IMPACT OF REDOX CHEMISTRY ON THE ENVIRONMENTAL FATE AND TRANSPORT OF ARSENIC AND URANIUM AT ABANDONED URANIUM MINES IN HARDING COUNTY, SD

Soil samples from the North Cave Hills abandoned uranium mine site in Harding County, SD were analyzed to determine the chemical mechanisms controlling the transport of arsenic and uranium, previously found at above background level concentrations, from the mine tailings to the surrounding watershed. Soil cores were collected anoxically, divided into 5 cm sections, and prepared for analysis. X-ray Absorption Near-Edge Spectroscopy (XANES) analyses show that the presence of As(III) is directly correlated with the soil redox potential (i.e., low E_h). XANES also indicates that the As found in the tailings is in a mineral form as opposed to being adsorbed to iron oxy (hydr)oxides at other sampling locations These results suggest remobilization of arsenic from the tailing impoundment is occurring and likely serves as a source of arsenic for the downgradient watershed. High uranium concentrations were present in reduced sediments collected within a sedimentation pond at the base of the tailings, indicating uranium immobilization is occurring, and can be attributed to the chemical reduction of U(VI) to insoluble U(IV). Sequential chemical extraction data indicate that arsenic and uranium are primarily associated with the iron oxide, manganese oxide, and clay fraction of the watershed sediments. Mineralogical characterization of iron-bearing clays and iron oxy (hydr)oxides of selected samples was performed using X-ray Diffraction (XRD) and Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS). Preliminary results suggest that the anaerobic sedimentation pond efficiently sequesters uranium but is likely releasing As due to reductive processes. Iron minerals seem to play an important role in controlling the fate of arsenic and uranium downstream of the sedimentation pond.