SULFATE REDUCTION RATES IMPACT ON ARSENIC CONCENTRATIONS IN CAMBODIA

Arsenic is a widespread natural contaminant in drinking water in South and Southeast Asia. Most commonly, arsenic contamination in groundwater is thought to arise through reductive dissolution of arsenic-bearing iron minerals in aquifer-bearing sediments; however, sulfur redox cycling may also influence arsenic phase partitioning tby sequestering arsenic in sulfide phases of facilitating iron reduction. Here, we examine the potential impact of sulfate reduction in regulating arsenic concentrations in the Mekong delta region of Cambodia. As in Bangladesh and other areas, arsenic contamination in sampled wells is associated with reducing conditions (Eh < 0 V, more than 1 mg/L dissolved Fe), but many of the wells that sampled reduced groundwater contained little or no measurable arsenic. As a result, reduction potential and dissolved iron is not strongly correlated to dissolved arsenic levels. Arsenic concentrations instead were associated with the combination of reducing conditions and low concentrations of dissolved sulfate. Based on these data, we infer that arsenic is effectively trapped in sulfide minerals under reducing conditions in which adequate sulfate is available for sulfate reduction, but is released into groundwater when sulfate levels are sufficiently low to impede sulfate reduction relative to iron reduction. Thus, it appears that the kinetics of sulfate reduction modulates arsenic fate.