Redox Controlled Biogeochemical Processes Affecting Arsenic Solubility In a Shallow Aquifer In Semi-Arid Cache Valley, Utah.

The poisoning of millions of people in West Bengal, Bangladesh and Southeastern Asia from arsenic in groundwater has focused research attention on the sources and geochemistry of arsenic in these humid regions. Few studies however have been performed to determine the sources and mechanisms of solubilization of As in semi-arid environments. The shallow aquifer throughout the Cache Valley, Utah, contains As concentrations that exceed EPA’s drinking water limit. Two continuous cores, from the soil surface to 1.5 m below the water table, were collected from the center of the valley in order to describe the biogeochemistry that controls the solubility of naturally occurring arsenic. General soil properties, pore water chemistry, and solid phase characterization of arsenic, using sequential extractions, have been determined. Geologic arsenic contents were present throughout the two profiles and arsenic was released into the pore water. The mineral association of arsenic changed with depth, with arsenic accumulating in the redox transition zone. Arsenic in the surface soils and depletion zone solids was associated with recalcitrant minerals, while in the redox transition zone, the majority of arsenic was associated with amorphous metal oxides and carbonates. The heterogeneity in the distribution of speciation and mineralogies of arsenic has been confirmed by synchrotron-base X-ray absorption spectroscopy. The dominant mechanism of arsenic release depended on the redox of the solids. We present evidence to show that the surface soil contains leachable arsenic and the redox transition zone plays a dual role in controlling solubility by sequestering As leached from the surface then releasing this As under altering redox state with the raising and lowering of the water table. DNA/RNA extraction and analysis for arsenic reductase and oxidase gene expression will be performed in order to describe the contribution of direct microbial reduction and oxidation in controlling arsenic in the profile.