Redox Controlled Biogeochemical Processes Affecting Arsenic Solubility Down a Sediment Profile In Cache Valley, Utah.

Arsenic concentration in groundwater throughout Cache County exceeds the drinking water limit. Previous studies of aquifer solids collected from an area near the Logan landfill revealed that the As in the groundwater is from geologic sources. In order to determine the geologic sources of arsenic and to determine the driving force(s) for arsenic release/retention that controls the spatial variability in As levels in the groundwater, four cores, consisting of vadose zone, redox transition zone and saturated zone solids, from the soil surface to the depth of groundwater, were collected from the same area. Geologic As was not confined to the deep aquifer solids but was also in the surficial materials. Primary As minerals may be continuously deposited potentially via weathering of volcanic rock from the Salt Lake Formation located along the base of the Wellsville Mountains or leaching of As-containing minerals from the Bear River Range. In the redox transition zone, arsenic was sorbed or co-precipitated with Fe oxides and carbonate minerals. DNA/RNA extraction and analyses indicated that microbes capable of As reduction was present in this system. Reductive dissolution of Fe oxides working in conjunction with direct microbial As reduction caused solubilization of As. Arsenic content reached its maximal value in the saturation zone. The accumulation in this zone may be due to the retention of As by reformed Fe-sulfides under sulfate-reducing conditions. Understanding the behavior of geologic arsenic in this location is important because these processes may also affect other regions in northern Utah due to similarity in the geology.