ARSENIC AND URANIUM IMPACTED SEDIMENT BEHAVIOR WITHIN THE BOWMAN-HALEY RESERVOIR, BOWMAN COUNTY, NORTH DAKOTA

The purpose of this study was to determine the impacts of regional historical uranium mining activities (from the Cave Hills region and UMTRA Griffin uranium processing site) within the Bowman-Haley Reservoir sediments. The Bowman-Haley reservoir is a 20,400 acre feet US ACE facility built at the confluences of the North Fork of the Grand River, Alkali Creek, and Spring Creek. The extent of anthropogenic-influenced watershed impacts were quantified through the determination of sediment metals concentrations and metal enrichment factors to evaluate the potential of geochemically-influenced As and U remobilization within the reservoir sediments. Sediment cores were collected at five locations within the reservoir: Spring Creek confluence, Alkali Creek confluence, two locations within the North Fork of the Grant River confluence, and adjacent to the reservoir outfall. Sediments cores were divided into 2 cm slices to depth (up to 1 m) and analyzed for trace metal and carbon concentrations. Pearson product moment correlations established inter-core metal correlations, while sediment enrichment factors were determined relative to background concentrations found within Alkali Creek, a watershed with no historical mining activities. Enrichment factor results suggest all sampling sites may be classified as ‘minor to moderately enriched’ for As and U. Metal behavior for the three reservoir inlets indicate similar metal loading sources and post-depositional behavior, while metals migration at the reservoir outlet appear controlled by geochemical and/or physical mechanisms. For the reservoir outlet, As and U normalized Al ratios suggest vertical migration of As is occurring above the reservoir spillway, while surface-bound U remobilization is apparent within the water column immediately above the sediments. The highest U concentrations were found within the Spring Creek reservoir, and the source of this loading appears to be from the Griffin, ND uranium processing facility operations and cleanup. Overall, sediment As enrichment exists within the reservoir inlets, and elevated sediment U was found adjacent to the reservoir spillway, suggesting that As loading within the reservoir appears controlled by sediment deposition, whereas U transport appears controlled by aqueous dissolution processes.