THE ROLE OF POOR SANITATION IN ARSENIC CONTAMINATION OF GROUNDWATER

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. The objective of this research is to determine whether the excessive nitrate from poor sanitation can mobilize As from sediment into groundwater. Since poor sanitation (inadequate separation between sanitary facilities and water supply) is a global phenomenon, this research has the potential to have global consequences for public health. Laboratory experiments are being conducted to see whether nitrate has the ability to displace As from sediment sorption sites into water. The experiments involve four sediment types (hematite, limonite, smectite, quartz), three sediment sizes (< 0.063 mm (silt and clay), 0.063 – 0. 125 mm (very fine sand), 0.125 – 0.250 mm (fine sand)) and four K nitrate solutions with nitrate concentrations of 0, 10, 100, and 1000 mg/L, for a total of 48 experiments. Each mixture of 6 g of sediment to 270 mL of K nitrate solution is being stirred for 24 hours, after which samples are filtered through 6 micron filter paper to remove excess sediment. The filtrates are being analyzed for As using the silver diethyldithiocarbamate method (U.S. EPA Standard Method 3500-As) with the Hach DR- 2700 Spectrophotometer. Initial results using smectite in the silt / clay size fraction show that As in the filtrate increases from As = 0.078 mg/L at nitrate concentration of 0 mg/L to As = 0.179 mg/L at nitrate concentration of 1000 mg/L, suggesting an ability of nitrate to displace As. Further experiments will be carried out using solutions of Ca nitrate and organically complexed nitrate to explore possible additive and competing effects of oxyanion displacement, flocculation by divalent cations and organic complexation in displacing As from sediment into water. Results will be reported at the meeting.