Hydrogeological controls of uranium and arsenic mobility in groundwater of the Pine Ridge Reservation, South Dakota

Abstract

This study integrates geochemical modeling, spatial analysis and several statistical methods including principal component analysis, multivariate regression and cluster analysis to investigate hydrogeologic controls of arsenic and uranium contamination within groundwater of the Arikaree aquifer on the Pine Ridge Reservation (PRR). Located in southwestern South Dakota, geologic strata on the PRR are enriched with uranium and arsenic due to volcanic ash deposits emplaced into the White River Group, which unconformably underlies the Arikaree Group. Groundwater samples were obtained for over 250 wellbores through collaboration with the Oglala Sioux Tribe and Indian Health Service. Cluster analysis was used to delineate differences in groundwater chemistry in these data, and spatial analysis identified four regions, which represent upgradient, intermediate, and downgradient portions of the Arikaree aquifer. Groundwater alkalinity, sodium, and pH levels increase along flowpaths in the Arikaree aquifer despite rising carbonate mineral saturation indices, indicating that volcanic ash may act as a secondary source of alkalinity in the aquifer. Elevated alkalinity and pH levels are the primary measured drivers of arsenic and uranium mobility within the Arikaree aquifer, indicating that downgradient sections of the aquifer in the northern portions of the PRR are most likely to face impacts from groundwater contamination.