MODELING THE FORMATION AND DISTRIBUTION OF AQUEOUS CALCIUM-URANIUM-CARBONATE COMPLEXES IN THE KINGS GROUNDWATER BASIN, CENTRAL VALLEY, CALIFORNIA

Groundwater concentrations of uranium (U) in the Kings Groundwater Basin in the Central Valley, California exceed the Environmental Protection Agency’s Maximum Contaminant Level of 20 pCi/L (~30 μg/L). U concentrations above 20 pCi/L can result in an array of human health hazards when consumed via drinking water and present a financial challenge to water municipalities. U naturally exists in reduced form, U(IV), in aquifer minerals weathered from the Sierra Nevada east of the Central Valley. However, U(IV) can be oxidized to U(VI), uranyl (UO₂²⁺), a soluble, mobile ion in groundwater. In this study, we used groundwater data from the Groundwater Ambient Monitoring and Assessment (GAMA) program in the Kings Groundwater Basin with two objectives: 1) to identify regions with elevated U in groundwater, and 2) to model the geochemical speciation of U in groundwater to characterize U stability. Results show that U concentrations in shallow groundwater are up to 2.7 times higher than in deep groundwater, with a maximum of 665 pCi/L. U concentrations are higher on the western margin of the Kings Groundwater Basin, where groundwater is concomitantly elevated in alkalinity and calcium (Ca). Using geochemical modeling, we show that U speciation is dominated by the formation of aqueous Ca-UO₂-CO₃ complexes (CaUO₂(CO₃)₃^2- and Ca₂UO₂(CO₃)₃^o) in regions exceeding MCLs, while in regions with U below the MCL, UO₂-CO₃ complexes dominate (mostly UO₂(CO3)₂^2- and UO₂(CO3)₃^4-). The combination of Ca²⁺ and UO₂-CO₃ species reduces the charge of the UO₂-CO₃ complex, which decreases the ability of the complex to adsorb to positively charged surfaces mineral surfaces, especially iron oxides. The shift in charge state of the complex via calcium availability is the mechanism most responsible for observed changes in the concentration of U throughout the valley, further confirmed by regional assessments of Ca and U. The formation of Ca-UO₂-CO₃ complexes is highly sensitive to Ca²⁺ concentrations, whereas the concentrations of CO₃^2- minimally affect speciation when other variables are held constant. Our results suggest that elevated Ca²⁺ and the subsequent formation of Ca-UO₂-CO₃ is a dominant control on observed concentrations of U above MCLs in the Kings Groundwater Basin.