Processes Controlling Arsenic Fate in Asian Aquifers From the Yangtze to the Mekong.

Presently more than 100 million people in Asia are exposed to groundwater with arsenic concentrations exceeding standards recommended by the World Health Organization. Arsenic originating from Himalayan sediment transported down major rivers and deposited within low-lying basins and deltas contaminates underlying aquifers. The governing biogeochemical processes controlling the release of arsenic from sediments and potential secondary sinks remain in question. Further, it remains unclear whether a single mechanism or a combination of processes controls arsenic cycling across the large affected geographic areas. Here we provide a detailed evaluation of aquifer chemistry within the Jianghan Plain of the Yangtze River, China, followed by a comparison to the biogeochemistry of the Mekong Delta of Cambodia. Groundwater and sediments from the Jianghan plain show aqueous arsenic concentrations in the shallow aquifer exceed 1000 μg L^-1. Arsenic X-ray absorption spectroscopy (XAS) analysis of unaltered sediments revealed that throughout the sediment profile As(III) dominates As speciation, but that 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. Arsenic sulfide phases are observed in a limited number of samples despite high dissolved sulfur levels in the aquifer. Batch incubations of sediment amended with 2 mM glucose or with deionized (DI) water demonstrated that the greatest As release is coincident with elevated solid-phase As at depths between 18 and 23 m, also where As(V) dominates speciation. Furthermore glucose amendment enhances As release at these depths by a factor of ~2 compared to DI water, but marginally increases As liberation at other depths. These results contrast findings from Cambodia where dissolved sulfur concentrations are generally lower and zones of high solid-phase arsenic are associated appreciably with sulfide, implying that the arsenic release mechanism(s) may differ between the two basins.