5.30 455 74.4 61.93 <5.2 10.59 0.06324 Enyinasie BH/HP 27.3 6.07 263 460.0 8.64 69.4 4.69 0.9325 Enyinasie BH/HP 26.2 6.23 262 276.0 9.45 <5.2 2.17 0.6126 Akyem BH/HP 26.5 6.13 307 375.0 7.01 <5.2 0.25 0.4227 Akyem BH/HP 26.0 6.06 337 275.0 7.39 <5.2 0.19 0.5028 Adieyie BH/HP 28.5 6.31 350 355.0 8.73 <5.2 0.069 0.2929 Mile 8 BH/HP 26.0 6.92 223 468.0 26.12 <5.2 0.076 0.9730 Teberebe DW/HP 26.5 5.18 440 119.5 30.97 <5.2 0.029 0.07531 Teberebe BH/HP 25.9 5.33 399 83.2 36.66 <5.2 0.16 0.9232 Huniso BH/HP 26.9 4.44 523 681.0 2175.33 <5.2 0.014 0.6833 Huniso BH/HP 26.2 6.11 444 638.0 11.32 <5.2 0.004 1.0734 Abekoase DW/HP 26.4 5.56 421 109.5 12.31 <5.2 0.012 0.09335 New Atuabo DW/HP 27.0 5.82 285 130.1 5.54 <5.2 10.84 0.2936 Koduakrom BH/HP 26.6 6.87 239 454.0 8.47 <5.2 0.10 0.3937 Damang BH/HP 26.1 4.75 474 32.9 44.35 <5.2 0.004 0.0438 NewKyekyewere BH/HP 25.9 5.85 439 11.0 3.95 <5.2 0.001 0.0439 Huni Valley BH/HP 26.2 5.71 490 113.4 5.46 <5.2 0.004 0.0440 Bompieso BH/HP 26.2 6.67 234 471.0 9.32 <5.2 0.28 0.7141 Bompieso BH/HP 26.5 6.51 228 400.0 8.01 <5.2 0.48 0.6042 Akoon BH/HP 26.9 4.19 489 423.0 593.81 <5.2 0.006 2.04

⁄Abbreviations: BH: Borehole; DW: Dugwell; HP: Handpump.

P. Bhattacharya et al. / Journal of African Earth Sciences 66–67 (2012) 72–84 77

(Table 4), occurs at two locations and might be of principal healthconcern for drinking water supplies.

Statistical comparison of the major ion chemistry of the ground-water samples from shallow and deep wells is in Fig. 5. There arehigher pH values (Fig. 6a) and lower Eh values (Fig. 6b) for ground-water from deep wells. This is consistent with longer residencetimes of groundwater in deep wells with resulting advanced neu-tralisation in reactions with carbonates and silicates and consump-tion of electron acceptors such as oxygen. Longer residence time indeep wells also results in higher EC values (Fig. 6c) and higherbicarbonate concentrations (Fig. 6d). Concentrations of dissolvedFe and Mn (Fig. 7) are also higher in deep wells as can be expecteddue to their increased mobility under more reducing conditions.

4.3. Trace element characteristics

Statistical characteristics for dissolved Al and As(tot) are shownin Fig. 7. A total of 17 wells have higher metal content than WHOguidelines concerning As(tot), Mn, Fe and Al. Total As [As(tot)] ex-ceeds the drinking water guidelines at two locations, Samahu,15.6 lg/l and Eyinaise, 69.4 lg/l (Table 5). Manganese is the majorcontaminant and among the 17 wells with high concentrations ofmetals, 14 has elevated Mn-levels. Iron exceeds the guideline in se-ven wells and Al is exceeding the guideline at two locations, Hun-iso and Akoon. These wells also display the two lowest measuredpH values, 4.44 and 4.19, respectively. Nitrate is exceeding theguidelines in the same two wells in Huniso and Akoon. The wells

with elevated metal content are presented in Table 5. All the wellswith metal concentrations exceeding WHO guidelines are bore-holes except New Atuabo which is a hand dug well.

Both Mn and Fe show similarity in distribution pattern, almostall areas with Fe concentrations (Fig. 9c) above WHO guidelinesalso have high Mn concentrations (Fig. 9d). However, there is novisible trend between these parameters when plotted (R2 = 0.09)(not shown). For Fe and SO2�

4 it is difficult to see any trend(Fig. 8a), but there is a positive trend for Mn and SO2�

4 (Fig. 8b).The trend between Mn and SO2�

4 could originate from dissolutionof carbonate minerals like kutnohorite, Ca(Fe, Mn)(CO3)2, duringneutralisation of AMD. For certain areas such as Akoon, New Atu-abo, Enyinasie, and Dadwen high concentrations of both Fe andSO2�

4 and low pH values indicate the impact of acid mine drainage.For Akoon, the location that display the highest levels of both Feand Mn, there were small-scale mining activities just about100 m from sampled well. In Aboso, high concentration of NHþ4and relatively low concentration of SO2�

4 may indicate presenceof other sources of pollution than acid mine drainage. Precipitationof Fe-oxyhydroxides can explain low correlation between Fe andSO2�

4 . Oxidation of Fe(II) and precipitation of Fe-oxyhydroxides oc-curs at lower redox level than oxidation of Mn(II) and precipitationof Mn-oxyhydroxides and, thus, Mn remains dissolved even underrelatively oxidising conditions, when most of Fe has already pre-cipitated (Drever, 1997). However, many samples display bothlow Fe and SO2�

4 values, and thus, they are not affected by acidmine drainage.

Table 4Concentrations of major ions, the water types, dissolved organic carbon (DOC) and ammonium.

No. Location HCO�3 Cl� NO�3 SO2�4 PO3�

4Ca2+ Mg2+ Na+ K+ Water type DOC NHþ4

mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l lg/l

3 Simpa 119.3 84.2 2.35 36.75 0.30 47.58 9.16 44.58 1.00 Ca–Na–Cl–HCO3 1.40 84.024 Dadwen 57.1 6.9 0 6.56 1.08 6.72 5.39 10.32 0.37 Na–Mg–Ca–HCO3 1.00 05 Dompim 82.5 10.85 0.18 6.86 0.75 18.38 3.97 11.43 0.54 Ca–Na–HCO3 0.87 06 Dompim 87.1 27 1.74 9.74 0.48 20.27 6.71 20.31 0.53 Ca–Na–Mg–HCO3–Cl 1.08 07 Atwerboanda 150.2 49.3 4.7 45.9 0.05 45.10 20.73 21.51 5.66 Ca–Mg–HCO3–Cl 0.87 08 Odumase 294.9 5.05 0.18 1.43 0.06 66.82 14.02 10.91 0.76 Ca–Mg–HCO3 0.93 09 Nsuaem 377.6 58.35 35.9 30.4 0 111.18 22.45 29.69 7.85 Ca–Mg–HCO3 0.76 0

10 Aboso 230.3 26.18 0 18.62 0.48 47.36 18.31 25.39 0.59 Ca–Mg–Na–HCO3 1.06 26.211 Kokoase 262.4 8.92 0 10.28 0.31 43.29 17.79 23.82 0.62 Ca–Mg–Na–HCO3 0.93 012 Samahu 81.5 3.65 2.01 1.43 0 21.03 2.40 3.02 0.82 Ca–HCO3 0.73 013 Samahu 328.7 5.08 0.19 1.47 0.04 67.29 11.12 24.52 0.82 Ca–HCO3 0.86 014 Yaryeyaw 79.3 5.14 3.31 0.54 0.16 13.92 4.16 8.65 0.27 Ca–Na–Mg–HCO3 0.71 1.215 Suwinso 81.9 5.07 0 0.88 0.22 14.12 2.61 11.03 0.41 Ca–Na–HCO3 0.94 016 Gordon 65.4 7.87 11.60 0.55 0.30 9.68 5.95 10.30 0.38 Mg–Ca–Na–HCO3 0.81 1.717 Akotomu 258.8 12.57 0.19 1.89 0.24 55.42 11.74 18.16 0.59 Ca–Mg–HCO3 0.87 018 Kofi GyanCamp 36.6 10.2 18.7 1.16 0.22 7.20 4.22 9.63 1.04 Na–Ca–Mg–HCO3–Cl–NO3 0.99 019 Kofi Gyankrom. 95.0 12.17 0 0.73 0.09 16.50 5.54 12.39 0.34 Ca–Na–Mg–HCO3 0.96 020 Huniano n 1 150.5 17.83 0.27 7.46 0.09 42.01 4.54 11.32 5.16 Ca–HCO3 2.68 021 Tarkwa Banso 59.3 4.8 2.45 1.18 0.17 7.18 5.16 7.28 0.38 Mg–Ca–Na–HCO3 1.05 022 T. Banso 162.1 3.43 0 0.65 0.33 34.82 5.16 11.71 1.60 Ca–HCO3 0.94 023 Domeabra 18.9 5.91 17.89 1.22 0 8.60 0.91 3.70 0.53 Ca–Na–HCO3–NO3–Cl 1.03 024 Enyinasie 152.3 47.36 1.42 68.52 0.41 50.42 20.83 20.26 1.28 Ca–Mg–HCO3–SO4–Cl 0.90 025 Enyinasie 156.8 6.47 0 2.75 0.30 34.03 4.83 10.53 3.67 Ca–HCO3 0.96 026 Akyem 145.2 32.5 7.3 33.45 0.10 41.97 9.50 24.78 2.24 Ca–Na–HCO3–Cl 0.97 027 Akyem 158.8 8.18 0 7.74 0.20 30.91 8.89 13.07 1.11 Ca–Mg–HCO3 0.83 24.128 Adieyie 230.9 4.59 0.22 1.45 0.08 40.65 12.99 16.01 0.73 Ca–Mg–HCO3 1.31 029 Mile 8 312.9 3.89 0.22 1.35 0 77.32 5.73 16.83 0.68 Ca–HCO3 1.23 030 Teberebe 26.0 13.85 25.4 2.7 0.04 9.93 1.28 11.50 2.56 Na–Ca–HCO3–Cl–NO3 2.10 031 Teberebe 20.3 14.6 0.16 4.62 0.04 4.15 1.10 9.51 2.81 Na–Ca–Cl–HCO3 3.89 032 Huniso 0.0 117.4 146.4 10.4 0.61 23.69 10.63 69.70 22.69 Na–Ca–Cl–NO3 1.09 033 Huniso 221.3 76.55 25.35 15.1 0.04 86.28 9.10 29.37 1.12 Ca–HCO3–Cl 0.89 034 Abekoase 69.2 3.18 0.78 1.24 0 19.91 0.58 2.19 1.05 Ca–HCO3 0.94 035 New Atuabo 46.4 5.72 0.23 6.35 0 14.11 0.64 3.44 2.00 Ca–HCO3 1.53 261.836 Koduakrom 292.3 4.2 0.22 1.63 0 58.02 5.82 30.26 2.42 Ca–Na–HCO3 0.65 037 Damang 12.0 3.16 2.41 0.22 0.13 1.26 0.89 2.56 0.55 Na–Mg–Ca–HCO3–Cl 0.70 038 NewKyekyewere 82.7 3.48 0 0.39 0.21 12.16 2.20 11.57 1.73 Ca–Na–HCO3 0.73 039 Huni Valley 75.3 5.39 0 1.13 0.25 9.13 3.44 11.68 1.78 Na–Ca–Mg–HCO3 0.64 040 Bompieso 258.2 23.94 0 6.16 0.11 68.65 6.67 21.17 0.82 Ca–HCO3 0.86 041 Bompieso 228.3 14.12 5.82 0.09 55.88 6.13 18.34 0.60 Ca–HCO3 0.83 042 Akoon 0.0 46.15 66.35 55.35 0.06 22.51 5.80 31.55 11.24 Na–Ca–Cl–SO4–NO3 1.30 28.3

78P.Bhattacharya

etal./Journal

ofA

fricanEarth

Sciences66–

67(2012)

72–84

80

60

40

20

Mg

80

60

40

20

80 60 40 20 20 40 60 80

Ca Na+K HCO3 Cl

80

60

40

20

SO4

80

60

40

20

Legend

Deep wellsShallow wells

Fig. 4. Piper diagram showing composition of groundwater in the deep (bore) andthe shallow (dug) wells.

P. Bhattacharya et al. / Journal of African Earth Sciences 66–67 (2012) 72–84 79

Iron shows only a weak negative correlation with Ca (Fig. 8c).This is consistent with expected removal of Fe during neutralisa-tion of AMD by carbonates, which are the source of Ca. Manganeseshows strong positive correlations with Ca (Fig. 8d). Sample 42(Akoon) shows a strongly deviant value. If this value is excludedthe R2-value increases to 0.64. This supports the hypothesis aboutMn-carbonate as a source of Mn.

Ca shows a strong correlation with pH and HCO�3 shown inFig. 8e and f, respectively. Samples 32 and 42 were not includeddue to probable acid mine drainage impact. This indicates thatCaCO3 is the source of Ca.

Spatial distribution based on kriging of selected species concen-trations and pH is shown in Fig. 9a–e. Based on the comparisonwith geological map (Fig. 2), it can be seen that Fe, Mn, As andSO2�

4 concentrations do not exhibit any major differences in thepattern of distribution in the Tarkwaian and Birimain system ofrocks. Concentrations of Al exceeding WHO guidelines are foundlocally in the Tarkwaian system (Fig. 9a). This is most likely a resultof the low pH in two wells (Fig. 9d) and is not related to the differ-ences in geology between the two systems.

4.4. Geochemical modelling

Samples 13, 24, 32 and 42 were selected for geochemical speci-ation modelling. This choice was based on their concentrations of

0,01

0,10

1,00

10,00

Con

cent

ratio

ns (m

eq/l)

0,01

0,10

1,00

10,00a

Ca Mg Na K HCO3 Cl SO4 NO3

Fig. 5. Major ion compositions in groundwater samples from (a) deep bore w

metals and pH values. All selected samples are from drilled wellslocated in the following geological formations:

� Sample 13: The Tarkwaian system, Huni Sandstone: (a quartz-ite) 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 ofgreen lava. Lava and tuff dominate this part. Several band ofphyllite occurs in this zone and are manganiferous in places.� Sample 32: The Tarkwaian system, Huni Sandstone: (a quartz-

ite) 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 andgondite

Table 6 shows the saturation indices (SI) for selected mineralphases. For most minerals only the results in the interval(�2 < SI < 2) are shown. For reactive minerals, as calcite, dolo-mite and gypsum, SI values are presented regardless of theirmagnitude.

The issue of principal interest was the precipitation and stabil-ity of oxides and hydroxides of Fe, Al, and Mn. Groundwater wasgenerally supersaturated with respect to Al/Fe-oxides and hydrox-ides and different silicate minerals. Various minerals containing Alsuch as oxides/hydroxides can precipitate, but precipitation ofbohemite, diaspore, and kaolinite is generally kinetically con-strained (Appelo and Postma, 1999). Bohemite and diaspore arerather formed by re-crystallization of precipitated amorphousAl(OH)3, but ground water is undersaturated with respect to thismineral phase. However, ground water is supersaturated with re-spect to some Fe oxides/hydroxides. For example, goethite can pre-cipitate, providing sites for adsorption. Groundwater isundersaturated with respect to Mn oxides and hydroxides andtheir precipitation is therefore unlikely.

Minerals like siderite, vivanite, and rhodochrosite are sinks fordissolved Fe and Mn and their precipitation can disturb correlationbetween Fe, Mn, and As (Sracek et al., 2004; Hasan et al., 2007,2009; von Brömssen et al., 2008; Bhattacharya et al., 2009). How-ever, groundwater from selected wells is undersaturated with re-spect to these minerals.

Saturation indices (SIs) show that groundwater is undersatu-rated with respect to most potential secondary minerals. This sug-gests that the groundwater has short residence time and naturalequilibrium with these minerals is not reached. In large-scale min-ing lime is used, but groundwater is undersaturated with respect tocalcite.

LegendMax.

75 percentile

Median

25 percentile

Min.

b

Ca Mg Na K HCO3 Cl SO4 NO3

ells and (b) shallow dug wells. Note the concentration units are in meq/l.

100

200

300

400

500

600

4.0

4.6

5.2

5.8

6.4

7.0

0

160

320

480

640

800

0

80

160

240

320

400Max.

75 percentile

Median

25 percentile

Min.

ba

dc

HC

O3- (

mg/

l)E

h(m

V)

pHS

EC

(µS

/cm

)

Deep wells Shallow wells Deep wells Shallow wells

Deep wells Shallow wellsDeep wells Shallow wells

Fig. 6. Box plots of selected parameters (a) pH, (b) Eh, (c) SEC, and (d) HCO�3 for deep and shallow wells.

1

10

100

1000

10000

1

10

100

1000

10000Legend

Max.

75 percentile

Median

25 percentile

Min.

Con

cent

ratio

n(µ

g/l)

Con

cent

ratio

n(µ

g/l)

a

bFe Mn Al As(tot)

Fig. 7. Distribution of Fe, Mn, Al and As(tot) in (a) deep (bore) wells and (b) shallow(dug) wells.

80 P. Bhattacharya et al. / Journal of African Earth Sciences 66–67 (2012) 72–84

Groundwater is generally undersaturated with respect to theminerals containing sulphates. Only in well 32 and 42 water sam-ples were found to be supersaturated with respect to Al-sulphatemineral (alunite). These wells have very low pH values and thisindicates an impact of acid mine drainage. In wells with low pH,ground water is supersaturated with respect to some silicate min-erals, which could provide a sink for Al. The dominant aqueousspecies are Fe2+, Mn2+, Al3+ and H2AsO�4 .

Arsenic is present as oxidised anionic species as As(V), which ismore adsorbed than As(III) under the observed pH conditions(Bhattacharya et al., 2002; Smedley and Kinniburgh, 2002).

5. Discussion and conclusions

The metal concentrations in the study area are generally lowerthan expected on the basis of large scale mining activities. Theintensive mining industry and the study by Kortatsi (2004) in anearby area indicated higher values (values above WHO guidelinesfor Al, As, Cd, Cr, Fe, Mn, Ni, Pb and Zn). The groundwaters in somewells in the study area have values of Mn, Fe, As, and Al exceedingthe WHO guidelines. Mn and Fe account for almost all the elevatedvalues. These metals do not have the same serious health effect asheavy metals like Cd, Cr, Hg, and Pb. The groundwater quality istherefore better than expected. The groundwaters generally haveneutral to acidic pH and are oxidising. The dominant major ionsare calcium, sodium, and bicarbonate.

Mining activities probably affect the groundwater through acidmine drainage (AMD) in areas where high concentrations of Fe andSO2�

4 and low pH coincide. Principal areas affected by AMD are NewAtuabo, Akoon, Teberebe, Huniano no 1, Dadwen and Aboso. Theoccurrence of As at three sampled sites is most probably of naturalorigin and is not considered as a major problem. The rest of themetals exceeding the guidelines are all components of common

Table 5Samples exceeding WHO guideline values for safe drinking purposes (WHO, 1996, 2011a).

No. LocationWHO guidelinevalue

As(tot)10.0(lg/l)

Fe3.0(mg/l)

Al0.2(mg/l)

Mn0.4(mg/l)*

NO�350(mg/l)

3 Simpa bdl 3.41 0.9164 Dadwen bdl 7.295 Dompim bdl 5.208 Odumase bdl 3.37 0.5679 Nsuaem bdl 0.851

10 Aboso bdl 4.33 0.85013 Samahu 15.6 0.56517 Akotomu bdl 0.64024 Eyinaise 69.4 4.69 0.92625 Eyinaise bdl 0.60529 Mile 8 bdl 0.96832 Huniso bdl 2.18 0.676 14633 Huniso bdl 1.0735 New Atuabo bdl 10.840 Bompieso bdl 0.71341 Bompieso bdl 0.60242 Akoon bdl 0.594 2.04 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 ofhealth concern (WHO, 2011b).

y = 0.0157x + 0.2706R2 = 0.401

0

0.5

1

1.5

2

2.5

y = 0.026x + 0.8951R2 = 0.0324

0

2

4

6

8

10

12

0 20 40 60 800 20 40 60 80

y = 41.31x - 216.97R2 = 0.6262

0

20

40

60

80

100

120

4.50 5.00 5.50 6.00 6.50 7.00

y = 0.2436x - 0.9703R2 = 0.8527

0

20

40

60

80

100

120

0.0 100.0 200.0 300.0 400.0

y = -0.0064x + 1.3815R2 = 0.005

0

2

4

6

8

10

12

Fe (m

g/l)

Mn

(mg/

l)

y = 0.0087x + 0.1347R2 = 0.3137

0

0.5

1

1.5

2

2.5

0 50 100 0 50 100

Fe (m

g/l)

Mn

(mg/

l)

Ca

(mg/

l)

Ca

(mg/

l)ba

c d

e f

SO42- (mg/l) SO4

2- (mg/l)

Ca (mg/l)

HCO3- (mg/l)

Ca (mg/l)

pH

Fig. 8. Bivariate plots showing correlations of: (a) Fe vs SO2�4 , (b) Mn vs SO2�

4 , (c) Fe vs Ca, (d) Mn vs Ca, (e) Ca vs pH, and (f) Ca vs HCO�3 in groundwater from the investigated wells.

P. Bhattacharya et al. / Journal of African Earth Sciences 66–67 (2012) 72–84 81

-2.1 -2.05 -2 -1.95 -1.9 -1.85

5.15

5.2

5.25

5.3

5.35

5.4

5.45

5.5

0100200300400500600700800900100011001200130014001500160017001800190020002100

Dadwen

Dompim

AtwerboandaOdumaseNsuaem

AbosoKokoase

Samahu

Yaryeyaw

Suwinso

Gordon Akotomu

Kofi Gyan CampKofi Gyankrom

Huniano no 1

Tarkwa Banso

DomeabraEnyinasie

AkyemAdieyie

Teberebe

Huniso

New Atuabo

DamangNew Kyekyewere

Huni Valley

Bompieso

Akoon

-2.1 -2.05 -2 -1.95 -1.9 -1.85

5.2

5.3

5.4

5.5

-200-1000100200300400500600700800900100011001200130014001500160017001800190020002100

Dadwen

Dompim

AtwerboandaOdumaseNsuaem

AbosoKokoase

Samahu

Yaryeyaw

Suwinso

Gordon Akotomu

Kofi Gyan CampKofi Gyankrom

Huniano no 1

Tarkwa Banso

DomeabraEnyinasie

AkyemAdieyie

Teberebe

Huniso

New Atuabo

DamangNew Kyekyewere

Huni Valley

Bompieso

Akoon

5.15

5.25

5.35

5.45

-2.1 -2.05 -2 -1.95 -1.9 -1.85

5.15

5.2

5.25

5.3

5.35

5.4

5.45

5.5

-500050010001500200025003000350040004500500055006000650070007500800085009000950010000

Dadwen

Dompim

AtwerboandaOdumaseNsuaem

AbosoKokoase

Samahu

Yaryeyaw

Suwinso

Gordon Akotomu

Kofi Gyan CampKofi Gyankrom

Huniano no 1

Tarkwa Banso

DomeabraEnyinasie

AkyemAdieyie

Teberebe

Huniso

New Atuabo

DamangNew Kyekyewere

Huni Valley

Bompieso

Akoon

-1.95 -1.9

5.2

5.3

5.4

5.5

4.24.34.44.54.64.74.84.955.15.25.35.45.55.65.75.85.966.16.26.36.46.56.66.76.86.9

Dadwen

Dompim

AtwerboandaOdumaseNsuaem

AbosoKokoase

Samahu

Yaryeyaw

Suwinso

Gordon Akotomu

Kofi Gyan CampKofi Gyankrom

Huniano no 1

Tarkwa Banso

DomeabraEnyinasie

AkyemAdieyie

Teberebe

Huniso

New Atuabo

DamangNew Kyekyewere

Huni Valley

Bompieso

Akoon

-2.1 -2.05 -2 -1.85

5.15

5.25

5.35

5.45

-2.1 -2.05 -1.95 -1.9

5.15

5.2

5.25

5.3

5.35

5.4

5.45

5.5

-5

0

5

10

15

20

25

30

35

40

45

50

55

60

65

Dadwen

Dompim

AtwerboandaOdumaseNsuaem

AbosoKokoase

Samahu

Yaryeyaw

Suwinso

Gordon Akotomu

Kofi Gyan CampKofi Gyankrom

Huniano no 1

Tarkwa Banso

DomeabraEnyinasie

AkyemAdieyie

Teberebe

Huniso

New Atuabo

DamangNew Kyekyewere

Huni Valley

Bompieso

Akoon

-2 -1.85

a cb

d e

MnAl Fe

SO42-pH

Fig. 9. Concentration of (a) Al (lg/l), (b) Fe (lg/l), (c) Mn (lg/l), (d) pH and (e) SO2�4 (mg/l) in the deep wells within the study area. Bold lines indicate drinking water guideline

values (WHO, 1996).

Table 6Result 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) �1.13 Al(OH)3(a) �1.05 Al(OH)3(a) �1.5 Al(OH)3(a) �2.78Barite �0.72 Alunite �0.64 AlOHSO4 �1.11 AlOHSO4 �1.1Barite �0.72 Anhydrite �1.81 Barite �0.26 Alunite 0.75Boehmite 0.67 Barite �0.02 Boehmite 0.3 Anhydrite �2.19Calcite �0.23 Boehmite 0.76 Chalcedony �0.07 Barite 0.25Chalcedony 0.23 Calcite �1.48 Diaspore 1.99 Boehmite �0.97CO2(g) �1.35 Chalcedony 0.28 Ferrihydrite �2.58 Chalcedony �0.48Diaspore 2.37 CO2(g) �0.87 Gibbsite(C) 0.09 Diaspore 0.72Dolomite �0.96 Dolomite �3.07 Goethite 1.87 Ferrihydrite �4.25Fe3(OH)8 �0.96 Diaspore 2.45 Gypsum �2.77 Gibbsite(C) �1.19Ferrihydrite 0.79 Fe3(OH)8 �1.56 Halloysite 1.55 Goethite 0.21Gibbsite(C) 0.47 Ferrihydrite 0.5 Lepidocrocite 0.94 Gypsum �2Goethite 5.22 Gibbsite(C) 0.54 Manganite �8.38 Halloysite �1.81Gypsum �3.15 Goethite 4.97 MnHPO4(C) �0.56 Kaolinite 1.44Hydroxyapatite 0.21 Gypsum �1.63 Montmorillonite 1.79 Lepidocrocite �0.73Magnesite �1.23 Magnesite �2.07 Pyrolusite �11.1 Manganite �9.2MnHPO4(C) 1.31 Manganite �7.75 Quartz 0.41 MnHPO4(C) �0.31Pyrolusite �11.2 MnHPO4(C) 1.97 SiO2(am) �0.88 Quartz 0Quartz 0.71 Pyrolusite �13.2 Strengite �1.02 SiO2(am) �1.29Rhodochrosite �0.52 Quartz 0.75 Vivianite �17.3 Vivianite �19.3Siderite �1.05 Rhodochrosite �1.43SiO2(am) �0.58 Siderite �0.57Strengite �0.57 SiO2(am) �0.53Strontianite �1.21 Strengite 1.19Vivianite �5.63 Vivianite �1.57

82 P. Bhattacharya et al. / Journal of African Earth Sciences 66–67 (2012) 72–84

P. Bhattacharya et al. / Journal of African Earth Sciences 66–67 (2012) 72–84 83

minerals and they probably origin from natural processes. For Althere is a strong correlation with pH since it precipitates as amor-phous Al(OH)3 at pH higher than 4.0 (Appelo and Postma, 1999). Instrongly alkaline environment Al becomes mobile again because ispresent as AlðOHÞ�4 , but such high pH values are not found in thestudy area. Concentrations of Mn and Fe are inversely correlatedto Eh, which determines their oxidation state and, thus, theirmobility.

There are a number of reasons that may explain relatively lowobserved dissolved metal concentrations:

� Adsorption processes are probably very important and can con-siderably lower the metal concentrations in groundwater. Allsoil types in the Tarkwaian system are clayey and the soils ofthe 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 pro-vide significant sites for sorption. Heavy metals as Cu, Pb, Hg,and Cd are strongly bounded to these sites and this explainstheir low dissolved concentrations. The pH is the most impor-tant parameters concerning metal mobility. Value of pHZPC forgoethite is 6.0–7.0 compared to much lower value about 2.5for montmorillonite (Drever, 1997). However, ferric mineralsare more efficient adsorbents of heavy metals than clays. In thisstudy, among all investigated species, only Al and Ca are foundto 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 systemswith short groundwater residence times. This is consistent withnegative saturation indices for many potential secondary min-erals. The local groundwater systems also prevent mining toaffect larger groundwater systems on a regional scale. However,there is a possibility that at some sites local mining pollutantshave not reached yet the wells and the groundwater qualityin 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 dilutingeffect 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 conductivityand Eh values between the shallow and deep wells may be ex-plained 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 exceededthe guideline in two wells. Elevated levels of Mn, Fe and Al are rel-atively easy to treat. In the most affected areas aeration and adjust-ment of pH should improve the drinking water quality.

For further investigations in the study area more informationabout the depth of the wells, groundwater flow pattern, locationof small-scale mining activities and detailed geological informationof the sampling positions would provide a better understanding ofthe processes governing the groundwater quality. This would makea more comprehensive assessment of the groundwater vulnerabil-ity concerning mining pollution possible. Although results of thisstudy generally indicate good drinking water quality, contamina-tion of groundwater from mining activities have been found atsome locations and further contamination is possible.

Acknowledgements

We acknowledge the financial support provided by the SwedishInternational Development Cooperation Agency (Sida-SAREC) forthe research project on Contamination of Water Resources inTarkwa Mining Area of Ghana (SWE-2003-245). We thank Ann

Fylkner and Monica Löwen of the Department of Land and WaterResources Engineering (KTH) for chemical analyses. BE and RAacknowledge the financial support provided by the InternationalProgrammes Office (IPK), Stockholm and the Swedish InternationalDevelopment Cooperation Agency (Sida) in the form of Minor FieldStudy grant during 2004. The help received from E. Kumo duringthe field work carried out in the Tarkwa mining area duringAugust–October, 2004 is deeply appreciated. We are thankful tothe anonymous reviewer and the editors of the journal for theirthoughtful comments on an earlier version of this manuscript.

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