North-Central Section - 50th Annual Meeting - 2016

Paper No. 34-2
Presentation Time: 1:50 PM


JEMISON, Noah, Geology Department, University of Illinois- Urbana Champaign, 152 Computing Applications Bldg., 605 E. Springfield Ave., Champaign, IL 61820, JOHNSON, Thomas M., Geology, University of Illinois, 156 Computing Applications Building, 605 E. Springfield Ave, Champaign, IL 61820, LUNDSTROM, Craig C., Geology, University of Illinois at Urbana-Champaign, 63 Computing Applications Building, Champaign, IL 61820 and DAVIS, James A., Earth Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, MS 74R316C, Berkeley, CA 94720,

Uranium (U) mining and milling operations in the western United States have produced dozens of sites with U contaminated groundwater. U is redox active, primarily present in the environment in two valence states (+4 and +6). U(VI) is mobile in groundwater as soluble aqueous complexes, while U(IV) is usually insoluble and immobile, occurring primarily in sediments as U(IV) –bearing solids. Many U contaminated sites contain naturally reduced zones (NRZ’s) that also have high U(IV) concentrations. During seasonal shifts in redox conditions (i.e., changes in concentration of dissolved oxygen (DO) and nitrate), U(IV) from these NRZ’s can leach into groundwater as U(VI). These NRZ’s may prolong the long-term remediation of U at contaminated sites by slowly releasing U(VI) into the groundwater.

The use of the U isotope ratio 238U/235U allows us to better understand the addition of U to groundwater due to oxidation of NRZ’s. U becomes sequestered in NRZ’s via microbial reduction of dissolved U(VI) to U(IV). This process causes an isotopic shift in U; produced U(IV) is shifted to higher 238U/235U. We measured 238U/235U in solid materials of an NRZ and confirmed the existence of high-238U/235U U(IV). Oxidation of NRZ’s can be detected with 238U/235U measurements, because this high-238U/235U U is remobilized, driving aqueous U to higher 238U/235U. We measured a doubling of U(VI) concentration and an increase of 0.3 per mil in 238U/235U in groundwater as DO was injected into an NRZ at a U contaminated site in Rifle, CO. The data confirmed that oxidative remobilization is accompanied by a 238U/235U increase. Future work includes using reactive transport modeling to study NRZ evolution and oxidation.