Statistical, Geospatial, and Geochemical Modeling of Geogenic Uranium in Groundwater in the Kings Groundwater Basin, Central Valley, California

Uranium (U) concentrations exceed 30 μg/L, the maximum contaminant level, in some shallow groundwater aquifers in the Kings Groundwater Basin in Central California. We used statistical and spatial analyses, geochemical modeling, and sequential sediment extractions to evaluate the mechanisms responsible for elevated U in groundwater. Thirty-four percent of shallow wells (<74.3 m below land surface) exceeded 30 μg/L, whereas wells deeper than 74.3 m did not exceed 30 μg/L. Forty-to-sixty percent of total U in sediments was soluble via oxidation. Reducible and carbonate-bound U were not sources of U in groundwater. Ion-exchangeable U was <4% of the total U in all samples. Uranium <30 μg/L was observed in the presence of dissolved oxygen, suggesting some aerobic oxidative dissolution of U(IV) to U(VI). Uranium >30 μg/L co-occurred with elevated concentrations of nitrate, calcium, and alkalinity, which are linked to agricultural development. However, U was <30 μg/L in groundwater with elevated calcium and alkalinity if nitrate was <7 mg/L NO3–N. We propose a two-step mechanism for U concentrations exceeding 30 μg/L: (1) U dissolution is initiated via microbial oxidation of U(IV) by nitrate to the soluble uranyl ion, and (2) U solubility is enhanced by the formation of aqueous (calcium−) uranyl–carbonate complexes.