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.