Isotopic Insights into Redox Processes Driving Uranium Distribution in Eastern Karnataka Groundwater

Uranium (U) contamination in groundwater becomes a critical concern when its concentration exceeds permissible limits. Groundwater samples (n = 46) from Eastern Karnataka exhibit U concentrations ranging from 1.9 to 2744 μg/L, with 78% and 66% of sites exceeding the United States Environmental Protection Agency (USEPA) and Atomic Energy Regulatory Board (AERB), India thresholds, respectively. These high U levels are primarily geogenic. The δ238U and U activity ratios (234U/238U) were determined on the same set of samples to understand the redox processes governing groundwater U cycling. The δ238U isoscape displays a variation of 1.3‰ from −0.95‰ to 0.3‰, providing evidence of a generally oxidizing groundwater environment, with localized U-reducing conditions in the southern part of the study area. The (234U/238U) of groundwater varies from 1.2 to 5.4, indicating a dynamic interplay of α-recoil effects, preferential leaching of 234U, and the dissolution of high U zone(s) in granitic rocks. Statistical analysis of geochemical and field parameters reveals three distinct redox environments: oxidized, intermediate, and U(VI) reducing, across the study area. Integrating U isotope compositions and additional redox indicators with statistical analysis in this manner provides a robust approach to understand the subsurface environment controlling U cycling in large, data-limited regions.