Stream sediment geochemistry of the upper Cheyenne River watershed within the abandoned uranium mining region of the southern Black Hills, South Dakota, USA

Abstract

Abandoned uranium (U) mine workings and tailings deposits are found throughout the southern Black Hills of South Dakota. The close proximity of the mine sites to the Cheyenne River (CR) appears to promote elevated metal and radionuclide transport within the watershed, however, the extent of their contribution is unknown. Sixty sediment and soil samples were collected from potentially impacted locations within the watershed to elucidate contaminant occurrence and transport mechanisms. The degree of sediment contaminant impacts were assessed by establishing enrichment factors (EF) and pollution load indices (PLI) criterion as compared to non-mining impacted reference sites. Herron classifications suggest that the sediments collected from the southern Black Hills are immature, consistent with the presence of detrital feldspar. Generally, sediment Sr content were found depleted and intermittently enriched in Rb/Sr ratios compared to upper continental crust concentrations, suggesting physical weathering mechanisms predominate within the CR catchment up gradient of the Angostura Reservoir. Increasing EF and PLI indices coupled with low strontium (Sr) values suggest sediment contamination found along the CR and Angostura Reservoir delta appear attributed to anthropogenic activities and not natural weathering processes.