ARSENIC AND OTHER HEAVY METALS IN THE NIGER RIVER, MALI, WEST AFRICA

Previous research in Mali (West Africa) showed both elevated As (geometric mean As = 0.057 mg/L) and a positive correlation between As and nitrate (R² = 0.39) in 36 shallow (depth ~ 10 m), uncased, hand-dug wells located within 10 – 15 m of unlined sanitation pits. Of the 36 hand-dug wells, As levels exceeded the WHO drinking water standard (As = 0.01 mg/L) in 100% of samples. These findings raised the possibility that excessive nitrate resulting from poor sanitation could be displacing As from sediment sorption sites. The implication is that As contamination of groundwater in Mali is simply a local issue that could be addressed by appropriate casing of wells and separation between wells and sanitation pits. The objective of this study was to determine whether As contamination of groundwater could, in fact, be regional and naturally occurring in Mali. The concern of the study was motivated by also finding elevated As in three stream samples (geometric mean As = 0.089 mg/L), including two samples of the Niger River in the capitol city of Bamako, and seven 70 – 80 m-deep drilled wells (geometric mean As = 0.062 mg/L). The Niger River between Bamako and Mopti was assumed to contain a representative sample of surface water and groundwater in southern Mali. Therefore, the objective was addressed by collecting 50 water samples while traveling 530 km by boat along the Niger River and Bani River between Bamako and Mopti. Additional water samples were collected from both hand-dug and drilled wells in towns along the main rivers. Water temperature, electrical conductivity, dissolved oxygen and pH were measured on site. Samples are now being analyzed for As, nitrate, phosphate, sulfate and the transition elements normally associated with As (Fe, Cu, Ni, Co, Mn, Cr, Zn) using the Hach DR-2700 Spectrophotometer. Results will be reported at the meeting.