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