INSIGHT INTO U TRANSPORT AND IMMOBILIZATION: 238U/235U MEASUREMENTS OF AN ANOMALOUS U CONCENTRATION PEAK AT THE RIFLE INTEGRATED FIELD RESEARCH CHALLENGE SITE

The Rifle Integrated Field Research Challenge Site at Rifle, CO is located in an area of residual uranium groundwater contamination from a uranium mill tailings pile. Several stimulated uranium bioremediation experiments have been successfully conducted at the site, characteristically decreasing dissolved uranium concentrations only days after acetate injection. Previous work using high precision MCICPMS methods has shown that as U is reduced in these groundwaters, the 238U/235U (δ238U) of the remaining U(VI) in solution increases (Bopp et al., EST, 2010).

Here we present δ238U results for a set of groundwater samples taken from the “Big Rusty” experiment in 2008 in which concentrations of U(VI)(aq) dropped after initial acetate injection then rebounded sharply after a 7 day interruption in acetate amendment and remained elevated for over a month. Was this anomalous increase due to reoxidation, mixing in of upgradient groundwater, or desorption? Results show that δ238U increases from -0.75 to 0 in concert with U(VI)(aq) concentration but there is a lag time between the decrease of U(VI)(aq) and the onset of lower δ238U. Upgradient control wells during this time period show no change in U(VI)(aq) or δ238U. Using an isotopic calibration of U reduction at the Rifle site based on Bopp et al. (2010) and recent results for isotopic fractionation with sorption (Wasylenki et al., 2010), we evaluate the three possible explanations. Based on other conservative tracers, mixing cannot explain the anomalous peak. The δ238U results appear inconsistent with sorption effects but are in line with expectations of reoxidation. Specifically, the "stair-step" shape of the δ238U profile may be characteristic of a reoxidation process; however more work is needed to constrain the effects of adsorption/desorption and reoxidation on 238U/235U to verify this interpretation. Note that this reoxidation occurs under anaerobic conditions; this may reflect reoxidation by reactive and thermodynamically favorable ferric phases (Ginder-Vogel, 2006; 2010). With further work, the 238U/235U method may provide a reliable tracer of short-term uranium remobilization due to oxidation.