EVALUATING GEOCHEMICAL AND MICROBIAL INFLUENCES ON URANIUM MOBILIZATION IN CENTRAL VALLEY, CALIFORNIA

The Central Valley aquifer of California contains sediment with natural uranium (U) as the mineral uraninite. While uraninite is generally insoluble, oxidation of U(IV) to the soluble U(VI) uranyl ion (UO22+) can mobilize U into groundwater. One prevalent oxidant in the Central Valley triggering this reaction is nitrate (NO3-), commonly used in agricultural fertilizers. Furthermore, elevated concentrations of carbonate (CO32-) can enhance U dissolution from sediments via formation of aqueous uranyl carbonate complexes. Dissolved calcium (Ca2+) may further stabilize U in groundwater via formation of increasingly more stable calcium uranyl carbonate complexes. Although each of these mechanisms may individually cause elevated U in groundwater, the primary cause and/or interplay between these reactions is not well characterized, and it is unknown if these reactions are microbially facilitated, occur abiotically, or both. This study aims to characterize local microbial communities within sediment from the Central Valley responsible for facilitating the mobilization of U. The sediment samples used came from both the saturated and unsaturated zones of the aquifer, with U concentrations of 20 mg/kg and 5 mg/kg, respectively. We established microcosms in anaerobic environments with six treatments and amendments: NO3-, CO32-, NO3- and CO32-, Ca2+ and CO32-, NO3- with Ca2+ and CO32-, and a control. Each treatment had both sterile and live subsets. After ~3 weeks, we quantified dissolved uranium and characterized the microbial community in each microcosm with DNA extraction and sequencing. We expect to determine which mechanism(s) are primarily responsible for the mobilization of U, and if the reactions are performed abiotically or biotically. If microbial activity appears to facilitate U mobilization, we aim to identify the community responsible for U dissolution. Results from this study could provide more insight into the processes that mobilize U in groundwater and inform the development of mitigation strategies.