Stanford University

(As) concentrations exceeding World Health Organization standards. The biogeochemical processes controlling As release from sediments and potential secondary sinks remain in question. Further, it remains unclear whether a single mechanism or a combination of processes control As cycling across the large affected geographic areas. Here we provide an evaluation of aquifer chemistry within the Jianghan Plain of the Yangtze River, China, focusing on coupling solid-phase and aqueous measurements. Groundwater As concentrations exceed 1000 μg L-1 in some locations within the Jianghan Plain. Arsenic X-ray absorption spectroscopy (XAS) analysis of these sediments reveals that although As(III) is the dominant species throughout the profile, As appears to be accumulating in an oxidized layer ~20 m below the surface where the solid-phase arsenic concentration exceeds 100 mg kg-1. Batch incubations of sediment amended with 2 mM glucose or with deionized (DI) water demonstrates that As is most labile in this oxidized layer ~20 m. Glucose amendment enhances As release in this layer by a factor of two compared to DI water, but marginally increases As liberation at other depths. XAS analysis of glucose amended sediment post-incubation shows that when sulfur is present, As transitions from an oxoanion to As-S precipitate, preventing As release to solution. In sulfur-depleted sediment As is released to solution following glucose amendment. Our results highlight the vulnerability of groundwater resources in the Jianghan Plain to changing geochemical conditions.