2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 14-10
Presentation Time: 10:40 AM

OXIDATIVE DISSOLUTION OF NATURALLY-OCCURRING URANIUM IN SHALLOW AQUIFERS


NOLAN, Jason1, PAN, Donald2, HEALY, Olivia2, KIAT, Rebecca2, JUNG, Soyung2, STANGE, Marty3, CAMPBELL, Kate M.4 and WEBER, Karrie A.5, (1)Department of Earth and Atmospheric Sciences, University of Nebraska-Lincoln, Lincoln, NE 68588, (2)School of Biological Sciences, University of Nebraska-Lincoln, Lincoln, NE 68588, (3)Hastings Utilities, City of Hastings, Hastings, NE 68901, (4)U.S. Geological Survey, Boulder Labs, 3215 Marine St, Boulder, CO 80303, (5)School of Biological Sciences and Department of Earth & Atmospheric Sciences, University of Nebraska-Lincoln, Lincoln, NE 68588-0340, Jason.nolan@unl.edu

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.