Journal of African Earth Sciences 66–67 (2012) 72–84 Contents lists available at SciVerse ScienceDirect Journal of African Earth Sciences journal homepage: www.elsevier.com/locate/jafrearsci 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 b Department of Geology, Faculty of Science, Palacky´ University, 17 listopadu 12, 771 46 Olomouc, Czech Republic c OPV s.r.o. (Protection of Groundwater Ltd.), Belohorská 31, 169 00 Praha 6, Czech Republic 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 article info abstract Article history: The aim of this study was to investigate the groundwater chemistry with special concern to metal pol- Received 27 May 2011 lution in selected communities in the Wassa West district, Ghana. In this mining area, 40 ground water Received in revised form 12 March 2012 samples, mainly from drilled wells, were collected. The groundwaters have generally from neutral to Accepted 15 March 2012 acidic pH values and their Eh values indicate oxidising conditions. The dominating ions are calcium, Available online 24 March 2012 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 at Keywords: least one metal. The main contaminants are manganese and iron, but arsenic and aluminium also exceed Groundwater the guidelines in some wells probably affected by acid mine drainage (AMD). Metal concentrations in the Mining Hydrogeochemistry groundwater seem to be controlled by the adsorption processes. Hydrogeochemical modelling indicates Metal pollution supersaturation of groundwater with respect to several mineral phases including iron-hydroxides/oxides, Arsenic suggesting that adsorption on these minerals may control heavy metal and arsenic concentrations in Tarkwa groundwater. 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 a strong impact of acid mine drainage on groundwater systems in a regional scale. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Both small-scale miners and large-scale mining are currently operating in Ghana and about 237 (154 Ghanaian and 83 foreign) West Africa has been one of the world’s most important gold enterprises are prospecting for gold and another 18 are operating mining regions for centuries. Today the most significant gold pro- gold mines (Hilson, 2002a,b). Large-scale mining in Tarkwa region ducing country in the area is Ghana (Hilson, 2002a). The earliest is conducted as surface mining. Cyanidation is the most common European attempts to extract gold on a large scale were concen- technique in the region and is used for treatment of non-sulphidic trated in Tarkwa and Prestea regions in the late 19th century. A palaeoplacer ore (Akosa et al., 2002; Kortatsi, 2004). The manage- gold rush in the early 20th century was followed by a mass in- ment of waste from large scale mining is done in accordance to ap- crease in gold production. After Ghana gained independence proved environmental plans. The waste rock heaps are stabilised 1957 the industry collapsed and reached a 50-year low in 1982. and re-vegetated. Tailing slurries are channelled into tailing dams In 1983 the government started the Economic Recovery that also are re-vegetated. Reagent containers and packing materi- Programme (ERP) under guidance of WHO. After this the mining als are sold out to contractors for further disposal, however, the industry has seen a phenomenal growth and the gold production monitoring of these activities is poor. Small-scale mining in Ghana has increased by 700% (Hilson, 2002a). 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 ⇑ Corresponding author. Tel.: +46 8 790 7399; fax: +46 8 790 6857. more persons’’ (Government of Ghana, 1989). In the Tarkwa area, E-mail address: [email protected] (P. Bhattacharya). small-scale mining is found all around, both in the forest and along 1464-343X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jafrearsci.2012.03.005 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 the lies within the main gold belt of Ghana that stretches from Axin in Wassa West district through out the year. Among these small-scale the southwest, to Konongo in the northeast (Kortatsi, 2004). Loca- miners about 90% are illegal. Currently, 168 small-scale mining tion of the Wassa West district and the study area is shown in Fig. 1. concessions are valid in the region (Asklund and Eldvall, 2005; Balfors et al., 2007). 2.2. Climatic characteristics The general ore processing techniques are handpicking, amal- gamation, cyanidation, flotation, electroextraction, and roasting of The climate of the area is tropical and is characterised by sea- ore (Akosa et al., 2002). The technique differs between large- and sonal weather patterns. The Wassa West district is situated at small-scale mining and also varies depending on the type of deposit the border of two climatic regions. The south part belongs to the and its location (Ntibery et al., 2003). The area has three main gold south western equatorial climatic region and the northern part deposits. Placer or alluvial deposit, non-sulphidic paleoplacer or free has a wet semi-equatorial climate (Dickson and Benneh, 1980). milling ore and oxidised ore (Kortatsi, 2004). The area is characterised by double wet season during the months This study is focused on an area in southwestern Ghana that has of April–June and October–November. The first and largest peak a long history of mining activities where groundwater serves as the occurs in June, whilst the second and smaller peak occurs in Octo- main source of drinking water supply for local population. Most ber. Around 53% of all rain in the region falls between March and major towns in the area except Tarkwa rely solely on groundwater. July. The mean annual rainfall is approximately 1874 mm with To match the demand for potable water the number of boreholes max and min values of 1449 mm and 2608 mm, respectively. The and hand dug wells is increasing rapidly (Kortatsi, 2004). There area is very humid and warm with temperatures between 28– are apprehensions that the mining activity is causing serious metal 30 °C during the wet season and 31–33 °C during the dry season pollution to the water resources by contaminants such as arsenic, (Dickson and Benneh, 1980; GSR, 2004). The mean pH of the rain lead, cadmium, mercury, and cyanide. Earlier studies have shown water in the area during 2000–2001 was around 6.1 (Kortatsi, that metal levels in groundwater exceed WHO guidelines for drink- 2004), and temperature was between 26 and 30 °C. ing water in many areas in western Ghana (Kortatsi, 2004; Kuma, 2004). 2.3. Geological and geomorphologic characteristics Estimated 5 tonnes of mercury (Hg) are released from small- scale mining operations in Ghana each year (Hilson, 2001). High The regional geology of Ghana is represented by a wide variety concentrations of Hg have been found in sediments and fish in of Precambrian igneous and metamorphic rock comprising the the vicinity of small-scale mining activities using amalgamation Basement Complex and covers about 54% of the country, mainly as the main technique. The concentration in most fish fillets in the southern and western parts (Fig. 2). these areas exceeds the recommendations of the United States The geomorphology of the Tarkwa–Prestea area consists of a Food and Drug Agency (Babut et al., 2003). In general, the manage- series of ridges and valleys parallel to each other and to the strike ment of waste in small-scale mines, particularly the illegal ones, of the rocks. The strike of the rock are generally in north–south lacks waste management plan and simply leave the waste. Addi- direction (Kortatsi, 2004). Both the Tarkwaian and Birimian sys- tionally, mining has led to conflicts among communities, displaced tems are folded along axes that trend northeast (Gyau-Boakye by mining operations, and health and social problems, pollution of and Dapahh-Siakwan, 2000). The general type of topography re- the community water sources, and depletion of groundwater re- flects underlying geology (Kortatsi, 2004). The soil in the Tarkwa sources (Fonseca, 2004). area consists of mostly silty-sands with minor patches of laterite, Groundwater in mining areas as the Tarkwa–Prestea area is mainly in hilly areas (Kuma and Younger, 2001). known to be vulnerable to pollution from mining that may have The Basement complex is divided into different sub provinces a serious effect on human health. In gold mining areas sulphides including the metamorphosed and folded rocks of the Birimian oxidation leads to low pH in the groundwater that encourages and Tarwaian system (Gyau-Boakye and Dapahh-Siakwan, 2000) the mobility of trace metals which are found in the groundwater with gneiss, phyllites, schists, migmatites, granite-gneiss and quar- in very high concentrations (Kortatsi, 2004).