OXIDATIVE DISSOLUTION OF NATURALLY-OCCURRING URANIUM IN SHALLOW AQUIFERS

Naturally occurring uranium (U) groundwater (GW) contamination linked to nitrate has been recognized in major US aquifers. U minerals are ubiquitous in soil and sediment (~2-4 mg/kg) existing as insoluble reduced U(IV) minerals, e.g. uraninite. Oxidants such as nitrate, a common GW contaminant, oxidatively dissolve reduced U(IV) minerals leading to U mobilization and GW contamination (EPA MCL 30 ug/L). Oxidized U(VI) is highly soluble when oxidized. Here we geochemically characterize sediment and GW in the saturated zone of an alluvial aquifer to investigate the potential of U mobilization in a region where aqueous U(VI) and nitrate co-exist below the MCL. However U and nitrate concentrations in excess of the MCL have been measured in this aquifer. Sediment cores (0-170ft) collected by sonic drilling were immediately processed in an anoxic atmosphere (100% argon). GW U and nitrate concentrations in the borehole did not exceed the MCL (≤15 ug/L; ≤8 mg/L). Sedimentary U was measured as high as 1.8 mg/kg where 80-95% of U is reduced, suggesting the potential for oxidative dissolution. Alkalinity’s role in U mobility was determined by differences in adsorbed U(VI) by bicarbonate extraction under anoxic and oxic atmospheres (2.1 – 82.1 vs 19.2 – 182.5 ug/L) indicating a redox shift influencing U(VI) adsorption. A surface complexation model with aquifer Pco₂ (0.5 - 1.1), pH (7.8 - 8.2), and sediment surface area (2.2 - 6.2 m²/g), revealed alkalinity accounts for up to 10 – 30% of GW U(VI) concentrations. The potential for oxidative dissolution of U minerals was investigated with packed up-flow bioreactors amended with U(IV). Effluent concentrations of U(VI) increased by 250 ug/L after nitrate addition, and a two fold increase in total oxidized U. The role of microbial catalysis in U mobility via most-probable-number enumeration identified microorganisms capable of direct/indirect U(IV) oxidation: anaerobic nitrate-dependent U(IV) oxidation (1.5x10⁵ - 1.1x10⁷ cells/g sediment), nitrate-dependent Fe(II) oxidizers (2.1x10⁴ - 2.3x10⁷ cells/g sediment), heterotrophic nitrate reducers (3.6x10⁴ - 1.1x10⁹ cells/g sediment). Together the results suggest oxidative dissolution of reduced U(IV) minerals as a plausible mechanism that could lead to increased GW U concentrations in this aquifer, rather than alkalinity alone.