HYDROLOGIC INSIGHTS INTO THE DRIVERS OF ARSENIC AND URANIUM CONTAMINATION IN THE US NORTHERN PLAINS

Contamination of drinking water with arsenic (As) and uranium (U) affects more than 100 million people globally, including millions of people across the US. In rural populations this issue is particularly acute as drinking water is often obtained from unregulated private wells. One specific location greatly affected by high levels of As and U in surface and groundwater is the US Northern Plains. Groundwater recharge sources and the flow path and source of water supplying surface water can strongly influence the mobilization and accumulation of aqueous As and U. However, the environmental and anthropogenic factors responsible for the mobilization and heterogeneous distribution of As and U in the waters of this region remain poorly characterized. Thus, characterizing both the chemical and physical hydrology is important to the overall understanding of As and U contamination. To address these questions, we collected surface water samples from the White River watershed, which drains 26,000 km² across Nebraska and South Dakota. We used our measurements of stable water isotopes (δ²H, δ¹⁸O, δ¹⁷O) and trace and major elemental composition of the water, along with streamflow data to: (1) determine the sources of recharge/inflow to both surface water and groundwater; (2) gain insight into the mechanisms responsible for As and U mobilization; and (3) identify the drivers of the spatial and temporal evolution of As and U as water travels downstream in the White River. Moving downstream we observe large increases in As and U concentrations, with As often starting at levels well below EPA guidelines (10 µg/L) in upstream areas reaching levels greatly in exceedance of EPA guidelines further downstream. We use our measurements of stable water isotopes and dissolved ions to track the evaporative history of water as it travels downstream and find that evaporative concentration can explain the majority of the increases in As and U in the White River. Our results highlight that an upstream contaminant source that initially results in contaminant levels below regulatory guidelines may ultimately exceed those guidelines further downstream without any additional contaminant input. These results are particularly relevant to semi-arid and arid regions as well as regions where losing river reaches may extend for significant distances.