USING URANIUM ISOTOPES TO TRACK OXIDATIVE REMOBILIZATION OF U(IV) FROM NATURALLY REDUCED ZONES

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.