Isoscapes as a Regional-Scale Tool for Tracing Groundwater Uranium Cycling in the Northern Plains, United States

Groundwater uranium (U) contamination poses a significant health risk, particularly in Native American communities reliant on private wells. This study examines groundwater U cycling in a tribal region in South Dakota that participates in the Strong Heart Water Study based on samples from 140 private wells. We measured U concentrations, δ238U, (234U/238U), and redox-sensitive elements, including iron (Fe), manganese (Mn), nitrate (NO3–), selenium (Se), and vanadium (V). Uranium concentrations range from 0.4 to 48.2 μg/L, with 5% exceeding the U.S. EPA maximum contaminant level of 30 μg/L. Spatial patterns in δ238U and (234U/238U) delineate distinct redox regimes: oxidizing zones in the northeast show higher U (median = 18 μg/L) and positive δ238U values (from 0.08 to 0.30‰), while reducing zones in the southwest display lower U (median = 10 μg/L) and large negative δ238U values (from −0.61 to −0.30‰). The (234U/238U) values (from 1.53 to 3.07, median of 2.03) serve as a tracer of source proximity, with lower values (1.53–1.80) indicating shorter travel distance relative to the U source and higher values (1.80–2.50) reflecting U transported farther along flow paths. Cluster and uniform manifold approximation and projection (UMAP) analyses identify three geochemical environments consistent with oxidizing, reducing, and intermediate redox conditions. Constructing the first δ238U and (234U/238U) isoscapes for a sandstone aquifer, we show that U is released by oxidative dissolution in the northeast and removed under reducing conditions in the southwest and that the northeastern zone may require continuous monitoring and intervention for exposure reduction.