The Mobilization of Geologic Arsenic in Aquifers of Cache Valley, Northern Utah.

The concentrations of arsenic in groundwater at many locations throughout Cache Valley, Northern Utah, exceed the drinking water limit. Aquifer solids were collected from an area with a network of monitoring wells near a municipal landfill where the source and the mechanism of arsenic solubilization are not known. Chemical extraction procedures were used to determine the mineral association of arsenic in the local geology. Anaerobic microcosms with two selected aquifer solids, site-oxidized and site-reduced, and groundwater collected near the landfill, groundwater plus glucose, and landfill leachate, were constructed and analyzed over time to evaluate the mechanism of arsenic solubilization. Microcosms were sacrificed over a 54 day period for analysis of arsenic and iron in the solution and solid phases, arsenic and iron speciations, changes in mineralogy and water chemistry parameters. In the site-oxidized aquifer solid, regardless of treatment, As solubilization was linked with Fe reduction. The addition of glucose enhanced this microbially driven process. As(III) was the dominant As species in solution and the proportion of As(III) associated with the 0.5 M HCl extractable minerals increased with time. In the site-reduced aquifer solid, there was no evidence of Fe reduction regardless of treatment. However, the solubilization of As was observed with the groundwater and leachate treatments. Addition of glucose decreased soluble As concentration, as As sorbed onto or precipitated with the solid phase. Results from this study illustrate that for the oxidized aquifer solid, the solubilization of As was coupled with the reductive dissolution of Fe minerals, whereas in the site-reduced aquifer solid As solubilization and reduction were independent of iron reduction. The role of arsenic reducing bacteria in these aquifer solids is presently being explored.