Arsenic in groundwater from mineralised Proterozoic basement rocks of Burkina Faso |
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| Authors: | PL Smedley J Knudsen D Maiga |
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| Institution: | 1. British Geological Survey, Wallingford, Oxfordshire OX10 8BB, UK;2. COWI Consulting Engineers, Parallelwej 2, 2800 Kongens Lyngby, Copenhagen, Denmark;3. Direction de l’Approvisionnement en Eau Potable et de L’Assainissement (DAEPA), 03 BP 7025 Ouagadougou 3, Burkina Faso |
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| Abstract: | This study describes the hydrogeochemistry and distributions of As in groundwater from a newly investigated area of Burkina Faso. Groundwaters have been sampled from hand-pumped boreholes and dug wells close to the town of Ouahigouya in northern Burkina Faso. Although most analysed groundwaters have As concentrations of less than 10 μg L−1, they have a large range from <0.5 to 1630 μg L−1. The highest concentrations are found in borehole waters; all dug wells analysed in this study have As concentrations of <10 μg L−1. Skin disorders (melanosis, keratosis and more rare skin tumour) have been identified among the populations in three villages in northern Burkina Faso, two within the study area. Although detailed epidemiological studies have not been carried out, similarities with documented symptoms in other parts of the world suggest that these are likely to be linked to high concentrations of As in drinking water. The high-As groundwaters observed derive from zones of Au mineralisation in Birimian (Lower Proterozoic) volcano-sedimentary rocks, the Au occurring in vein structures along with quartz and altered sulphide minerals (pyrite, chalcopyrite, arsenopyrite). However, the spatial variability in As concentrations in the mineralised zones is large and the degree of testing both laterally and with depth so far is limited. Hence, concentrations are difficult to predict on a local scale. From available data, the groundwater appears to be mainly oxic and the dissolved As occurs almost entirely as As(V) although concentrations are highest in groundwaters with dissolved-O2 concentrations <2 mg L−1. The source is likely to be the oxidised sulphide minerals and secondary Fe oxides in the mineralised zones. Positive correlations are observed between dissolved As and both Mo and W which are also believed to be derived from ore minerals and oxides in the mineralised zones. The discovery of high As concentrations in some groundwaters from the Birimian rocks of northern Burkina Faso reiterates the need for reconnaissance surveys in mineralised areas of crystalline basement. |
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