GEOSPATIAL AND STATISTICAL ANALYSIS OF URANIUM IN GROUNDWATER IN CENTRAL CALIFORNIA

Groundwater in the Kings Groundwater Basin (KGWB) of the Central Valley of California has uranium (U) concentrations >30 μg/L, the maximum contaminant level (MCL) set by the U.S. Environmental Protection Agency. U originates in aquifer minerals as uraninite. Several hypotheses have been proposed for the mobilization of U from uraninite, including oxidation by dissolved oxygen and/or nitrate, the formation of aqueous uranyl-carbonate complexes, and/or the formation of ternary aqueous calcium-uranyl-carbonate complexes. In this study, we used statistical and quantitative spatial analyses of groundwater chemistry data, geochemical modeling, and sequential chemical extractions of aquifer sediments to evaluate the mechanisms responsible for elevated U in KGWB groundwater. 34% of shallow wells assessed exceeded the 30 μg/L MCL, whereas wells screened only at deep intervals were below the MCL. Spatial analysis indicates that as much as 58% of shallow groundwater in the KGWB is unsafe with respect to U. Sequential extractions show that 40-60% of U in sediments was soluble via oxidation. Neither carbonate-bound nor reducible U contributed to U dissolution. U <30 μg/L in groundwater was observed in the presence of dissolved oxygen, suggesting aerobic oxidation may play a moderate role in U mobility. Regions where U was >30 μg/L in groundwater coincided with elevated concentrations of nitrate, calcium, and alkalinity. However, U concentrations were low in groundwater with calcium and alkalinity if nitrate was lacking. Geochemical modeling predicts that the dominant U species in the KGWB are CaUO2(CO3)32- and Ca2UO2(CO3)3. From these results, we propose that a two-step mechanism facilitates the dissolution of U to concentrations >30 μg/L: 1) U dissolution is initiated via microbially catalyzed 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. U groundwater contamination presents a unique challenge to unregulated private wells and municipal treatment centers. Carbonate and nitrate levels are inextricably linked to agricultural and human activity, both with potential to further exacerbate U contamination without continued regulatory action on nitrate contamination.