Spatial variability of groundwater arsenic and uranium across the US Great Plains: Geochemical and isotopic insights

Contamination of drinking water with arsenic (As) and uranium (U) affects millions of people across the United States (US) and upwards of 200 million people globally. Rural regions often rely on domestic wells, which are generally untreated, and thus tend to have higher rates of As and U exposure. In the High Plains of the US, many people source their drinking water from domestic wells, and the region has high levels of As and U in the groundwater. However, the environmental and geologic factors responsible for the mobilization and heterogeneous distribution of As and U in the groundwater of the High Plains remains poorly characterized. In this work we examine how groundwater recharge sources and groundwater flow paths influence the mobilization and accumulation of As and U across this region. We used stable water isotopes (δ2H, δ18O) along with groundwater geochemistry (e.g., F, Cl, Br, SO4, NO3, Se, V) to characterize groundwater recharge sources, flow paths, and the potential mobilization mechanisms and geologic sources that lead to As and U contamination. To do so we collected more than 1000 groundwater, tap water, and river water samples from across the High Plains and measured stable water isotopes, major ions, and trace elements. We observed that stable water isotopes along with trace and major elements exhibit distinct spatial patterns that shed light on the underlying geochemical processes occurring in this region's aquifer systems. These insights into the groundwater geochemical processes help to explain the heterogeneous distribution of As and U. Our results suggest that, (1) As and U exhibit a strong positive correlation across much of the region, (2) mobilization is generally occurring under oxic conditions and (3) mobilization is delocalized with concentrations likely accumulating along groundwater flow paths. These results allow us to better understand how groundwater recharge sources and flow paths can help identify areas that are at a high risk for contamination.