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

January 8, 2021

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.

Citation Information

Publication Year 2021
Title Uranium(VI) attenuation in a carbonate-bearing oxic alluvial aquifer
DOI 10.1016/j.jhazmat.2021.125089
Authors PJ Nolan, S Bone, Kate M. Campbell, David Pannell, O Healy, M Stange, J Bargar, KA Weber
Publication Type Article
Publication Subtype Journal Article
Series Title Journal of Hazardous Materials
Index ID 70217756
Record Source USGS Publications Warehouse
USGS Organization Geology, Geophysics, and Geochemistry Science Center