Y. Zheng, D. Nemergut and D. McKnight Elevated concentrations of arsenic (As) in groundwater in many sedimentary aquifers have been observed to occur under anoxic conditions, posing significant threats to human health in some regions of the world. Microbial reduction of As-bearing Fe(III)-oxides has been invoked to explain the enrichment of As, although the mechanisms of Fe(III)-oxide reduction are not fully resolved. One potentially important factor in microbial Fe(III)-oxide reduction is the availability and abundance of complex organic molecules called humic substances which can shuttle electrons between microbes and Fe(III)-oxides. This study evaluates the role of electron-shuttling by humic substances in FeIII-reduction and subsequent As release in Bangladesh aquifers over a variety of chemical, spatial and seasonal gradients. The chemistry of the seasonally variable recharge is hypothesized to structure microbial communities. The microbes, in turn, are hypothesized to regulate local rates of Fe reduction through electron shuttling by humics, ultimately influencing As release to the aquifer. The study seeks to ascertain if Fe(III) oxide reduction is enhanced by electron shuttling of humic substances during recharge to the shallow aquifer, and if electron shuttling is limited in the generally organic-poor sediment further down the flow path . The study takes place in Araihazar, Bangladesh, where the shallow aquifer has shown a range of As concentrations and recharges rates. In addition to assaying the chemical (pH, Eh, DO, nitrate, Fe, Mn, sulphate) and redox properties (FeII/FeIII, AsIII/AsV) of water and sediment, the study employs fluorescence spectroscopy to evaluate redox state of humics in the field and molecular phylogenetic methods for microbial community characterization. Sediment electron shuttling capacities will also be measured using incubation initiated in the field. This study provides critical data (e.g., proxies for concentrations of electron shuttling compounds and microbial community shifts) to enhance the understanding of the mechanisms of Fe(III) reduction in a system where seasonal flooding, agriculture, and human waste are simultaneously influencing carbon dynamics. Because of the many important natural biogeochemical consequences of Fe-reduction, our study will have implications for other deltaic environments far beyond the interpretation of As mobility.
