THE ROLE OF POOR SANITATION IN THE MOBILIZATION OF ARSENIC IN MALI, WEST AFRICA

In Mali, West Africa, shallow (depth ~ 10 m), uncased, hand-dug wells are normally located within 10 – 15 m of unlined sanitation pits. In an arid environment untreated sewage can be a significant source of recharge to neighboring wells. In addition to the well-known detrimental effects of drinking water contaminated by bacteria from human waste, the possibility exists that the elevated nitrate and phosphate from untreated sewage could mobilize arsenic by displacing arsenic from sediment or soil sorption sites. In March 2010 a team from Utah Valley University and the University of Bamako carried out the first survey of groundwater and surface water arsenic in Mali. Water samples were collected from 36 hand-dug wells, seven drilled wells (70 – 80 m deep), seven public faucets and two cisterns fed by drilled wells, three streams and one lake in the capitol city of Bamako and the rural commune of Ouelessebougou. Electrical conductivity, pH and water temperature were measured on site. Concentrations of iron, copper, potassium, sulfate, nitrate, phosphate, hardness and fluoride were measured in Mali with a Hach DR-2700 Spectrophotometer. Samples were brought back to UVU for measurement of arsenic and chloride. Arsenic concentrations exceeded the World Health Organization standard (As = 0.01 mg/L) in 100% of samples. Maximum and geometric mean concentrations in hand-dug wells were As = 1.096 mg/L and As = 0.057 mg/L, respectively, while maximum and geometric mean concentrations in drilled wells and public faucets were As = 0.378 mg/L and As = 0.062 mg/L, respectively. Among hand-dug wells there was a positive correlation (R² = 0.39) between arsenic and nitrate, while arsenic and phosphate were uncorrelated (R² = 0.03), suggesting that nitrate is more effective at displacing arsenic due to its higher mobility. Among drilled wells and public faucets there was a positive correlation between iron and arsenic (R² = 0.33), while arsenic and sulfate were uncorrelated (R² = 0.08), which is consistent with the reductive dissolution model for arsenic release into groundwater. Our tentative recommendation is that, in light of the elevated arsenic in drilled wells, instead of focusing on drilling deep wells, there needs to be development of shallow wells with proper protection from sewage.