FROM ROCKS TO SEDIMENTS TO GROUNDWATER: THE WEATHERING AND MICROBIAL INFLUENCES ON URANIUM MOBILITY IN THE CENTRAL VALLEY, CALIFORNIA

Groundwater in the Central Valley, California experiences uranium (U) concentrations fifteen times greater than the US Environmental Protection Agency’s safe threshold of 30 μg/L. U in groundwater is dissolved from uraninite in aquifer sediments. Uraninite originates from the Sierra Nevada batholith. While U in uraninite is generally insoluble, oxidative dissolution by nitrate (via human and agricultural activity) and the formation of (calcium)-uranyl-carbonate complexes can act as mobilization mechanisms for U. To better understand the chemical evolution of U from source rock to sediments to groundwater, we compared the chemistry of Sierra Nevada bedrock, freshly weathered saprolite, and unconsolidated sediments in the Central Valley. We performed a microcosm study on the aquifer sediments to evaluate the biogeochemical controls on U mobility.

Results show that the bulk of chemical alteration occurs early in the weathering and transport process. Chemical weathering indices for saprolite and aquifer sediments are nearly indistinguishable. U is preferentially weathered from the source rock, occurring in higher concentrations in sediment than in Sierra Nevada rocks.

Microcosm experiments were performed anaerobically under six conditions, with live and sterile subsets. Treatments included amendments to sediments with: NO₃^-, CO₃^2-, NO₃^-/CO₃^2-, Ca²⁺/CO₃^2-, NO₃^-/Ca²⁺/CO₃^2-, and a control. Samples with an active native microbial community released significantly more U than sterile samples. U concentrations were highest in the live treatments containing nitrate and carbonate; dissolved U in the NO₃^-/Ca²⁺/CO₃^2- and NO^3-/CO₃^2- treatments was 289 μg/L and 252 μg/L, respectively. DNA amplicon sequencing shows microbial and fungal communities are rich in microbes with redox-sensitive metabolisms with metabolite profiles further supporting redox sensitive reaction pathways. Our observations suggest that U mobility is facilitated by a combination of microbially driven denitrification, followed by the formation of aqueous complexes with (Ca)^-CO₃. This study provides insight into the processes that mobilize U in groundwater and inform the development of mitigation strategies. Finally, regular DNA profiling of well water coupled with water chemistry has the potential to help mitigate contamination, and understand and predict trends in water chemistry.