Concentration of Trace Metals in Boreholes in the Ankobra Basin, Ghana

Analysis of trace metals in groundwater from the Ankobra basin revealed high levels of iron, manganese and aluminium. Approximately 40% of boreholes had total iron concentration exceeding 1000 μg l-1 (maximum WHO permissible limit). Aluminium concentration varied from 0.1 μg l-1 to 2510 μg l-1 with a median value of 10.0 μg l-1. Approximately 20% of the boreholes had aluminium concentration exceeding the WHO maximum acceptable limit (200 μg l-1) for drinking water. Manganese concentration was in the range 6–2510 μg l-1 with a median of 356 μg l-1. Roughly 25% of the boreholes had manganese concentration higher than 500 μg l-1, which is the WHO maximum acceptable limit for drinking water. The concentration of mercury was higher than 1.0 μg l-1 (WHO maximum acceptable limit) in 60% of the boreholes during the rainy season but below detection limit in the dry season, suggesting anthropogenic origin for mercury in the groundwater. Other trace metals that occurred, but in insignificant concentration in boreholes, include lead, arsenic, nickel and selenium. Most of the boreholes with high trace metal concentrations were located in and around the Bawdie-Bogoso-Prestea area. Introduction Ankobra basin is one of the main mining areas in Ghana. The major minerals mined in this area include gold, manganese, bauxite and diamond. Gold mining in this basin dates about 500 years ago but written records have restricted the period to 120 years (Marston et al., 1992). The intense and uncontrolled mining activities in the basin have led to environmental degradation, including the pollution of surface water sources. Ankobra river, for instance, is docu-mented to be highly polluted due to mining activities (WRRI, 1986). The Government of Ghana, increasingly apprehensive of the extent of pollution of surface water resources, has been providing communities with drilled borehole and hand dug wells as alternative source of potable water supply. Consequently, groundwater has become the principal and, sometimes, the only source of drinking water supply in the Ankobra basin. Apart from Tarkwa all major towns in the area, notably Bogoso, Aboso, Prestea, Huni Valley and almost all rural communities, rely solely on untreated groundwater as source of drinking water. The preference of groundwater to surface water as a source of drinking water was based on the fact that groundwater excluded from the atmosphere would be less susceptible to pollution. This is true to an extent. However, groundwater in hard-rock aquifers, particularly in mining areas, is known to be vulnerable to quality problems that may have serious impact on human health. The rocks are often carbonate-deficient and give rise to poorly buffered water (Smedley et al., 1995). Secondly, in gold and base metal mining areas, sulphides oxidation resulting from chemical and biogeochemical processes leads to the production of low pH ground-water that encourages the dissolution of trace metals into the groundwater system in very high concentrations. The groundwater, thus, becomes dangerous for human consumption. Trace metals such as Cr, Cd, Hg, Pb and U are known to be powerful nephrotoxins (Doul et al., 1980). Renal injury caused by exposure to As, Au, Fe, Pt, Sb and Tl has also been demonstrated (Maher, 1976). The main gold ore associated with the Birimian formation is refractory quartz-Fe/As sulphide lode gold (Marston et al., 1992). Junner et al. (1942) pointed out that pyrite is common in many of the igneous rocks and quartz veins that intrude the Birimian and the Tarkwaian rocks. Thus, the existence of sulphides in the rock matrix and the moderately low pH groundwaters within the Ankobra basin (Kortatsi, 2004) suggest that there is the high probability of mobilization of some toxic trace metals into the groundwater and, for that matter, the drinking water supply of the Ankobra basin particularly in the Birimian rock areas where the sulphide ores are common. In spite of this, there are only few studies concerning the quality of groundwater or drinking water supply in the Ankobra basin. The objective of this paper, therefore, is to examine the level of trace metals concentration in groundwater used for drinking in the Ankobra basin. Materials and methods Study area The Ankobra basin is approximately 8,400 km and lies between latitudes 4.8 N and 6.5 N, and longitudes 1.6 W and 2.3 W, respectively (Fig. 1). Physically, the Ankobra basin comprises series of ridges, which in some places reached the height of 200 m and separated by steep sided flat-bottomed valleys that vary in altitude from 46m in the south to approximately 76 m in the north. Most of the valleys are parallel to one another and to the strike of the rocks. Heavy rainfall and forest vegetation that prevent sheet erosion have reduced the whole area into highly dissected and uniformly moderate relief with a gentle slope to the south (Service, 1938). Erosion is mainly restricted to river channels that cut up the plateau surfaces, and the hills are usually capped with iron-pan (laterite) and bauxite (Dickson & Benneh, 1980). Sekondi Takoradi Apowa Dixcove Nsuaem Axim Essiama Dompem Tarkwa Aboso Huni Valley Tamso Kutukrom Nkwanta Ateiku Opon Valley Insu Bogoso Bawdie Ekuropon Ayanfure MansoAmenfi Dwokwaa Awaso Asawinso