Uranium(VI) attenuation in a carbonate-bearing oxic alluvial aquifer

• Published in: Journal of hazardous materials 2021-06, Vol.412, p.125089-125089, Article 125089
• Main Authors: Nolan, PJ, Bone, Sharon E., Campbell, Kate M., Pan, Donald, Healy, Olivia M., Stange, Marty, Bargar, John R., Weber, Karrie A.
• Format: Article
• Language: English
• Subjects: ENVIRONMENTAL SCIENCES; Geogenic contamination; Groundwater; Uranium; Uranium carbonate minerals
• ISSN: 0304-3894; 1873-3336
• DOI: 10.1016/j.jhazmat.2021.125089

Uranium minerals are commonly found in soils and sediment across the United States at an average concentration of 2–4 mg/kg. Uranium occurs in the environment primarily in two forms, the oxidized, mostly soluble uranium(VI) form, or the reduced, sparingly soluble reduced uranium(IV) form. Here we describe subsurface geochemical conditions that result in low uranium concentrations in an alluvial aquifer with naturally occurring uranium in soils and sediments in the presence of complexing ligands under oxidizing conditions. Groundwater was saturated with respect to calcite and contained calcium (78–90 mg/L) with elevated levels of carbonate alkalinity (291–416 mg/L as HCO3−). X-ray adsorption near edge structure (XANES) spectroscopy identified that sediment-associated uranium was oxidized as a uranium(VI) form (85%). Calcite was the predominant mineral by mass in the ultrafine fraction in uranium-bearing sediments (>16 mg/kg). Groundwater geochemical modeling indicated calcite and/or a calcium-uranyl-carbonate mineral such as liebigite in equilibrium with groundwater. The δ13C (0.57‰ ± 0.15‰) was indicative of abiotic carbonate deposition. Thus, solid-phase uranium(VI) associated with carbonate is likely maintaining uranium(VI) groundwater levels below the maximum contaminant level (MCL; 30 µg/L), presenting a deposition mechanism for uranium attenuation rather than solely a means of mobilization. [Display omitted] •Uranium(VI) can be sequestered with minerals in sediments of oxic carbonate-bearing aquifers.•Groundwater uranium concentration remains below MCL when uranium(VI) is sequestered with minerals.•Shifts in carbonate equilibrium may cause mineral dissolution leading to uranium mobilization.