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Contamination Of Water Resources In Tarkwa Mining Area Of Ghana




Contamination of water resources in Tarkwa mining area of Ghana Contamination of water resources in Tarkwa mining area of Ghana A Minor Field Study for Master of Science Thesis Royal Institute of Technology Ragnar Asklund and Björn Eldvall LTH, Ekosystemteknik Supervisors Professor P.W.K Yankson Assistant Professor John Koku Department of Geography and Resource Development University of Ghana Legon-Accra, Ghana Assoc. Prof. Prosun Bhattacharya Department of Land and Water Resources Engineering Kungliga Tekniska Högskolan Stockholm, Sweden Examiner Assoc. Prof. Gerhard Barmen Department of Engineering Geology Lunds Tekniska Högskola Lund, Sweden i Asklund and Eldvall LUTVDG/TVTG--5092--SE Authors Ragnar Asklund Björn Eldvall Title Contamination of water resources in Tarkwa mining area of Ghana Titel Förorening av vattenresurser i Tarkwa gruvområde, Ghana Keywords Ghana, Tarkwa, Water quality, Gold mining, Heavy metals Nyckelord Ghana, Tarkwa, Vattenkvalitet, Guldgruvor, Tungmetaller Published by Department of Engineering Geology Lund University Lund 2005 Printed by KFS AB, Lund 2005, Sweden ISRN LUTVDG/TVTG--5092--SE ii Contamination of water resources in Tarkwa mining area of Ghana Preface This study has been carried out within the framework of the Minor Field Studies Scholarship Programme, MFS, which is funded by the Swedish International Development Cooperation Agency, Sida. The MFS Scholarship Programme offers Swedish university students an opportunity to carry out two months’ field work in a Third World country. The results of the work are presented in a report at the Master’s Degree level, usually the student’s final degree project. Minor Field Studies are primarily conducted within subject areas that are important from a development perspective and in a country where Swedish international cooperation is ongoing. The main purpose of the MFS Programme is to enhance Swedish university students’ know-ledge and understanding of these countries and their problems and opportunities. MFS should provide the student with initial experience of conditions in such a country. A further purpose is to widen the Swedish human resources cadre for engagement in international development cooperation. The International Office at the Royal Institute of Technology, KTH, Stockholm, administers the MFS Programme for the faculties of engineering and natural sciences in Sweden. Sigrun Santesson Programme Officer MFS Programme iii Asklund and Eldvall LUTVDG/TVTG--5092--SE iv Contamination of water resources in Tarkwa mining area of Ghana Abstract Heavy metals in groundwaters cause serious health problems in many parts of the world. This study is carried out in an area in western Ghana that has a long history of mining activity. There are fears that the mining activity is causing serious metal pollution of the water resources by for example arsenic, lead, cadmium and mercury. Earlier studies have shown that metal concentrations in groundwater exceed WHO:s guidelines. This study is part of a bigger project linking technical, socio-economical and gender dimensions to the contamination of water resources in Tarkwa mining area. The aim of this study is to investigate the groundwater chemistry with special concern to metal pollution in selected communities. The result of this Minor Field Study will help to determine the most crucial areas for further investigation and contribute with more physical data for the continuous work within the bigger project. Another objective is to conduct a literature study of the area. 42 water samples, mainly from drilled wells, were collected. The groundwaters are generally neutral to acidic and oxidizing. The dominating ions are sodium and bicarbonate. The metal concentrations in the study area are generally lower than expected. 17 wells show metal concentrations exceeding WHO:s guidelines of at least one metal. The main contaminants are manganese and iron but arsenic and aluminium are also exceeding the guidelines in some wells. The composition of the groundwater indicates influence of Acid Mine Drainage in some wells. Sorption processes are probably crucial for determining metal concentrations in the groundwater in the area. Hydrochemical modelling indicates that some minerals containing aluminium- and iron-hydroxides/oxides are supersaturated. This suggests that precipitation controls heavy metal concentrations in the groundwater. The occurrence of arsenic in three wells is most probably natural and not considered a major problem in the area. Manganese, iron and aluminium are all parts of common minerals and they probably origin from dissolution of minerals. The area is very hilly which causes a lot of water divides. The groundwater system will therefore not be strongly affected by weathering of minerals due to short residence time. The local groundwater systems also prevent mines to affect larger groundwater systems on a regional scale. Seven locations have been selected as the most interesting sites for further studies. The locations are: Simpa, Aboso, Samahu, Eyinaise, Huniso, New Atuabo and Akoon. v Asklund and Eldvall LUTVDG/TVTG--5092--SE Sammanfattning Tungmetaller i grundvatten är ett allvarligt hälsoproblem i många delar av världen. Denna studie är utförd i ett område i västra Ghana som har en lång tradition av gruvdrift. Det finns misstankar om att områdets vattenresurser allvarligt påverkas av gruvdriften genom utsläpp av tungmetaller, t ex arsenik, bly, kadmium och kvicksilver. Tidigare studier har påvisat metallhalter som överstiger WHO:s gränsvärden. Denna studie är en del av ett större projekt som sammankopplar föroreningen av vattenresurser i området med tekniska, socioekonomiska och genusfrågor. Syftet med studien är att undersöka grundvattenkemin i utvalda områden med särskild hänsyn till tungmetaller. Resultatet från denna ”Minor Field Study” ska ligga till grund för vidare undersökningar i området och även bidra med information till det större projektets fortgående. Ytterligare ett syfte är att genomföra en litteraturstudie över området. 42 vattenprover samlades in, huvudsakligen från borrade brunnar. Grundvattnet är generellt neutralt till surt och är oxiderande. De dominerande jonerna är natrium och bikarbonat. Halterna av metaller är generellt lägre än väntat. 17 brunnar har metallkoncentrationer som överstiger WHO:s gränsvärden, i huvudsak magnesium och järn. Även arsenik och aluminium har förhöjda värden i vissa brunnar. Vattnets sammansättning tyder på påverkan från ”Acid Mine Drainage” i vissa fall. Adsorption är troligen en mycket viktig process för grundvattnets metallkoncentrationer. Hydrokemisk modellering tyder på att vissa mineral innehållande aluminium- och järn- hydroxider/oxider är övermättade. Troligen är utfällning av dessa en viktig faktor för metallkoncentrationen i grundvattnet. Förekomsten av arsenik är antagligen naturlig och anses inte vara ett stort problem. Mangan, järn och aluminium förekommer i vanliga mineral och härstammar troligen från vittring. Området är väldigt kuperat vilket ger upphov till många vattendelare. Grundvattnet får då korta upphållstider och mängden metaller som härstammar från vittring är låg. De lokala grundvattenssytemen förhindrar att gruvindustrin har en regional påverkan. Följande sju områden har bedömts vara mest intressanta för fortsatta studier: Simpa, Aboso, Samahu, Eyinaise, Huniso, New Atuabo och Akoon. vi Contamination of water resources in Tarkwa mining area of Ghana Acknowledgements A thousand thanks to our supervisors at KTH, Assoc. professor Prosun Bhattacharya and our supervisors at university of Ghana, professor P.Yanksson, assistant professor J.Koko who made this thesis possible and took very good care of us during our time in Ghana. Thanks a lot to S.Kufogbe. Lots of thanks to The Wassa West District Assembly who made this study possible by arranging transport and guidance in Tarkwa during the field study. Thank you, Dr J.S. Kuma at Western University Collage for valuable information concerning the sampling plan and for allowing us to use his Hach pH-meter. We are also very grateful to Dr B. Kortatsi at Water Research Resource Institute for valuable tips in preparing the sampling plan. Thank you, Dr. Ondra Sracek for helping us to interpret the hydrochemical modelling. We are very grateful to Ann Fylkner for her big support in the laboratory. At last, we like to thank associate professor Gerhard Barmen. vii Asklund and Eldvall LUTVDG/TVTG--5092--SE Table of contents 1 Introduction 1.1 Contamination of water resources in Tarkwa Mining Area of Ghana: Linking Technical, Social-Economical and Gender Dimensions 1.2 Objectives 2 Mining in Ghana 2.1 Historical background 2.2 Contribution to national economy 2.3 Methods of mining 2.3.1 Large-scale mining 2.3.2 Small-scale mining 2.4 Impact of mining and environmental degradation 2.4.1 Large-scale mining 2.4.2 Small-scale mining 3 Location of the study area and geographical characteristics 3.1 The Wassa West district 3.2 Climate 3.3 Geology 3.3.1 The Birimian system 3.3.2 The Tarkwaian system 3.3.3 Granitoids 3.3.4 Basic intrusives 3.4 Hydrogeology 3.5 Soils 4 Environmental geochemistry 4.1 Oxidation of sulphide minerals and Acid Mine Drainage 4.2 Factors effecting the mobility of heavy metals in the environment 4.2.1 Sorption Processes 4.2.2 Precipitation 4.2.3 Redox potential 4.2.4 Grouping of heavy metals 4.2.5 Anions 4.3 Major metallic contaminants related to mining 4.3.1 Aluminium 4.3.2 Arsenic 4.3.3 Cadmium 4.3.4 Chromium 4.3.5 Iron 4.3.6 Lead 4.3.7 Manganese 4.3.8 Mercury 4.3.9 Nickel 4.3.10 Nitrate and nitrite 4.3.11 Sulphate viii 1 1 1 3 3 4 5 5 6 6 7 7 9 9 9 9 12 13 14 15 15 16 17 17 19 19 20 20 21 22 22 22 22 23 23 24 24 24 25 25 25 25 Contamination of water resources in Tarkwa mining area of Ghana 5 Materials and methods 5.1 Sampling plan 5.2 Field methods 5.2.1 Groundwater sampling 5.2.2 Surface water sampling 5.3 Water analysis 5.4 Treatment of analytical data 6 Results and discussion 6.1 Field measured parameters 6.2 Major ions 6.3 Trace elements 6.4 Differences between deep and shallow wells 6.5 Hydro-chemical modelling 6.6 Surface water chemistry 7 Conclusion 8 Recommendation References Appendix 1: Field measured parameters Appendix 2: Trace elements Appendix 3: Major Ions, NPOC, NH4+ and water type Appendix 4: Ion Balance Appendix 5: Correlation matrix, all wells Appendix 6: Correlation matrix, deep wells Appendix 7: Correlation matrix, shallow wells ix 27 27 27 27 28 28 29 31 31 32 33 38 39 42 43 45 46 50 52 56 57 60 61 62 Asklund and Eldvall LUTVDG/TVTG--5092--SE x Contamination of water resources in Tarkwa mining area of Ghana 1 Introduction The Hungarian Nobel Prize winner Albert Szent-Gyorgyi once said, "Water is life's mater and matrix, mother and medium. There is no life without water.” If the water resources are contaminated, so is life. Providing clean drinking water for the growing population of the world is one of the most pressing issues we stands against in the 21th century. Both anthropogenic and natural processes can affect the water quality. Except from the metals man has created through nuclear reactions the rest has been on earth since the planet was formed. There are a few examples of local metal pollutions through natural weathering but in most cases metals become an environmental and health issue because of anthropogenic activity. Mainly mining and smelting plant release metals from the bed-rock (Walker & Sibly 2001). This study is focused on an area in western Ghana that has a long history of mining activity. There are fears that the mining activity is causing serious metal pollution to the water resources by for example arsenic, lead, cadmium, mercury and cyanide. Earlier studies have shown that metal levels in groundwater exceed WHO:s guidelines for drinking water in many areas in western Ghana. That represents a serious threat to public health. This study is part of a bigger project “Contamination of water resources in Tarkwa Mining Area of Ghana: Linking Technical, Social-Economical and Gender Dimensions” which is a collaboration between KTH and the University of Ghana. 1.1 Contamination of water resources in Tarkwa Mining Area of Ghana: Linking Technical, Social-Economical and Gender Dimensions Contamination of water resources in Tarkwa mining area of Ghana: linking technical, social-economical and gender dimensions is an interdisciplinary study which is a collaboration between the Department of Land and Water Resources Engineering, Royal Institute of Technology (Sweden), and the Department of Geography and Resource Development, University of Ghana (Ghana). The general goal of this study is to gain a deeper understanding of the extent of the problem, by linking technical knowledge on the subject with socio-economic and gender perspectives. From the above stated general goal, four focus areas have been identified to constitute specific objectives of the study. These include: i) investigate metal pollution in water resources in selected communities. ii) examine the exposure pathways of the heavy metals and how that differs among socioeconomic and gender groups in local communities. iii) assess the influence of socioeconomic/cultural factors on people’s perception of and attitude to these metal-related problems and the remediation measures being applied in the areas; iv) suggest measures that are technically and socio-culturally appropriate for addressing the problem of contaminated water in the local communities. The proposed project is planned to cover three years starting January 2004 continuing to December 2006 (Jacks et al 2003). This minor field study covers the first objective. 1.2 Objectives The aim of this study is to investigate the groundwater chemistry with special concern to metal pollution in the water resources in selected communities. The result of this MFS study will help to determine the most crucial areas for further investigation and contribute with more physical data for the continuous work within the bigger project “Contamination 1 Asklund and Eldvall LUTVDG/TVTG--5092--SE of water resources in Tarkwa Mining Area of Ghana: Linking Technical, SocialEconomical and Gender Dimensions”. Another objective is to conduct a literature study of the area. As much information as possible concerning the mining industry, geology and hydrogeology conditions has been collected. We also hope that this thesis can be used as a comprehensive review of metals in groundwater by the Wassa West District Assembly. 2 Contamination of water resources in Tarkwa mining area of Ghana 2 Mining in Ghana 2.1 Historical background West Africa has for centuries been one of the world’s most important gold mining regions. Today the most significant gold producing country in this area is Ghana. Prospective gold regions are localized in the western part of the country. Numerous hard rock deposits can be found and significant quantities have also been re-deposited in local water-bodies, alluvial gold. These gold deposits enhanced the development of many successful ancient West African civilizations, and attracted both Arabic and European merchants. The country of Ghana has taken its name from the Ancient Kingdom of Ghana, which was located about 800 km north of present Accra. Ghana was first mentioned in an Arabic source 788-93 when the trans-Saharan trade with the western savannah started. The gold trade brought increased wealth to West Africa but ancient Ghanaian society was already in an advanced economical and political state (Hilson 2002a). Pre-colonial gold mining operations were extremely simple. Alluvial mining was most widespread and practised along rivers. Sediment was scooped from the shores, stored in canoes or bowls and washed repeatedly to separate gold particles. Shallow-pit surface mining and deep shaft mining also occurred. At the beginning of the Trans Saharan trade gold was collected as dust or nuggets by rural inhabitants, but increasing demand from Arabic traders intensified gold production. For 700 years the Islamic world was the only external influence on West Africa (Hilson 2002a). Because of acute gold shortage during the 15th and 16th centuries, Europe’s interest in West Africa increased. In 1471 the Portuguese reached present day Ghana and gained control of the West African gold trade. The arrival of the Europeans simulated a shift in activity towards the Gulf of Guinea coastlines. The Portuguese settlement in West Africa lasted for some 100 years. They constructed a number of forts along the coast facilitating transcontinental gold trade and preventing other Europeans from being engaged in the trade. In 1595 the Dutch landed in the Gold Coast and the overtaking of fort Elmina in 1642 signified the end of the Portuguese occupation. The English soon challenged the Dutch. During the 16th, 17th and 18th centuries the Dutch West India Company and the British African Company of Merchants were extremely active in gold mining and trade in West Africa. Towards the end of the 16th century the slave trade began. Trading of gold decreased but was not abandoned though no advancements and improvements of mine design and extraction were made. Britain finally got control of Ghana in the mid-1800s, establishing the Gold Coast Colony in 1874 (Hilson 2002a). The earliest European attempts to extract gold on a large scale were concentrated in Tarkwa and Prestea in the late 19th century and the first official European gold mining company was the African Gold Coast Company, registered February 18th 1878. A number of other companies were established in Tarkwa at this time. The majority of these companies failed due to various reasons and it was not until 1895, when a series of gold mines opened in Obasi, increases in gold production occurred. A gold rush in the early 20th century was followed a mass increase in gold production. After this the gold production decreased, but experienced a gold rush after the First World War. Because of the unwillingness of 3 Asklund and Eldvall LUTVDG/TVTG--5092--SE Ghanaians to work for Europeans in late 1920s the British passed the Mercury Ordinance, which made it illegal for Ghanaians to own Mercury. The gold production fluctuated until Second World War. After Ghana gained independence in 1957 the industry collapsed (Hilson 2002a). The drastic decrease in gold production was due to the many problems resulting of the economic, financial, institutional and legal framework within which the sector operated (Aryee 2001). By 1966 all but one of the Ghanaian gold mines were nationalized. The industry experienced a continuous decrease in production and the rapid deterioration was the result of excessive state control. In the 1960s and 1970s Ghana developed one of the most centrally controlled economies outside Eastern Europe (Hilson 2002a). This resulted in a rise of illegal and uncontrolled artisanal mineral production and smuggling as well as declining mineral sector performance (Addy 1998). In 1972 the Government endorsed a new law emphasizing that 55% of equity capital of each company to be held by the government and payment of fair compensation, based on 55% of the company’s total assets also to be made to the government. It became extremely difficult for companies to become profitable. By 1976, gold mine production was about 60% compared to 1960 and a 50-years low record was reached in 1982 (Hilson 2002a). In 1981, a military coup lead by Rawlings overthrew the existing government and formed the Provisional National Defence Council (PNDC). The PNDC government soon sought help from IMF (international money fund) to prepare a plan for economic recovery (Hilson 2002a). In 1983 the government started the Economic Recovery Program (ERP) under guidance of IMF (Hilson 2002a). The objective of the program was to quickly attract investors to the mining sector and other key sectors, which had export potential, to turn around the general economy of the country (Aryee 2001). After the implementation of the ERP the mining industry has seen a phenomenal growth, which mainly can be attributed to the adoption of World Bank recommendations in a new national mineral policy through the 1986 Minerals and Mining Law. This law basically means that the government leaves the mine operation, management and ownership to the private sector (Addy 1998). In 1989 the Small Scale Gold Mining Law legalized small-scale gold mining as an industry in Ghana (Hilson 2002b). Records from the Minerals Commission show that US$4 billon of private investment capital was injected in the mining sector between 1983-1998. The gold production increased from 8.87 MT 1983 to 74.1 MT 1998 (Aryee 2001). Both large and small-scale projects have developed during the 1990s and a wide range of companies from Australia, Canada, the Netherlands, South Africa, the United Kingdom and the United States now hold controlling interest in most of the gold mines currently in operation (Addy 1998). From 1992 the mineral industry became the single largest foreign exchange earner and gold accounts for 95 % of this. Other big key sectors in Ghana are cocoa and forestry (Aryee 2001). 2.2 Contribution to national economy The UN definition of a mineral economy is those economies where mining generates at least 10 % of Gross Domestic Product (GDP) and mineral exports are at least 40 % of their 4 Contamination of water resources in Tarkwa mining area of Ghana foreign exchange earnings. Ghana is not exactly classified as a mineral economy by the UN definition. About 40 % of gross foreign exchange earnings come from the mining sector and it generates 5.7 % of GDP (Aryee 2001). The industry also has linkages to other sectors and is a major employer in rural areas. Mines also contribute to the development of these areas by engaging in community activities and adding to infrastructure by building schools, hospitals and roads (Addy 1998). The fairly low contribution to GDP conforms that while mining is important, the dependence of this sector is comparable or lower than a number of other mining countries (Aryee 2001). The ERP was well received by for example IMF and the World Bank and Ghana has received a mass inflow of loans and development funds. Since 1980 the gold production has increased with 700%. Gold mining has long been an important economic activity in Ghana and has recently become the main industry of the county (Hilson 2002a). The mining sector is very important to the nation’s economic recovery (Addy 1998). 2.3 Methods of mining In Ghana there are both small-scale miners and large-scale mining. The general processing techniques are handpicking, amalgamation, cyanidation, flotation, electrowinning and roasting of ore (Akosa et al. 2002). The technique differs between large- and small-scale mining and also varies depending on the type of deposit and its location (Ntibery et al. 2003). The area has three main gold deposits. Placer or alluvial deposit, non-sulphidic paleplacer or free milling ore and oxidized ore (Kortatsi 2004). 2.3.1 Large-scale mining Large-scale mining is today conducted as surface mining. Cyanidation is the most common technique in the study area and is used for non-sulphidic paleplacer ore (Akosa & Adimado 2002 and Kortatsi 2004). Non-sulphidic paleplacer ore occurs mainly in hard rock. It is particularly associated with the Banket conglomerates of the Tarkwa formation. Teberebie Goldfields Limited and the Ghana Australian Goldfields use this ore (Kortatsi 2004). This technology is typically conducted as drilling, blasting, haulage of the ore, crushing and screening, agglomeration, haulage and stacking. Lime (CaO) is now applied to the ore to raise the pH to between 10.5 and 11.0. Sodium cyanide solution (NaCN) is used for dissolution of the gold. The prepared ore is heaped into plastic lined pads but between 45450 l/day of sodium cyanide solution per hectare possibly leaks out into the environment (Kuma & Younger 2004). Finally gold is recovered through electro winning (Akosa et al. 2002 and Kortatsi 2004). Oxidized ore occurs in weathered rocks and is derived from sulphides, arsenopyrite, realgar (AsS), opiment (As2S3) pyrites etc. Roasting is used (Kortatsi 2004). In the Wassa west district there are in this moment seven large-scale mines which extract and process two metals, gold (six mines) and manganese (one mine). A number of these are located in our study area namely; Ghana Australian Goldfields (GAG), Teberebie Goldfields Limited (TGL), Goldfields Ghana limited (GGL) (Kuma & Younger 2004), Ghana Manganese Company in Nsuta (Kortatsi 2004) and also Abosso Goldfield at Damang (Nankara, T. 2004, pers. comm., 16 Sep). 5 Asklund and Eldvall LUTVDG/TVTG--5092--SE 2.3.2 Small-scale mining Small-scale mining in Ghana is defined as “mining by any method not involving substantial expenditure by any individual or group of persons not exceeding nine in number or by a cooperative society made up of ten or more persons” (Government of Ghana 1989). They are estimated to number over 150,000 in Ghana, of which many operate illegally on concessions belonging to large scale operators, or in restricted areas (Ghana academy of arts and sciences 2003). The illegal small-scale miners account for approximately 10% of the gold production in Ghana (Ntibery, B. 2004, pers. comm., 9 Sep). These are locally referred to as galamsey (Hilson 2002c). The technique mostly used for small-scale mining is amalgamation (Akosa 2002). In this process mercury is mixed with gold concentrate to form gold amalgam, which is heated to separate the gold (Ntibery et al. 2003). Both legal and illegal small scale mining is practised in the district (Avotri et al. 2002). In the Tarkwa area small-scale mining is found all around, both in the forest and along the rivers. It is practised all year around and number about 20 000 in the Wassa West district. Of these small-scale miners about 90 % are illegal. At the moment 168 small-scale mining concessions are valid in the region (Ntibery, B. 2004, pers. comm., 9 Sep). Figure 2-1:Blanket washing of milled ore (to the left). The concentrate from this gravity method is repeatedly washed and gold amalgam is formed when mercury has been added (to the right). At present there are totally about 237 companies (154 Ghanaian and 83 foreign) prospecting for gold and another 18 are operating gold mines in Ghana (Hilson 2002a). 2.4 Impact of mining and environmental degradation Large- and small-scale mining cause somewhat different environmental concerns. 6 Contamination of water resources in Tarkwa mining area of Ghana 2.4.1 Large-scale mining The major concerns observed in this area are (the following section is mostly collected from Akosa et al. (2002)): • Land degradation, for example removal of vegetative cover and destruction of flora and fauna • Impact due to processing technique includes contamination of water bodies and soil by release of cyanide (see below), arsenic, sulphates, and heavy metals as Pb, Cu, Zn and Fe • Cyanide spillage. There have been a number of accidental cyanide spillages in Ghana. The major spillages occurred in 1989, 1991, 1994, 1996, 1999 and 2001 • Roasting of ore containing pyrite gives a rise to the production of SO2 in the atmosphere which produces acid rain. The acid water then releases high levels of toxic ions from the rock matrix in the groundwater. This has been the main mode of extraction for the Prestea mine during the last decade. SO2 could also been transported with north-eastern winds from the Ashanti Goldfields in the northeast (Kortatsi 2004) • Noise and vibrations • Dust from blasting operations • AMD (Acid Mine Drainage) from solid waste from sulphidic ore leaching heavy metal and acidity into water and soil • Siltation of surface waters • Grease and oils from various activities in the mine The management of waste from large scale mining is done in accordance to approved environmental plans. The spent heap and waste rock heaps are stabilized and re-vegetated. Tailing slurries are channelled into tailing dams that also are re-vegetated. Reagent containers and packing materials are sold out to contractors who dispose of them. The monitoring of these contractors is poor. Spent oil and grease are sold to end-users. 2.4.2 Small-scale mining Illegal miners account for the most significant part of the environmental damage of the small-scale miners. Legal small-scale miners must have environmental permits and are monitored regularly by field officers. Amalgamation is the technique mostly used (Ntibery et al. 2003). The main environmental problems are mainly (the following section is mostly collected from Ntibery et al. (2003)): • Land degradation • Pollution of rivers and streams by mercury • Atmospheric impacts from mercury fumes during gold recovery and dust •Mercury in groundwater from accidental spillage during gold processing (Akosa et al. 2002) • AMD from solid waste from sulphidic ore leaching heavy metal and acidity into water and soil (Akosa et al. 2002) • Siltation of surface waters (Akosa et al. 2002) • Deforestation due to wood used for stabilizing mining shafts (Ntibery, B. 2004, pers. comm., 9 sep) • Damage to infrastructure due to undermining of roads and houses 7 Asklund and Eldvall LUTVDG/TVTG--5092--SE The management of waste on small-scale mines particularly illegal ones does not have a waste management plan but simply leave the waste. Estimated 5 tonnes mercury is released from small-scale mining operations in Ghana each year (Hilson 2001). High concentrations of mercury have been found in sediments and fish in the vicinity of small-scale mining activities using amalgamation as their main technique. The concentration in most fish fillets in these areas exceeds the recommendations of the United States Food and Drug Agency (Babut et al. 2003). 8 Contamination of water resources in Tarkwa mining area of Ghana 3 Location of the study area and geographical characteristics 3.1 The Wassa West district The Wassa West district occupies the mid-southern part of the Western region of Ghana with Tarkwa as its administrative capital. The population of the district is approximately 236 000 and is mainly composed of the indigenous Wassa tribe but all tribal entities in Ghana are well represented. Subsistence farming is the main occupation of the people although rubber, oil palm and cocoa are also produced Mining is the main industrial activity in the area (Avotri el al. 2002). The area lies within the main gold belt of Ghana that stretches from Axin in the southwest, to Konongo in the northeast (Kortatsi 2004). Location of the Wassa West district and the study area is shown in figure 3-1. Figure 3-1: Location of the study area 3.2 Climate Wassa west district is situated on the border of two climatic regions. The south part belongs to the south western equatorial climatic region and the northern part has a wet semiequatorial climate. Generally the rainfall pattern follows the northward advance and the southward retreat of the inter-tropical convergence zone that separates dry air from Sahara and the moisture-monsoon air from the Atlantic Ocean. The north air mass, locally called the Harmattan, brings in hot and dry weather during December to February ( Dickson & Benneh 1980). The area is characterized by double rainfall maxima. The first and largest peak occurs in June, whilst the second and smaller peak occurs in October. Around 53% of all rain in the region falls between March and July. The mean annual rainfall is approximately 1874mm with max and min values of 1449mm and 2608, respectively. The mean pH of the rain water in the area during 2000-2001 was 6.07 (Kortatsi 2004). The area is very humid and warm with temperatures between 26-30C°. ( Dickson & Benneh 1980) 3.3 Geology Ghana is underlain partly by what is known as the Basement Complex. It comprises a wide variety of Precambrian igneous and metamorphic rock which covers about 54% of the country, mainly the southern and western parts of the country, Figure 3-2. It consists mainly of gneiss, phyllites, schists, migmatites, granite-gneiss and quartites. The rest of the country is underlain by Palaeozoic consolidated sedimentary rocks referred to as the Voltaian Formation consisting mainly of sandstones, shale, mudstone, sandy and pebbly beds and limestones (Gyau-Boakye and Dapaah-Siakwan 1999). 9 Asklund and Eldvall LUTVDG/TVTG--5092--SE Figure 3-2: Simplified geological map of southwest Ghana (modified from Kuma 2004) The Basement complex is further divided into different sub provinces including the metamorphosed and folded rocks of the Birimian and Tarwaian system (Gyau-Boakye and Dapaah-Siakwan 2000). In several places these systems are intruded by sills and dykes of igneous rocks ranging from felsite and quartz porphyry to metadolerite, gabbro and norite (Kortatsi 2004). 10 Contamination of water resources in Tarkwa mining area of Ghana Figure 3-3: Geological map of the Tarkwa-Prestea area (modified from Kortatsi 2004) The geomorphology of the Tarkwa-Prestea area consists of a series of ridges and valleys parallel to each other and to the strike of the rocks. The strike of the rock are generally in north-south direction (Kortatsi 2004). Both the Tarkwaian and Birimian systems are folded along axes that trend northeast (Gyau-Boakye and Dapaah-Siakwan 2000). The general 11 Asklund and Eldvall LUTVDG/TVTG--5092--SE type of topography reflects underlying geology (Kortatsi 2004). The geology of the Tarkwa Prestea area is shown in Figure 3-3. The area has three main gold deposits. Placer or alluvial deposit, non-sulphidic paleplacer or free milling ore and oxidized ore (Kortatsi 2004). Alluvial deposits occur in streams draining areas with auriferous deposits where the bedrock only is slightly metamorphosed and intruded by Dixcove granite particularly in Biriminan rock areas (Kortatsi 2004). Non-sulphidic paleplacer ore occurs mainly in hard rock. It is particularly associated with Banket conglomerates of Tarkwa formation (Kortatsi 2004). Oxidized ore occurs in weathered rocks and is derived from sulphides, arsenopyrite, realgar (AsS), opiment (As2S3) pyrites etc (Kortatsi 2004). 3.3.1 The Birimian system The Birimian system consists of a great thickness of isoclinally folded metamorphosed sediments intercalated with metamorphosed tuff and lava. Large masses of granite have also intruded the Birimian system (Gyau-Boakye and Dapaah-Siakwan 2000). The Birimian system is largely folded. It is fissured to a larger extent compared to the Tarkwaian system. This system is divided into to upper and lower Birimian Series (Kortatsi 2004). The sediments are predominant in the lower part of the system. These sediments have been metamorphosed to schist, slate and phyllite (Gyau-Boakye and Dapaah-Siakwan, 2000). There are also some tuff and lava. The upper part of the system is dominantly of volcanic and pyroclastic origin. The rocks consist of bedded groups of green lava (Kortatsi 2004). Lava and tuff dominate this part (Gyau-Boakye and Dapaah-Siakwan 2000). Several bands of phyllite occur in this zone and are manganiferous in places. The thickest sequence occurs in Nsuta where manganese is being mined (Kortatsi 2004). In the Birimian system, gold occurs in five parallel, more than 300km long, northeasttrending volcanic belts. They are separated by basins containing pyroclastic and metasedimentary units. The gold occurrence is 2 to 30 ppm in quartz veins of laterally extensive major ore bodies. They deeply penetrate fissures and shear zones in contact between metasedimentary and meta-volcanic rocks. The veins consist of quartz with carbonate minerals, green sericite, carbonaceous partings and metallic sulphides and arsenides of Fe, As, Zn, Au, Cu, Sb and Pb (Dzigbodi-Adjimah 1993). The Birimian system has a higher content of heavy metals than the Tarkwaian system (Kortatsi 2004). 12 Contamination of water resources in Tarkwa mining area of Ghana 3.3.2 The Tarkwaian system The Tarkwaian system is an elongated and narrow syncline about 250 km long and 16 km wide (Kortatsi 2004). The system consists of slightly metamorphosed, shallow-water, 30 Awudua 5 25' 16 Pa m pam e Po n Prestea 45 50 43 15 tu Huniso M esa AN KO BRA Hu ni 37 Awiafu C 37 Adja Bippo esa m ak Nt a as AbosoF Huni Sandstone and Phyllite 30 5 20' 20 L E G E N D 55 50 45 10 Akontasi 20 16 65 15 20 30 20 14 15 Tarkwa 10 INTRUSIVE ROCKS Nsuta ben Upper Birrimian Epidiorite, Gabbro and allied Intrusions Dixcove Granite and Porphyry Faults D 46 Small Sills and Dykes 45 75 30 33 80 Achim 40 50 60 Road 80 80 5 15' Kawere Group BIRRIMIAN SYSTEM ona 50 40 Banket Series; Conglomerate Zone shown thus Anw Ahum abuni Ab om pu nu Tarkwa 60 Tarkwa Phyllite TARKWAIAN SYSTEM Railway River 45 30 10 50 59 55 50 18 wu iabe Bed 59 ng 43 po Su 46 A 15 50 ka Tra B 70 5 10' Settlement Agona O NS Strike and Dip 80 Bonsaso A GEOLOGICAL SECTIONS Scale:- Horizontal... 1cm = 2.50 km Vertical..... 1cm = 480 km B Wuruwuru R. Dompim Supong R. Ongwan Hill Traka R. W ur uw ur u 25 Bediabewu R. Enifutu R. Anwonaben R. Abompunu R. Ntakasa R. Huni R Ankobra R. Ahumabuni R. Line A-B Simpa 5 05' Line C-D 5 Miles 8 Kms. Railway 4 6 Mesamesa R. 3 4 Awiafutu R. 2 Huni R. 2 1 Pampam R. 0 Pone R. 0 Line E-F 2 10' 2 00' 2 05' 1 55' Figure 3-4: Geological map of the Tarkwa area. Coordinates are given as degrees North and West (Kortatsi 2004) 13 Asklund and Eldvall LUTVDG/TVTG--5092--SE sedimentary strata. It is chiefly sandstone, quartzite, shale and conglomerate and is resting on and derived from the Birimian system (Gyau-Boakye and Dapaah-Siakwan 2000). Intrusive igneous rocks contribute to about 20% of the total thickness of the Tarkwaian System in the Tarkwa area. These range from hypabyssal felsic to basic igneous rocks (Kuma 2004). Granitoids of the Dixcove Granitoids systems has also intruded the tarkwaian system in many places (Kortatsi 2004). A detailed geological map is presented in Figure 3-4. The rocks of the Tarkwaian system consist of the Kawere Group, The Banket Series, the Tarkwa Phyllite and the Huni Sandstone. Most of the rocks that resemble sandstone at the surface are weathered equivalents of parent quartzites (Kuma & Younger 2001). See table 3-1. Table 3-1: Division of the Tarkwaian system (from Kuma & Younger 2001) Series Kawere Group Banket Series Thickness (m) 250-700 120-160 Tarkwa Phyllite 120-400 Huni Sandstone 1370 Composite lithology Quartzites, grits, phyllites and conglomerates. Tarkwa phyllite transitional beds and sandstones, quartzites, grits breccias and conglomerates. Huni sandstone transitional beds, and greenish-grey phyllites and schists. Sandstones, grits and quartizes with bands of phyllite. The Kawere Group is the oldest group and the conglomerates consist of silicified Birimian greenstone and hornstone with minor jasper, quartz, quartz-porhyry, tourmaline-quartz rocks with Birimian phyllites and schists in a matrix with quartz, feldspar, chlorite, carbonate, epidote and magnetit (Kuma 2004). The Banket conglomerate consists of 90% quartz and the rest is Birimian schist, quartzite, hornstone, chert and gondite (Kuma 2004). It represents a fluviatile series (Kortatsi 2004). The Tarkwa Phyllite consists of chloritoid and magnetite or hematite with sericite and chlorite (Kuma 2004). Huni Sandstone (a quartzite) consists of variable amounts of feldspar, sericite, chlorite, ferriferous carbonate, magnetite or hematite and epidote. In the Tarkwaian system the gold occurs in various auriferous and quartz-pebble conglomerates (Ghana academy of arts and sciences 2003). 3.3.3 Granitoids Large masses of granitoids of the Cape Coast Granitoids Complex have intruded the Birimian system. It is well foliated and the potash rich rocks consist of muscovite and biotite granite and granodiorite ore porphyrblastic biotitic gneiss, applite and pegmatite. They are characterized by the presence of many enclaves of schists and gneisses. The Dixcove Granitoids Complex has intruded both the Birimian and Tarkwaian systems in many places. It consists of hornblende granite or granodiorite grading locally into quartz diorite and hornblende diorite. The complex forms non-foliated discordant or semidiscordant bodies in the enclosing country rock, generally Upper Birimian meta-volcanics. 14 Contamination of water resources in Tarkwa mining area of Ghana 3.3.4 Basic intrusives Intrusive igneous rocks contribute to about 20% of the total thickness of the Tarkwaian System in the Tarkwa area. These range from hypabyssal felsic to basic igneous rocks (Kuma 2004). The igneous rocks are mainly of basic composition. Their composition range from quartz-porphyry and albite through keratophyre, granophyric albite-dolerite to fresh olivine-gabbro and norite. Pyrite is common in many of the igneous rocks and quartz veins that intruded the Birimian and the Tarwaian rocks that underlie the area (Kortatsi 2004). 3.4 Hydrogeology Groundwater is the main source of water supply in the study area. Most major towns in the area except from Tarkwa rely solely on groundwater. To match the demand for potable water the number of boreholes and hand dug wells are increasing rapidly (Kortatsi 2004). Surface water taken from the River Bonsa at Bonsaso is treated and distributed to Tarkwa town. Some villages between Bonsaso and Tarkwa are also connected to the pipe (Nankara, T. 2004, pers. comm., 16 Sep). Yield varies from 0.4-18 m3h-1 with an average of 2.4 m3h-1. The depth varies between 18m to 75m with an average of 35.4m but has little or no effect on borehole yields (Kortatsi 2004). In the Tarkwa-Prestea area groundwater occurrence is associated with the development of secondary porosity through fissuring and weathering. The rock underlying the area lack primary porosity since they are consolidated. The weathering depth is greatest in the Birimian system where depths between 90m and 120 m have been reached. Also in granites, porhyrites, felsites and other intrusive rock the weathering depth is great. In the Tarkwa system however, and especially in the Banket series quartzites, grits, conglomerates and Tarkwa phyllite, the weathering depth rarely exceed 20 m. Clay, silts, sandy clays and clayey sands are mostly the result of the weathering. In this area two types of aquifers occur. The weathered aquifer occurs mainly above the transition zone between fresh and weathered rock. Due to the soils content of clay and silt, these aquifers have high porosity and storage but low permeability. The aquifer in the fractured/fissured zone occurs below the transition zone. They have relatively high transmissivity but low storage (Kortatsi 2004). The recharge of groundwater in the area occurs mainly by direct seepage or infiltration. In some places groundwater is in hydraulic contact with rivers and recharge from them can also take place (Kortatsi 2004). Groundwater circulation in the Tarkwa-Prestea area is mainly localized due to the numerous low hills that act as groundwater divides. Groundwater circulation is mainly restricted to quartz veins and fissures-faults-brecciated zones. Groundwater velocities are not known. It might be that pollution has not yet reached the domestic wells (Kortatsi 2004). Low conductivity values of the groundwater in the area indicate that the water is unable to react with the rock matrix to equilibrium which indicates short resident times (Kortatsi 2004). 15 Asklund and Eldvall LUTVDG/TVTG--5092--SE Groundwater in mining areas as the Tarkwa-Prestea area is known to be vulnerable to pollution from mining that may have a serious effect on human health. In gold mining areas sulphides oxidation leads to production of low pH in the groundwater that encourages the dissolution of trace metals in the groundwater in very high concentrations (Kortatsi 2004). 3.5 Soils There are mainly two types of soil in the Tarkwa-Prestea area, the forest oxysols in the south and the forest ochrosol-oxysol integrates in the north (Kortatsi 2004). The forest oxysols are porous, well drained, and generally loamy brown to orange. Due to the heavy and plentiful rainfalls in the south, a high degree of leaching and reduction of calcium, magnesium and other nutrients have occurred in the soil. This has made the soil acidic. The forest ochrosol-oxysol integrated is an intermediate between the forest oxysols and the forest orhrosol. The forest ochrosol-oxysol integrated is highly coloured as it is less leached, as a result of reduced rainfall in the north. It contains more of its nutrients and is therefore more alkaline then the forest oxysols in the south (Kortatsi 2004). The soil in the Tarkwa area consists of mostly silty-sands with minor patches of laterite, mainly on hilly areas (Kuma & Younger, 2001). The distribution of size fractions is given in Table 3-2. Table 3-2: Characteristics of soils in the Tarkwa area (from Kuma & Younger 2001) Soil type Banket Huni Kawere Tarkwa Phyllite Weathered dyke Texture Silty-Sand Laterite Silty-Sand Silt Sand Laterite Silt Percentage Gravel 2 69 2 0 69 3 Sand 59 14 55 47 9 20 Silt 20 10 33 40 13 64 All these soil types are considered to be clayey soil-types (Svensson 1999). 16 Clay 10 7 10 13 16 13 Contamination of water resources in Tarkwa mining area of Ghana 4 Environmental geochemistry The groundwater composition varies widely and is a combined result of the composition of the water entering the groundwater reservoir and the reactions with minerals present in the rock that may modify the water composition. Some minerals dissolve quickly and significally change the water composition, like carbonates, others dissolve slowly and have less effect on the water composition, like silicates. The retention time is also important in determining the water chemistry. Long residence times allow reactions to take place and these waters are likely to have higher concentrations of ions than water with short residence times (Appelo & Postma 1999). Usually in unaffected environments the concentration of most metals is very low and is mostly determined by the mineralogy and the weathering (Espeby & Gustafsson 2001). There are a few examples of local metal pollution through natural weathering but in most cases metals become an environmental and health issue because of anthropogenic activity. Mainly mining and smelting plants release metals from the bedrock (Walker & Sibly 2001). Soil concentration of adsorbing surfaces (oxide surfaces, clay mineral and humic substances) and the pH are very important parameters effecting the transportation of metals in the groundwater system (Espeby & Gustafsson 2001). 4.1 Oxidation of sulphide minerals and Acid Mine Drainage Oxidation of pyrite and other sulphide minerals by oxygen have a large environmental impact and play a key role in Acid Mine Drainage (AMD). It is a source of sulphate, acidity, and iron in groundwater and is a source of heavy metals in the environment. Sulphide mine tailings are a notorious source to contamination of both streams and groundwater by heavy metals (Appelo & Postma 1999) see Figure 4-1. Figure 4-1: Pyrite oxidation by oxygen supplied by purely advective flow. Groundwater saturated with O2 is transported through a pyritic layer. Oxygen is consumed and values of SO42- and Fe2+ are increased (Appelo & Postma 1999). 17 Asklund and Eldvall LUTVDG/TVTG--5092--SE Sulphide minerals can be oxidized through three different processes: chemical oxidation with oxygen, chemical oxidation with Fe3+ and oxidation catalyzed by micro organisms. All these reactions accelerate at low pH. (SEPA 2002) Chemical oxidation with oxygen: The following reaction describes the overall process FeS 2 (s ) + 15 7 O2 + H 2 O ⇒ Fe(OH )3 (s ) + 2SO42− (aq ) + 4 H + 4 2 This complete oxidation involves oxidation of both polysulphide S22- and Fe2+. Under natural conditions the oxidation often proceeds in two steps: 7 FeS 2 (s ) + O2 + H 2 O ⇒ Fe 2+ (aq ) + 2SO42− (aq ) + 2 H + 2 1 1 Fe 2+ (aq ) + O2 + H + ⇒ Fe 3+ (aq ) + H 2 O 4 2 Fe3+ may precipitate as FeOOH depending on pH. Energy yield of polysulphide oxidation is larger than that of Fe2+ oxidation and incomplete oxidation of pyrite, resulting in solutions rich of Fe2+ and SO42- occurs naturally as well as complete oxidation. The second mechanism for pyrite oxidation is by reaction with Fe3+: FeS 2 (s ) + 14 Fe 3+ (aq ) + 8H 2 O ⇒ 15Fe 2+ (aq ) + 2SO42− (aq ) + 16 H + This reaction is particularly important at low pH since the solubility of Fe3+ decreases as a third function with increasing pH. This reaction is about ten times faster than the first reaction. The process rapidly consumes all Fe3+ and oxidation would cease unless Fe3+ is replenished by oxidation of Fe2+ by oxygen: 1 1 Fe 2+ (aq ) + O2 + H + ⇒ Fe 3+ (aq ) + H 2 O 4 2 Below pH 4.5 this reaction is considerably slower than pyrite oxidation of Fe3+ and is the rate limiting step at these pH values (Appelo & Postma 1999). Iron oxidizing bacterias, as Thiobacillus ferrooxidans, are able to increase the Fe2+ oxidation rate up to four orders of magnitude which brings up this reaction to the same magnitude as pyrite oxidation by Fe2+ (Salmon 2003). There is a close connection between sulphide minerals, especially arsenopyrite, and gold in most parts of Ghana (Smedley 1996). AMD have been reported from a number of mines in Ghana. Monitoring of a large spoil heap in the Tarkwa area show water quality consistent with AMD characteristics. The pH is consistently below 4, has high concentrations of sulphate, silica, aluminium, iron, and manganese, and shows little variation during the year (Kuma 2003). The major minerals associated with AMD that occurs in the Tarkwa-Prestea area shown in Table 4-1. The problems associated with AMD can therefore be expected in gold mining areas in Ghana. The pH levels of the groundwater in the Tarkwa Prestea indicate AMD. Acid rain and acid geothermal waters cannot fully explain these low values (Kortatsi 2004). 18 Contamination of water resources in Tarkwa mining area of Ghana Table 4-1: Minerals associated with AMD in the study area (Kortatsi 2004) Mineral Arsenopyrite Bournonite Chalcopyrite Galena Pyrite Sphalerite Tennalite Composition FeS2, FeAs, FeAsS PbCuSbS3 CuFeS2 PbS FeS2 ZnS [(Cu, Fe, Zn,)As4S] 4.2 Factors effecting the mobility of heavy metals in the environment Besides the metals man have created through nuclear reactions the rest have been on earth since the planet was formed (Walker & Sibly 2001). The metals exist naturally in the bedrock and are released through weathering. In water, metals exist in different forms, both solved and suspended, depending on a number of different parameters. The solubility, transportation and toxicity differ between different metal species. The transportation of metals with groundwater is normally affected by sorption to solid aquifer material (Appelo & Postma 1999). The most important chemical retention mechanisms are sorption processes and precipitation (Espeby & Gustafsson 2001). Other chemical processes of importance are redox reactions and complexation. An increased aqueous complexation often makes an element more soluble, but the form is often less toxic. The redox status decides the speciation of some redox-sensitive elements. Different redox species have different retention capacity and the redox status is important for transport (Espeby & Gustafsson 2001). These mechanisms and the mobility of metals are affected by a number of different parameters e.g. the oxidation state of the metal ion, pH and Eh (Appelo & Postma 1999). Determining the mobility of heavy metals is a very complex matter. 4.2.1 Sorption Processes The pH is crucial for the extent of sorption. Anions adsorb more strongly with decreasing pH while the reverse is true for cations (Espeby & Gustafsson 2001). This is caused by the increase in H+, which binds to charged surfaces instead of metals. Since binding sites are limited, metals will go into solution. Sorption processes is a generic term for a number of different mechanisms. Adsorption indicates that a chemical adheres to the surface of the solid, absorption suggest that the chemical is taken up into the solid and exchange involves the replacement of one chemical for another at the solid surface. The major difference between adsorption and ion exchange is that ion exchange considers the concentration of two chemicals and adsorption considers one. (Appelo & Postma 1999). Ion Exchange: An ion in solution can be electrostatically attracted to a charged surface, and is adsorbed (Fetter 2001). Only electrostical forces cause the adsorption and the ion is situated at a certain distance from the surface and can easily be substituted by competing ions. Soil particles are mostly negatively charged and ion exchange is most important for cations but Fe- and Aloxides have positive charge, and adsorption of anions can occur 19 Asklund and Eldvall LUTVDG/TVTG--5092--SE (Espeby & Gustafsson 2001). Ion exchange sites are found primarily on clay and organic materials however all soils and sediments have some ion-exchange capacity (Fetter 2001) A general order of cation exchangeability in groundwater is (Fetter 2001 and Appelo & Postma 1999): Na > K > Fe > Mn > Mg > Ca Adsorption and absorption: Ions have different tendencies to form complexes with different substances. Many cations can form complexes with hydroxyl groups (OH), or carboxyl groups (COOH) and therefore these ions easily are adsorbed to surfaces with these groups. Many anions form complexes with surfaces containing Fe or Al and can be adsorbed by them (Espeby & Gustafsson 2001). In soils, only particles with large specific surface have the ability to adsorb ions significantly. Coarser particles have much smaller surface and are therefore insignificant for sorption processes. Examples of soils with large specific surface are clay-minerals and oxides (Espeby & Gustafsson 2001). The charge of clay minerals varies depending on protonation of of surface oxygen and deprotonation of surface hydroxyls. The surface charge and capacity of sorption is therefore pH-dependent (Appelo & Postma 1999). Table 4-2 shows adsorption of two cations, Calcium (mostly ion exchange), and Copper (mostly adsorption and absorption), and one anion, Arsenic, to different surfaces (Espeby & Gustafsson 2001). Table 4-2: Examples of adsorption by different soils Type of surface Dominating mechanism Clay minerals Fe/Al oxides Mn-oxide Ion exchange Adsorption or absorption Adsorption or absorption Adsorption of Calcium Large Very small Small Adsorption of Copper Large Medium Very large Adsorption of Arsenic Small Very large Medium 4.2.2 Precipitation The most important precipitations are different oxides/hydroxides and carbonates. Concerning Manganese, Aluminium, Crom and Iron, formation of oxides/hydroxides are important throughout the natural range of pH, but also for other elements when they occur in high concentrations. Some elements form carbonates and hydroxycarbonates but only at high pH>7-8. During reducing conditions sulphides can be very important for precipitation (Espeby & Gustafsson 2001). 4.2.3 Redox potential Redox reactions imply a electrone transfer from one atom to another. Redox processes are generally very slow (Appelo & Postma 1999). The solubility of many substances is governed by the redox state. Some examples of metals greatly effected by the redox state are: Manganese, Cromium, Arsenic, Selenium and Iron. Sulfate ions can be reduced to sufide and react with metals and form complexes which often have very low solubility. This can considerably decrease the mobility of metals such as, Iron Copper, Lead, Zink, Mercury, Cadmium and Nickel (Espeby & Gustafsson 2001). 20 Contamination of water resources in Tarkwa mining area of Ghana Different species are reduced in a specific sequence. O2 is reduced first, followed by reduction of nitrate, followed by reduction of Mn(IV) to Mn(II), followed by reduction of Fe(III) to Fe(II), followed by reduction of organic matter, SO42- reduction etc. (Appelo & Postma 1999) 4.2.4 Grouping of heavy metals To enhance the clearness of this review we have separated some of the most important metals into four groups. The following information is collected mostly from Espeby & Gustafsson (2001): Group 1: Metals forming hydroxides. Aluminium and Crom(III). Both these metals easily form complexes in the form of hydroxides, which dominate when pH> approx. 4-5. Precipitation of the hydroxides commonly determines the solubility above this pH. At lower pH the solubility is usually determined by adsorption. These metals form strong complexes with organic ligands and because of this, the metals are usually transported as organic complexes at pH> 4-5. The solubility of Cr(VI) is instead governed by adsorption to oxides and varies significally with pH. Group 2: Strongly adsorbing cations. Copper, Lead and Mercury. These metals are strongly bonded to variable charged surfaces, for example clay minerals, because they easily form complexes in the form of hydroxides. The transport of these metals is retarded mostly through adsorption to clay minerals and Fe/Al oxides. The adsorption of these metals is very strong. At high pH (pH>7-8) precipitations like Cu2(OH)2CO3 (malakite) can be important . The solubility of the free ions is very low and the metals are almost always transported in different complexes (mostly organic complexes). If conditions are reducing, the metals are strongly bonded as sulphides with very low solubility. Group 3: Cations which are adsorbed with average strength. Cadmium, Nickel and Zink These metals are adsorbed fairly strongly in the soil, both through specific adsorption and ion exchange. This group is not adsorbed as strongly as group 2. The solubility varies strongly as a function of pH (lower pH gives higher solubility), organic content and mineralogy. During reducing conditions these metals form sulphides with low solubility. Group 4: Weakly adsorbed cations. Calcium, Manganese, Potassium and Sodium. Potassium and Sodium are almost solely bonded electrostatically through ion exchange while Calcium and Manganese also form weak surface complex with organic materials. Arsenic Arsenic is a metalloid, behaving more like a non-metal then a metal. It forms compound with oxygen. That makes As mobile in both oxidizing and reducing environments and it is mainly controlled by adsorption. That makes solid concentrations of oxides and hydroxides of Fe, Al and Mn essential parameters for controlling As transportation (Smedley & Kinniburgh 2001). 21 Asklund and Eldvall LUTVDG/TVTG--5092--SE 4.2.5 Anions Anions as phosphate can be adsorbed strongly, mostly to oxide surfaces. Sulfate is weakly adsorbed, this mechanism is only important at low pH (pH<6). 4.3 Major metallic contaminants related to mining Several of the metals are essential to the human body. The metals are mainly utilised in enzymes to make them function properly. But we only need the metals in small quantities (WHO 1996). Some of them we need as trace elements and some are non-essential for us. Calcium, sodium and magnesium are essential metals and cobalt, molybdenum, selenium, chromium, nickel, vanadium and silicon are added as trace metals. Mercury and cadmium are examples of non-essential metals (Walker & Sibly 2001). The term heavy metals is used for metals with a density more than 5 g/cm3 (Walker & Sibly 2001). Heavy metals important in environmental and health issues are for example Arsenic, Lead, Cadmium, Copper, Chromium, Mercury, Zink, Cobalt, Nickel, Tin and Vanadium (SEPA 2003). Those are not normally a part of the human body and are more poisonous to us than other metals (WHO 1996). Many metals can be stored in living tissue and remain there for a long time (SEPA 2003). If a metal acts as a pollutant or becomes harmful to our health depends on both the properties of the metal and the environment it is acting in. Both humans and plants exhibit a big variation concerning both the need of essential metals and the sensitivity to non-essentials metals and to high levels of essential metals and trace metals. Some metals are harmful mostly to plants, for example zinc, nickel and chromium, and some mostly to animals, for example cadmium and molybdenum (Pettersson 1994). Previous work in the Tarkwa area has mainly been conducted by Kuma and Kortatsi. Some of their results are presented in the section below. 4.3.1 Aluminium Aluminium salts are widely used in water treatment as flocculants. An associated link between the Alzheimer disease and aluminium in drinking water has lately been suspected. The associated link is not confirmed and more studies need to be conducted. The epidemiological and physiological evidence do not at present support a health-based guideline value for aluminium. However aluminium concentration levels of 0.2 mg/l have been suggested as a good compromise between practical use and caution (WHO 1996). Previous studies show maximum levels in groundwater to be 2.51 mg/l (Kortatsi 2004) and maximum levels in surface water to be 0.22 mg/l (Kuma 2004). 4.3.2 Arsenic The results of available studies indicate that arsenic may be an essential element for several animal species, but there is no evidence that it is essential for humans. The level of arsenic in natural waters generally varies between 1 and 2 µg/l. Concentrations may be elevated, however, in areas containing natural sources; values as high as 12 mg/l have been reported. Inorganic arsenic compounds are classified as carcinogenic to humans. Lethal doses in humans range from 1.5 mg/kg to 500 mg/kg of body weight depending on the compound. 22 Contamination of water resources in Tarkwa mining area of Ghana Early clinical symptoms of acute intoxication include abdominal pain, vomiting, diarrhoea, muscular pain, and weakness, with flushing of the skin. These symptoms are often followed by numbness and tingling of the extremities, muscular cramping, and the appearance of a papular erythematous rash. Within a month, symptoms may include burning paraesthesias of the extremities, palmoplantar hyperkeratosis, Mee’s lines on fingernails, and progressive deterioration in motor and sensory responses. Signs of chronic arsenicalism, including dermal lesions, peripheral neuropathy, skin cancer, and peripheral vascular disease, have been observed in populations ingesting arsenic-contaminated drinking-water. In view of reducing the concentration of arsenic in drinking-water, a provisional guideline value of 0.01 mg/l is recommended. The guideline value has been derived on the basis of estimated lifetime cancer risk (WHO 1996). Previous studies show maximum levels in groundwater to be 0.046 mg/l (Kortatsi 2004) and maximum levels in surface water to be 0.137 mg/l (Kuma 2004). 4.3.3 Cadmium Cadmium is chemically similar to zinc and occurs naturally with zinc and lead in sulphide ores. Cadmium concentrations in unpolluted natural waters are usually below 1 µg/l. Median concentrations of dissolved cadmium measured at 110 stations around the world were less then 1 µg/l. The maximum value recorded being 100 µg/l in the Rio Rimao in Peru. Food is the main source of cadmium intake. Crops grown in polluted soil or irrigated with polluted water may contain increased concentrations, as may meat from animals grazing on contaminated pastures. The estimated lethal oral dose for humans is 350-3500 mg of cadmium; a dose of 3 mg of cadmium has no effects on adults. A guideline value for cadmium is calculated to 0.003 mg/l drinking-water (WHO 1996). Previous studies show maximum levels in groundwater to be 0.003 mg/l (Kortatsi 2004) and maximum levels in surface water to be <0.05 mg/l (Kuma 2004). 4.3.4 Chromium Chromium is widely distributed in the earth’s crust. In water, chromium(III) is a positive ion that forms hydroxides and complexes, and is adsorbed at relatively high pH values. The ratio of chromium(III) to chromium(VI) varies widely in surface water. In general, chromium(VI) salts are more soluble than those of chromium(III), making chromium(VI) relatively mobile. The daily chromium requirement for adults is estimated to be 0.5-2 µg of absorbable chromium(III). That equals to approximately 2-8 µg of chromium (III) per day since only about 25% can be absorbed. The average concentration of chromium in rainwater is approximately 0.2-1µg/l. Natural chromium concentrations in seawater have been measured to 0.04-0.7 µg/l. The chromium concentration in groundwater is generally low (<1 µg/l). The natural total chromium content of surface water is approximately 0.5-2 µg/l and the dissolved chromium content 0.02-0.3 µg/l. Most surface water contain between 1 and 10 µg of chromium per litre. In general, the chromium content of surface water reflects the extent of industrial activity. The health effects are mostly determined by the oxidation state. Therefore two different guidelines for chromium(III) and chromium(VI) should be derived. However, current analytical methods and the variable speciation of chromium in water favour a guideline value for total chromium. As a practical measure, the guideline is set to 0.05 mg/l, which is 23 Asklund and Eldvall LUTVDG/TVTG--5092--SE considered to be unlikely to give rise to significant risks to health (WHO 1996). Previous studies show maximum levels in groundwater to be 0.066 mg/l (Kortatsi 2004) and maximum levels in surface water to be 0.49 mg/l (Kuma 2004). 4.3.5 Iron Iron is an essential element in human nutrition. Estimates of the minimum daily requirement for iron depend on age, sex, physiological status and iron bioavailability and range from about 10 to 50 mg/day. In drinking-water supplies, iron(II) salts are unstable and are precipitated as insoluble iron(III)hydroxide, which settles out as a rust-coloured silt. Anaerobic groundwater may contain iron(II) at concentrations of up to several milligrams per litre without discolouration or turbidity in the water when directly pumped from a well. Turbidity and discolouration may develop in piped systems at iron levels above 0.05-0.1 mg/l, whereas levels of 0.3-3 mg/l are usually found acceptable. As a precaution against storage of excessive iron in the body a provisional maximum tolerable daily intake was calculated to about 2 mg/l drinking water. That level does not present a hazard to health. The taste and appearance of drinking water will usually be affected below this level, although iron concentrations of 1-3 mg/l can be acceptable for people drinking anaerobic well-water. No health-based guideline value for iron is proposed (WHO 1996). Previous studies show maximum levels in groundwater to be 18.3 mg/l and maximum levels in surface water to be 4.01 mg/l (Kuma 2004). 4.3.6 Lead Lead is the most common of the heavy elements, accounting for 13 mg/kg of the earth’s crust.More than 80% of the daily intake of lead is derived from the ingestion of food, dirt, and dust. That means that an average of 5 µg/l lead intake from water forms a relatively small proportion of the total daily intake for children and adults, but a significant one for bottle-fed infants. Lead is possible human carcinogen (evidence inadequate in humans, sufficient in animals) and it is also a cumulative poison so that any increase in the body burden of lead should be avoided. A provisional tolerable daily intake is set to 3.5 µg of lead per kg of body weight for infants lead to a calculated guideline value of 0.01 mg/l. As infants are considered to be the most sensitive subgroup of the population, this guideline value will also be protective for other age groups (WHO 1996). Previous studies show maximum levels in groundwater to be 0.026 mg/l (Kortatsi 2004) and maximum levels in surface water to be <0.05 mg/l (Kuma 2004). 4.3.7 Manganese Manganese concentrations above 0.1 mg/l impart an undesirable taste to drinking water. Even at about 0.02 mg/l, manganese will form coatings on piping that may later tear off as a black precipitate. When manganese(II) compounds in solution undergo oxidation, manganese is precipitated. Humans can consume as much as 20 mg/day without apparent ill effects. Manganese is believed to have a neurotoxic effect; a provisional health-based guideline value of 0.5 mg/l is proposed to protect public health (WHO 1996). Previous studies show maximum levels in groundwater to be 1.3 mg/l (Kortatsi 2004) and maximum levels in surface water to be 2.43 mg/l (Kuma 2004). 24 Contamination of water resources in Tarkwa mining area of Ghana 4.3.8 Mercury Almost all mercury in uncontaminated drinking water is thought to be in the form of Hg2+. It is only the carbon-mercury bond in organic mercury compounds that are chemically stable. The solubility of mercury compounds in water varies. Mercury(II) chloride is readily soluble, mercury(I) chloride much less soluble, mercury sulphide has a very low solubility and elemental mercury vapour is insoluble. Some anaerobic bacterias are capable of mercury methylation. Methyl mercury can then easily enter the food chain as a consequence of rapid diffusion and tight binding to proteins. Environmental levels of methyl mercury depend on the balance between bacterial methylation and demethylation. Naturally occurring levels of mercury in groundwater and surface water are less than 0.5 µg/l. The WHO guideline value for total mercury is 0.001 mg/l. Previous studies show maximum levels in groundwater to be 0.037 mg/l (Kortatsi 2004) and maximum levels in surface water to be 0.093 mg/l (Kuma 2004). 4.3.9 Nickel In aqueous solution, nickel occurs mostly as the green hexa-aquanickel(II) ion, Ni(H2O)62+. The nickel ion content of groundwater may increase as a result of the oxidation of natural nickel containing ferrosulphide deposits. Oxidation can occur if the groundwater table is lowered or if nitrate has leached from the soil. Nickel concentrations in drinking water around the world are normally below 20µg/l, although levels up to several hundred micrograms per litre in groundwater and drinking water have been reported. Leaching from nickel-chromium plated taps and fittings is also a factor. The nickel intake from food exceeds that from drinking water, even if a health-based guideline value for drinking water is calculated to 0.02 mg/l. That should provide sufficient protection even for nickelsensitive individuals (WHO 1996). Previous studies show maximum levels in groundwater to be 0.076 mg/l (Kortatsi 2004). 4.3.10 Nitrate and nitrite Nitrate and nitrite are naturally occurring ions that are part of the nitrogen cycle. The nitrate ion (NO3-) is the stable form and it can be reduced by microbial action to a nitrite ion (NO2-) which is a relatively unstable oxidation state for the ion. It is the nitrite ion that constitutes the toxicity to humans. It is involved in the oxidation of normal haemoglobin to methaemoglobin, which is unable to transport oxygen to the tissues. Therefore the health guideline for nitrate-nitrogen is set to 10 mg/l. This value should not be expressed in terms of nitrate-nitrogen but as nitrate itself which is the chemical entity of health concern, and the guideline value for nitrate alone is therefore 50 mg/l (WHO 1996). Previous studies show maximum levels in groundwater to be (NO3-) 27.0 mg/l (Kortatsi 2004) and maximum levels in surface water to be (NO3-) 60 mg/l (Kuma 2004). 4.3.11 Sulphate The presence of sulphate in drinking water results in a noticeable change of taste. The lowest taste threshold concentration for sulphate is approximately 250 mg/l. The physiological effects resulting from the intake of large quantities of sulphate are catharsis, dehydration, and gastrointestinal irritation. Water containing magnesium sulphate at levels above 600 mg/l acts as a purgative in humans. Sulfate may also contribute to the corrosion of distribution systems. Drinking water should not have sulphate levels exceeding 500mg/l 25 Asklund and Eldvall LUTVDG/TVTG--5092--SE (WHO 1996). Previous studies show maximum levels in groundwater to be 21.0 mg/l and maximum levels in surface water to be 490 mg/l (Kuma 2004). WHO:s guidelines for important elements are summarized in Table 4-3 Table 4-3: Summary of WHO:s guidelines for drinking water (µg/l) Element WHO Guideline Al 200 As(tot) 10 Ba 700 B 300 Cd 3 Cl 250 000 Cr 50 Cu 2000 Element WHO Guideline Fe 3000 Mn 500 Ni 20 NO350 000 Na 200 000 Pb 10 SO42500 000 Zn 3000 26 Contamination of water resources in Tarkwa mining area of Ghana 5 Materials and methods 5.1 Sampling plan Information for supporting the sampling plan was collected during spring and summer of 2004 in Sweden and at the University of Ghana during the period 24/8-8/9 2004. In Ghana, information was collected at the Department of Geography and Resource Development, the Department of Geology and the Water Research Resource Institute, CSIR. A review was made of previous work not to overlap our work with previous studies in the area made by Kuma 2004 and Kortatsi 2004. Our supervisors had areas of special interest which were also taken into account in preparing the sampling plan. In Tarkwa, the Wassa West District Assembly and Small Scale Mining Centre were visited to finalize the sampling plan. More information concerning water supply in different communities and borehole data were collected. During the field study the sampling plan was revised according to local conditions. 5.2 Field methods The fieldwork was conducted during September 2004. Both ground and surface water samples were collected. Wassa West Assembly supported us with a car, driver and a guide. This enabled us to visit remote locations and expand our sampling. Our sampling positions (marked red) and supplementary information is presented in Figure 5-1. 5.2.1 Groundwater sampling The most important aspects according to our sampling protocol include the following: •Clear pump before sampling to avoid any stagnant water in the pump system. •Pumping for 7 min is usually adequate. •Filter the sample through a 0.45 µm filter. •Rinse the sampling bottle with groundwater before taking a sample. •Avoid mixing water with air at sampling •Avoid sampling during heavy raining Each sample was collected in 100 and 50 ml polyeten bottles where the 50 ml bottle was acidified with concentrated HNO3. Measured field parameters are Eh, conductivity, pH and temperature using Ecoscan pH 6, Ecoscan Con 5 and Hach Sension 2. A flowcell was used for measuring the Eh value. Measurements were made until stable values were achieved. The sample sites were positioned with a Cobra GPS100 to get their exact location. A calibration of the pH-meter was conducted each morning and also during field studies if unusual measures were made. A calibration of the Eh-meter and the conductivity-meter were made in the mornings of the 10th, 13th and 15th of september. Field measurements and sample collections were done during the period 10/9-15/9 2004. This period is at the end of the rainy season and it usually rained at least once a day during the period. 27 Asklund and Eldvall LUTVDG/TVTG--5092--SE Information about the wells was also collected when possible in the field. Unfortunately there was no information about the depth or screening of the wells. Figure 5-1: Map with locations of sampling points. The numbers mark the sampling numbers (Modified from Kortatsi 2004) 5.2.2 Surface water sampling Treated water from the River Bonsa is distributed to Tarkwa and its surroundings. Both treated and untreated water was collected and field measurements were taken to examine the quality of the treatment and quality of distributed water. The flowcell was not used in for the untreated surface water sample. Otherwise the same parameters were analysed as for groundwater sampling. 5.3 Water analysis Determination of carbonate alkalinity was measured by following standard SS-EN ISO 9963-2. Automatic titration equipment ABU 80 Autoburette, PHM 82 Standard pH meter and TTT80 Titrator from Radiometer Copenhagen were used. Unacified samples were used. 28 Contamination of water resources in Tarkwa mining area of Ghana For organic carbon, NPOC, a TOC-5000 Shimadzu Total Organic Carbon Analyser was used. Cl-, NO3- and SO42- were analysed at KTH on Dionex DX-120 Ion Chromatograph. Unacified samples were used. PO43--P (acified samples) and NH4+-N (unacified samples) was determined using Aquatec 5400 Analyser and 5027 Sampler, Tecator following application note ASN 140-01/90 and ASN 146-01/90. These analyses were performed at the laboratory of the Department of Land and Water Resource Engineering at KTH. Trace elements were analysed at the Department of Geology and Geochemistry at Stockholm University. Acified samples were used. Element concentrations were measured on an ICP-OES (Optical Emission Spectroscopy) made by Varian (Varian Vista Ax Pro, equipped with a CCD camera) and an axial mounted torch. All runs were made on a system using a small concentric spray chamber and a seaspray nebulizer (RF power 1.3kW, 5s integration time, 0.9 L/minute nebulizer flow). The typical precision in analyses based on measurements of certified standards was typically better than 4%. The precision was only available if using different emission lines in different concentration ranges. This means that several emission lines must be measured for each element. For example for Ca; the Ca 396.847 nm emission line was used up to 3 ppm and 317.933 nm from 3 ppm and up. 5.4 Treatment of analytical data Analytical treatment of data was made in Excel, AquaChem, PHREEQC, Minteq, Surfer and Arcview. When analyzing in AquaChem, half of the detection limit was used for values below the detection limit. This is madein order not to exclude important minerals when modelling. For analyzing in Surfer and Arcview values below detection level were set to zero. The depths of drilled wells are set to 35 m which is the average depth in the area. These well are supposed to represent confined or semi-confined aquifers. Hand-dug well are approximated to a depth of 5 m are supposed to represent unconfined aquifers. Electro neutrality of anions and cations were calculated to determine the accuracy of chemical analysis. If samples depart more than 5% from electroneutrality a comparison of sums of anions, cations and conductivity were also made to determine where the error is likely to be situated (Appello and Postma 1999). A correlation matrix was made for all analysed parameters for three groups, all wells, deep wells and shallow wells. To keep a 95% confidence level (p=0.05) samples with five or less analyses were excluded. For the group shallow wells with six samples only, correlations above r2= 0.66 (r= 0.81) are significant (Håkansson & Peters 1995). Marked r-values in the correlation matrices are all statistically significant. Parameters that Aquachem can not handle are excluded from the correlation matrices. 29 Asklund and Eldvall LUTVDG/TVTG--5092--SE 30 Contamination of water resources in Tarkwa mining area of Ghana 6 Results and discussion Detailed data on groundwater chemistry is presented in Appendix 1-4. Statistical summary for groundwater chemistry is presented in Table 6-1. To be able to draw further conclusions on groundwater chemistry and dominating processes in the area the wells were divided, according to if they are drilled or hand dug, into two groups, deep and shallow wells. 6.1 Field measured parameters Average groundwater temperature was measured to 26.6 C° ranging from 25.4-28.5 C° The pH varies between 4.19 to 6.92 with an average of 5.38. The redox potential was measured within a range of 192-523 mV with an average of 357 mV. No samples have negative redox potentials. Electric conductivity ranges between 11.0 and 780 µS/cm with an average of 301 µS/cm. Table 6-1 shows a statistical summary of groundwater chemistry for all the wells. Elements with only a few samples above the detection limit are not used in further discussion and are not included. The positions of the wells are presented graphically in Figure 5.1 and in Appendix 1. Table 6-1: Statistical summary of groundwater chemistry (values exceeding WHO:s guidelines for drinking water are marked bold). Parameter pH Eh Cond Temp DOC HCO3NO3SO42PO43ClNH4+ Na+ K+ Ca2+ Mg2+ Al As(tot) B Ba Cd Cr Cu Fe Li Mn Ni Pb Si Sr Zn Unit mV µS/cm C° mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l µg/l µg/l µg/l µg/l µg/l µg/l µg/l µg/l µg/l µg/l µg/l µg/l µg/l µg/l µg/l Min 4.19 192 11.0 25.4 0.636 bld bdl 0.220 bdl 3.160 bdl 2.19 0.266 1.27 0.583 3.49 bdl bdl 7.24 bdl bdl bdl 1.16 bdl 5.50 bdl bdl 2940 32.5 5.95 Max 6.92 523 780 28.5 3.89 378 146 68.5 0.352 117 0.262 69.7 22.7 111. 22.4 2180 69.4 53.1 469 0.704 2.00 17.0 10800 28.2 2040 19.0 4.11 26000 3260 650 31 Average 6.00 357 301 26.6 1.09 139 9.70 10.3 0.061 20.1 0.0107 17.1 2.28 34.1 7.48 80.8 2.26 10.4 89.0 bdl bdl 2.68 1160 6.72 432 3.39 bdl 14200 398 56.9 Median 6.07 344 276 26.5 0.940 107 0.230 2.73 0.034 8.55 bld 12.1 0.912 27.3 5.77 8.35 bdl 9.08 39.6 bdl bdl bdl 102 5.73 303 bdl bdl 15800 198 19.9 St. Dev. 0.631 103 197 0.654 0.590 103 26.0 16.2 0.072 25.7 0.0422 12.6 3.98 25.9 5.91 352 11.2 10.5 101 0.163 0.415 5.25 2350 5.99 403 5.10 0.650 6350 625 124 Asklund and Eldvall LUTVDG/TVTG--5092--SE 6.2 Major ions Electro neutrality was examined and all but two samples, no. 23 and 37, showed smaller deviations than 5%, see Appendix 4. The results from the groundwater sampling are considered to be reliable. Comparison with conductivity showed equal deviation for sample 23 and for sample 37 it was concluded that the sum of cations were to low. Table 6-1 shows a statistical summary of groundwater chemistry. Results of groundwater sampling are presented in Piper diagram in Figure 6-1. Legend Legend 80 A C 80 60 80 40 A A A 20 A AC CA A A C A C AC A AA A A A AAA A AA A AAA A AA AA 20 SO4 80 60 60 40 40 20 80 60 40 20 40 60 80 Na+K A AA A C A A A A A A C A A C C A A A A A A C A A A A A A A 20 AA A AA AA A AAAA A A AA A A 20 A AA A CA C CA AC A ACA C Ca Shallow wells 60 40 Mg Deep wells HCO3 Cl Figure 6-1: Piper diagram showing composition of groundwater. Most of the samples, 95%, are of a bicarbonate type according to anions. Two samples have higher levels of Cl- and for these Cl- is the dominating anion. According to cations the classification is not as clear. 75% of the samples have Ca as dominating cation and the rest has Na followed by Mg as dominating cation. 67% of the hand dug wells have a Ca-HCO3 water type. The rest have Ca-Na-HCO3-NO3Cl or Na-Ca-HCO3-Cl-NO3. 21% of drilled wells have a Ca-HCO3 kind of water. This coincides with theory, since groundwater should develop following the water type sequence with increasing age (Bhattacharya, P. pers. comm. 2004-12-02): Ca-HCO3 → Ca-Cl-HCO3 → Na-Ca-Cl-HCO3 → Na-HCO3→Na-Cl 32 Contamination of water resources in Tarkwa mining area of Ghana This indicates that Calcium is replacing Sodium and to some extent Magnesium through ion exchange in the soil matrix as the groundwater increases in age (see chapter 4.2.1). For the drilled wells about 25% of the samples has Na or Mg as dominating cation,. For the hand dug wells there is one sample, about 17%, with Na as dominating cation, The differences between the groups should be interpreted with caution since we have very few wells belonging to the group shallow wells. For a complete list of water types see Appendix 3. NO3- is the only major ion that exceeds WHO:s guidelines. This occurs at two locations, see Table 6-2. 6.3 Trace elements A total of 17 wells have higher metal content than WHO:s guidelines when these are interpreted strictly regarding iron and aluminium. As(tot), Mn, Fe and Al show values exceeding WHO: s guidelines. As(tot) exceeds the guidelines at two locations, Samahu, 15.6 µg/l and Eyinaise, 69.4 µg/l. Mn is the major contaminant and of all 17 wells with high contents of metals, 14 has elevated Mn-levels. Fe exceeds the guideline in seven wells. Al is exceeding the guideline at two locations, Huniso and Akoon. These wells also display the two lowest pH-values measured. NO3- is exceeding the guidelines in the same two wells in Huniso and Akoon. The wells with elevated metal content are presented in Table 6-2. All the wells with metal concentrations exceeding WHO:s guidelines are boreholes except New Atuabo which is a hand dug well. Table 6-2: Samples exceeding WHO:s guidelines (mg/l). Location WHO guideline Simpa Dadwen Dompim Odumase Nsuaem Aboso Samahu Akotomu Eyinaise Eyinaise Mile 8 Huniso Huniso New Atuabo Bompieso Bompieso Akoon No 3 4 5 8 9 10 13 17 24 25 29 32 33 35 40 41 42 As(tot) 0.01 0.0156 0.0694 Mn 0.5 0.916 0.567 0.851 0.850 0.565 0.640 0.926 0.605 0.968 0.676 1.07 Fe 3 3.41 7.29 5.20 3.37 Al 0.2 NO3 50- 2.18 146 0.594 66.4 4.33 4.69 10.8 0.713 0.602 2.04 If iron and aluminium not is interpreted strictly 14 well exceed drinking water limits mainly because of high levels of manganese. Maps of metal concentration in groundwater are shown in Figure 6-2 and 6-3. Figure 6-4 and 6-5 shows other parameters associated with metal distribution. Kriging is used to 33 Asklund and Eldvall LUTVDG/TVTG--5092--SE interpolate concentrations. Sampling positions are marked with a cross. Coordinates are given as degrees North and West. Damang New Kyekyewere 5.5 Damang New Kyekyewere 5.5 Yaryeyaw Yaryeyaw Huni Valley Suwinso Akotomu Gordon 5.45 Kofi Gyan Camp Kofi Gyankrom Bompieso 5.4 Huniano no 1 Huniso Samahu Aboso Kokoase Nsuaem Odumase Atwerboanda New Atuabo Akoon Domeabra 5.35 5.3 Enyinasie Tarkwa Banso Teberebe 5.25 Akyem Adieyie 5.2 5.15 2100 2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0 -100 -200 60 55 Huniano no 1 Huniso 50 45 Samahu Aboso Kokoase Nsuaem Odumase Atwerboanda New Atuabo Akoon Domeabra 5.35 5.3 40 35 30 25 Enyinasie Tarkwa Banso 20 Teberebe 15 5.25 Akyem Adieyie 10 5 5.2 0 -5 Dompim Dadwen Dadwen -2.1 65 Kofi Gyan Camp Kofi Gyankrom Bompieso 5.4 5.15 Dompim Huni Valley Suwinso Akotomu Gordon 5.45 -2.05 -2 -1.95 -1.9 -2.1 -1.85 -2.05 -2 -1.95 -1.9 -1.85 Figure 6-2: Concentration of Al (µg/l) to the left and As(tot) (µg/l) to the right. Bold line indicates WHO:s guideline. Damang New Kyekyewere 5.5 Damang New Kyekyewere 5.5 Yaryeyaw Yaryeyaw Suwinso Akotomu Gordon 5.45 Huni Valley Kofi Gyan Camp Kofi Gyankrom Bompieso 5.4 Huniano no 1 Huniso Samahu Aboso Kokoase Nsuaem Odumase Atwerboanda New Atuabo Akoon Domeabra 5.35 5.3 Enyinasie Tarkwa Banso Teberebe 5.25 Akyem Adieyie 5.2 5.15 10000 9500 9000 8500 8000 7500 7000 6500 6000 5500 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0 -500 Huniano no 1 Huniso Samahu Aboso Kokoase Nsuaem Odumase Atwerboanda New Atuabo Akoon Domeabra 5.35 5.3 Enyinasie Tarkwa Banso Teberebe 5.25 Akyem Adieyie 5.2 Dompim Dadwen Dadwen -2.1 Huni Valley Kofi Gyan Camp Kofi Gyankrom Bompieso 5.4 5.15 Dompim Suwinso Akotomu Gordon 5.45 -2.05 -2 -1.95 -1.9 -2.1 -1.85 -2.05 -2 -1.95 -1.9 -1.85 Figure 6-3: Concentration of Fe (µg/l) to the left and Mn (µg/l) to the right. Bold line indicates WHO:s guideline. 34 2100 2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0 Contamination of water resources in Tarkwa mining area of Ghana Damang New Kyekyewere 5.5 Damang New Kyekyewere 5.5 Yaryeyaw Suwinso Akotomu Gordon 5.45 Yaryeyaw Huni Valley Kofi Gyan Camp Kofi Gyankrom Bompieso 5.4 Huniano no 1 Huniso Samahu Aboso Kokoase Nsuaem Odumase Atwerboanda New Atuabo Akoon Domeabra 5.35 5.3 Enyinasie Tarkwa Banso Teberebe 5.25 Akyem Adieyie 5.2 5.15 5.45 6.9 6.8 6.7 6.6 6.5 6.4 6.3 6.2 6.1 6 5.9 5.8 5.7 5.6 5.5 5.4 5.3 5.2 5.1 5 4.9 4.8 4.7 4.6 4.5 4.4 4.3 4.2 Huni Valley 560 540 Kofi Gyan Camp Kofi Gyankrom Bompieso 5.4 520 500 Huniano no 1 Huniso 480 460 Samahu Aboso Kokoase Nsuaem Odumase Atwerboanda New Atuabo Akoon Domeabra 5.35 5.3 Enyinasie Tarkwa Banso 5.25 420 400 380 360 320 300 Akyem Adieyie 440 340 Teberebe 280 260 240 5.2 220 200 5.15 Dompim Dompim Dadwen Dadwen -2.1 Suwinso Akotomu Gordon -2.05 -2 -1.95 -1.9 -2.1 -1.85 -2.05 -2 -1.95 -1.9 -1.85 Figure 6-4: pH, to the left, and Eh, to the right, in our study area Damang New Kyekyewere 5.5 Damang New Kyekyewere 5.5 Yaryeyaw Yaryeyaw Suwinso Akotomu Gordon 5.45 Huni Valley 65 Kofi Gyan Camp Kofi Gyankrom Bompieso 5.4 Aboso Kokoase Nsuaem Odumase Atwerboanda New Atuabo Akoon Domeabra 5.3 Enyinasie Tarkwa Banso Huniso 45 Akyem Adieyie 3.4 3.2 3 Aboso Kokoase Nsuaem Odumase Atwerboanda New Atuabo Akoon Domeabra 35 30 5.3 25 Enyinasie Tarkwa Banso 2 1 5.2 0 0.8 0.6 5.15 Dompim Dompim Dadwen Dadwen -2.1 2.2 1.2 -5 5.15 2.4 1.4 Akyem Adieyie 5 5.2 2.6 1.6 5.25 10 2.8 1.8 Teberebe 15 5.25 3.6 Samahu 5.35 40 20 Teberebe 3.8 Huniano no 1 50 Samahu 5.35 5.4 55 Huni Valley Kofi Gyan Camp Kofi Gyankrom Bompieso 60 Huniano no 1 Huniso Suwinso Akotomu Gordon 5.45 -2.05 -2 -1.95 -1.9 -2.1 -1.85 -2.05 -2 -1.95 -1.9 -1.85 Figure 6-5: Levels of SO42- (mg/l), to the left, and DOC (mg/l), to the right, in our study area Mn and Fe show similarity in distribution pattern, almost all areas with Fe-values above WHO:s guidelines, also have high Mn-values. However, there is no visible trend between 35 Asklund and Eldvall LUTVDG/TVTG--5092--SE these parameters when plotted (R= 0.09). Fe and Mn distribution also show a similarity with SO42- (R=0.18 resp. 0.63). These correlations are shown in Figure 6-6. 2.5 12 y = 0.026x + 0.8951 R2 = 0.0324 10 Mn (mg/l) 8 Fe (mg/l) y = 0.0157x + 0.2706 R2 = 0.401 2 6 1.5 1 4 0.5 2 0 0 0 20 40 60 0 80 20 40 60 80 SO4 (m g/l) SO4 (m g/l) Figure 6-6: Diagrams showing correlations for Mn and SO42- and for Fe and SO42For the correlations in Figure 6-6, there are no obvious outliers that affect the correlations. For Mn and SO42- a positive trend can be seen but for Fe and SO42- it is difficult to see any trend. The trend between Mn and SO42- could origin from dissolution of minerals or from AMD. For certain areas as Akoon, New Atuabo, Aboso, Enyinasie and Dadwen high levels of both Fe and SO42- and low pH indicate acid mine drainage. For Akoon, the location that display the highest levels of both Fe and Mn, there were small-scale mining activities just about 100m from the well. These activities are not seen in Figure 5-1 and small-scale mining activities are difficult to locate since the majority are illegal. Samples with high Fe content and low SO42- can also indicate AMD since SO42- can be transformed to H2S through redox reactions. AMD can therefore not be excluded on basis on low SO42--values. This can also explain the low correlation between Fe and SO42-. But it is not possible to see any trend between SO42- and Eh and the correlation is very low (R2=0.007).However it must be kept in mind that field measured Eh is an unreliable parameter. Many samples display both low Fe and SO42- values. This can be explained by reducing conditions. H2S can precipitate by forming iron sulphides. This is not the case for Mn and SO42- as Mnsulphides are much more soluble. Fe and Mn can also act as redox couples. Mn is reduced, Mn4+→Mn2+, and Fe is oxidized, Fe2+→Fe3+. Fe3+ can precipitate as for example ferryhydrit, which is supersaturated in some samples according to the hydrochemical modelling, see chapter 6.5. This can also act as a sink for Fe(tot). Another reaction that can govern the concentration of Fe2+, especially in sedimentary aquifers with reducing conditions is precipitation of siderite (FeCO3) (Bhattacharya, P. pers.comm. 2004-12-10). Precipitation of different minerals can disturb the correlations. Measurements of Eh show no reducing conditions in our sampling area, and modelling shows that siderite is undersaturated. However the field measured Eh is an unreliable measurement and the modelling is only conducted at four wells. It is not unlikely that there are reducing conditions and that this process could be of importance. 36 Contamination of water resources in Tarkwa mining area of Ghana Al show similarity in distribution patterns with pH and Eh (R=-0.53 resp. 0.32). This is due to the strong depency of pH for Al mobility. At higher pH-values Al is precipitated as hydroxides. 2 .5 12 y = - 0 .0 0 6 4 x + 1 .3 8 1 5 R 2 = 0 .0 0 5 10 y = 0 .0 0 8 7 x + 0 .13 4 7 R 2 = 0 .3 1 3 7 2 Mn (mg/l) Fe (mg/l) 8 6 4 1 .5 1 0 .5 2 0 0 50 0 100 0 50 C a ( m g /l) 100 C a ( m g /l) Figure 6-7: Diagrams showing the correlations for Mn and Ca and Fe and Ca As shown in Figure 6-7, Mn show strong positive correlations with Ca, and the other major cations. Sample 42 (Akoon) show a strongly deviant value. If this value is exluded the R2value is increased to 0.64. Fe show weak negative correlations with Ca. This could indicate that Fe is replaced by Ca at exchange sites, but not Mn. The correlation between Mn and Ca could origin for dissolution minerals containing both elements. 120 100 120 y = 41.31x - 216.97 R2 = 0.6262 80 Ca (mg/l) Ca (mg/l) 80 60 40 20 0 4.50 y = 0.2436x - 0.9703 R2 = 0.8527 100 60 40 20 0 5.00 5.50 6.00 6.50 7.00 pH 0.0 100.0 200.0 300.0 400.0 HCO3 (m g/l) Figure 6-8: Diagrams showing correlations between Ca and pH and Ca and HCO3-. Ca shows a strong correlation with pH and HCO3- shown in Figure 6-8. Sample 32 and 42 are not included due to probable acid mine drainage. This indicates that CaCO3 is the origin of the Ca content. The quantity Ca is considerably lower than the quantity HCO3-. Na, Mg, Mn and Ba also show high correlation with HCO3- (R= 0.62, 0.66, 0.70 and 0.66). Therefore it is plausible that there are some minerals containing Na, Mg, Mn, Ba and HCO3- which are contributing with ions to the groundwater. 37 Asklund and Eldvall LUTVDG/TVTG--5092--SE The As(tot) kriging should be interpreted with extreme caution since it is based only on three samples. If the Figures 6-2 to 6-5 are compared with the geological map of the Tarkwa-Presea area, Figure 3-3, it can be seen that Fe, Mn, As and SO42- does not show any major differences in distribution between the Tarkwaian and Birimain system. For Al the values exceeding WHO:s guidelines is found only in the Tarkwaian system. This is most likely a result of the low pH in these two wells and not an effect of the difference in geology between the two systems. 6.4 Differences between deep and shallow wells To be able to draw further conclusions on groundwater chemistry and dominating processes in the area the wells were divided, according to depths, into two groups. Since there is no data on how deep the wells are, the groups are called deep wells and shallow wells depending on if they are drilled or hand dug. The shallow wells are number 12, 20, 23, 30, 34 and 35. These groups have been investigated separately to investigate differences in parameters between deep and shallow wells. The results are presented in Figure 6-9. Only parameters showing differences between the groups are shown. There are differences as can be seen from Figure 6-9 in Mn, HCO, pH and Cond. The differences in median value between the groups can have different reasons. • pH: There is a distinct difference in pH between the shallow and deep wells. Large scale mines in the area uses roasting of ore as processing method. This can give raise to acidified rain. But the low pH can not solely be explained by acid rain. The measured pH of the rain water in the area is higher than the pH of shallow wells. It could be that AMD is causing the low pH. However, the lowest recorded pH values are found in the deep wells. • HCO3-: These waters also have very low alkalinity which supports the AMD theory. In the deep wells buffering reactions with carbonates or silicates can have a more pronounced neutralizing effect thanks to longer residence times. • Conductivity and Mn: These parameters show higher values for deep wells. This is probably due to higher dissolution of minerals due to longer residence times of these groundwaters. • Eh: The shallow wells have slightly higher Eh. This could be due to that oxygen consuming materials in the aquifers have affected the groundwater more due to longer residence times in the deep groundwaters. With regard to minerals in the area associated with AMD, see Table 4-1, the following correlations between the different groups have been investigated: Fe-SO4, Cu-SO4, Zn-SO4, Cu-Fe, Fe-Mn, and Fe-Zn. Appendix 4-6 contains three correlation matrices one for all samples, one for deep well and one for shallow wells. None of the groups, deep, shallow and all well show any strong, significant correlation between these parameters. Considerations of outliers have been taken into account. Shallow wells are only six compared to 34 deep wells. The results should therefore be interpreted with great caution because many correlations are determined by one dominating value. There is generally no major difference between the correlation matrix for deep wells 38 Contamination of water resources in Tarkwa mining area of Ghana and the one for all wells. To be able to draw conclusions about the difference between deep and shallow wells a more even distribution between the groups should have been accounted for in the sampling plan. 2.2 Max. 75 percentile Mn mg/l 1.8 Median 25 percentile 1.3 Min. 0.9 0.4 0.0 Shallow wells 800 320 640 Cond uS/cm HCO3 mg/l Deep wells 400 240 160 80 480 320 160 0 0 Deep wells Shallow wells 600 6.4 500 5.8 400 pH Eh mV 7.0 5.2 4.6 Deep wells Shallow wells Deep wells Shallow wells 300 200 4.0 100 Deep wells Shallow wells Figure 6-9: Box plots of selected parameters for deep and shallow wells. 6.5 Hydro-chemical modelling Sample 13, 24, 32 and 42 was chosen to be investigated by hydro-chemical modelling. This was based on levels of metals and the pH, see Table 6-2. All are drilled wells. Comparison with map materials gives that these wells are located in the following geological formations: 39 Asklund and Eldvall LUTVDG/TVTG--5092--SE • Sample 13: The Tarkwaian system, Huni Sandstone: (a quartzite) consists of variable amounts of feldspar, sericite, chlorite, ferriferous carbonate, magnetite or hematite and epidote. • Sample 24: The upper Birimian system: dominantly of volcanic and pyroclastic origin. The rocks consist of bedded group of green lava. Lava and tuff dominate this part. Several band of phyllite occurs in this zone and are manganiferous in places. • Sample 32: The Tarkwaian system, Huni Sandstone: (a quartzite) consists of variable amounts of feldspar, sericite, chlorite, ferriferous carbonate, magnetite or hematite and epidote. • Sample 42: The Tarkwaian system, Banket Series: 90% quartz and the rest is Birimian schist, quartzite, hornstone, chert and gondite Table 6-3: Result of modeling with Phreeqc Interactive. Sample 13 Phase Al(OH)3(a) Albite(low) Analbite Analcime Annite Aragonite Barite Boehmite Calcite Chalcedony CO2(g) Diaspore Dolomite Fe3(OH)8 Ferrihydrite Gibbsite(C) Goethite Gypsum Hercynite Hydroxyapatite Laumontite Leucite Lime Magnesite Microcline MnHPO4(C) Nsutite Pyrolusite Quartz Rhodochrosite Sanidine(H) Siderite SiO2(a) SiO2(am) Strengite Strontianite Vivianite ZnSiO3 SI -1.13 0.64 -0.27 -0.19 2.04 -0.37 -0.72 0.67 -0.23 0.23 -1.35 2.37 -0.96 -0.96 0.79 0.47 5.22 -3.15 -0.71 0.21 1.69 -1.61 -21.9 -1.23 0.89 1.31 -12.9 -11.2 0.71 -0.52 0.45 -1.05 -0.28 -0.58 -0.57 -1.21 -5.63 0.15 Sample 24 Phase Al(OH)3(a) Albite(low) Alunite Analbite Analcime Anhydrite Annite Aragonite Barite Boehmite Calcite Chalcedony CO2(g) Dolomite Diaspore Fe3(OH)8 Ferrihydrite Gibbsite(C) Goethite Gypsum Hercynite Jarosite-K Jarosite-Na Laumontite Leucite Lime Magnesite Manganite Microcline MnHPO4(C) Nsutite Pyrolusite Quartz Rhodochrosite Sanidine(H) Siderite SiO2(a) SiO2(am) Strengite Vivianite ZnSiO3 SI -1.05 0.01 -0.64 -0.89 -0.87 -1.81 1.8 -1.62 -0.02 0.76 -1.48 0.28 -0.87 -3.07 2.45 -1.56 0.5 0.54 4.97 -1.63 -0.47 1.63 -0.51 0.37 -2.01 -23.5 -2.07 -7.75 0.53 1.97 -15.0 -13.2 0.75 -1.43 0.09 -0.57 -0.22 -0.53 1.19 -1.57 -0.93 40 Sample 32 Phase Al(OH)3(a) AlOHSO4 Barite Boehmite Chalcedony CupricFerrite Diaspore Ferrihydrite Gibbsite(C) Goethite Gypsum Halloysite Leonhardite Lepidocrocite Maghemite Manganite Microcline MnHPO4(C) Montmorillonite Nsutite Pyrolusite Quartz Sanidine(H) SiO2(a) SiO2(am) Strengite Vivianite SI -1.5 -1.11 -0.26 0.3 -0.07 0.79 1.99 -2.58 0.09 1.87 -2.77 1.55 1.3 0.94 -1.77 -8.38 -1.36 -0.56 1.79 -12.9 -11.1 0.41 -1.8 -0.57 -0.88 -1.02 -17.3 Sample 42 Phase Al(OH)3(a) AlOHSO4 Alunite Anhydrite Barite Boehmite Chalcedony Diaspore Fe(OH)2.7Cl0.3 Ferrihydrite Gibbsite(C) Goethite Gypsum Halloysite Kaolinite Lepidocrocite Lime Magnetite Manganite MnHPO4(C) Muscovite Nsutite Pyrophyllite Quartz SiO2(a) SiO2(am) Vivianite SI -2.78 -1.1 0.75 -2.19 0.25 -0.97 -0.48 0.72 1.55 -4.25 -1.19 0.21 -2 -1.81 1.44 -0.73 -27.6 -1 -9.2 -0.31 -1.61 -14.5 0.64 0 -0.98 -1.29 -19.3 Contamination of water resources in Tarkwa mining area of Ghana The modelling is conducted on four wells and the results and interpretation are to be seen as suggestions to which processes that can be important for the area. For the wells that are not used in the modelling, other processes can be dominating. Table 6-3 shows the saturation indicies for these wells for selected minerals. Intervall (-2