STIMULATING MICROBIAL SULFATE REDUCTION TO LOWER ARSENIC CONCENTRATION IN GROUNDWATER

Natural contamination of groundwater with arsenic poses a significant threat to human health worldwide. Previous research has shown that we may be able to remediate arsenic contamination in situ by stimulating microbial sulfate reduction. The approach takes advantage of naturally occurring microbes within the aquifers themselves. Microorganisms that reduce iron in oxide minerals and dissolved sulfate generate ferrous iron (Fe(II)) and sulfide (S(-II)), respectively, which can form sulfide minerals such as mackinawite (FeS) and pyrite (FeS₂). As these minerals form, they can take-up arsenic, causing groundwater arsenic concentration to decrease. In many contaminated aquifers, sulfide mineral formation is limited by the availability of sulfate. Hence, by resupplying sulfate, we may be able stimulate the microbial reaction. We tested this strategy in laboratory bioreactors containing sediment and groundwater from an aquifer in West Bengal, which contains high levels of dissolved arsenic. We amended the groundwater in test bioreactors with sulfate (0.25 mM) as well as acetate (10 µM), which can serve as an electron donor for microorganisms. For control bioreactors, we included acetate but no sulfate. Preliminary results of our experiments show that microbial sulfate reduction only occurred in the test reactors and that arsenic concentration was 48% lower in test reactors compared to corresponding controls on average. These results suggest that stimulating sulfate reduction through sulfate amendment can be a successful strategy to lower arsenic concentrations in groundwater from anoxic aquifers. Our ongoing efforts seek to further constrain water chemistry and the composition of the microbial community in our bioreactors.