ARSENIC AND HEAVY METALS IN GROUNDWATER AND SURFACE WATER IN MALI, WEST AFRICA

The objective of this study was to carry out the first survey of As and heavy metals in groundwater and surface water in Mali (West Africa). Surface water samples were collected along the Niger and Bani Rivers from the capitol city of Bamako to Mopti and from streams in Bamako and Ouelessebougou, while groundwater samples were collected from both hand-dug and drilled wells in Bamako, Ouelessebougou, Mopti, and villages along the Niger River between Bamako and Mopti, for a total of 87 samples from hand-dug wells, 23 samples from drilled wells, and 44 samples from surface water. Water samples have been analyzed for nitrate, phosphate, sulfate and fluoride using the Hach DR-2700 Spectrophotometer and are being analyzed for As and the associated sulfide-forming elements Ag, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Ti and Zn using the Optima 8000 ICP-OES (Inductively Coupled Plasma - Optical Emission Spectrometer). Based on 47 analyses from hand-dug wells, the WHO drinking water standard for As (0.010 mg/L) was exceeded in 45% of wells, with maximum and geometric mean As concentrations of 0.341 mg/L and 0.007 mg/L, respectively. Based on 21 analyses from drilled wells, the WHO As standard was exceeded in 38% of wells, with maximum and geometric mean concentrations of 0.115 mg/L and 0.006 mg/L, respectively. Based on 31 surface water analyses, the WHO As standard was exceeded in 68% of samples, with maximum and geometric mean concentrations of 0.238 mg/L and 0.014 mg/L, respectively. For hand-dug wells, As was positively correlated with nitrate (R² = 0.24, P < 0.001) and Pb (R² = 0.21, P < 0.001), which suggests that As is being mobilized by anionic displacement by nitrate from adjacent sanitation pits. This phenomenon has not been previously reported in the literature and may be an issue for other areas with improper sanitation. For drilled wells, As was positively correlated with Fe (R² = 0.35, P = 0.001) and Ti (R² = 0.69, P = 0.003), which is consistent with subsurface oxidation of pyrite and related minerals. For surface water, As was positively correlated with pH (R² = 0.29) and temperature (R² = 0.19, P = 0.008), consistent with water-sediment sorption reactions, and positively correlated with Cu (R² = 0.28, P < 0.001), which is consistent with sulfide oxidation and possible impact of mining on surface water. Further results will be reported at the meeting.