T88 Uranium in Groundwater: Biogeochemical Controls on Mobility and Persistence

Monday, 10 October 2011
8:00 AM - 12:00 PM
Room 211AB

FIELD-SCALE DETERMINATION OF URANIUM ISOTOPE FRACTIONATION INDUCED BY U(VI) ADSORPTION AND DESORPTION: THE SUPER 8 EXPERIMENT AT DOE'S RIFLE, COLORADO SITE

Paper number: T88-14
Program: Technical Sessions
Time: 11:15 AM – 11:30 AM

Authors

Abstract

238U/235U shows promise as an indicator of U(VI) reduction in groundwater contaminated by natural U (e.g., from mining and milling sites). Like other stable isotope ratios which indicate redox reactions, 238U and 235U (radioactively stable on decadal time scales) can be fractionated during U(VI) reduction. Previous work on both field-scale bioremediation and ancient redox fronts preserved as ore deposits show ~1 per mil shift to lower 238U/235U in the water as U(IV) is removed as a solid. The sense of shift is opposite to that observed with many lighter elements, presumably because nuclear field shift effects dominate over mass-dependent effects. Identifying U(VI) reduction is critically important, both as a means of tracking bioremediation of uranium contamination in groundwater and as a possible signature of U redox cycling during Earth history.

Recent laboratory work, however, indicates small but significant 238U/235U shifts induced by adsorption of U(VI) onto manganese oxides, introducing a possible confounding effect for using 238U/235U variations to infer or quantify U(VI) reduction. The present study examines 238U/235U variation during a field experiment at the Rifle, CO, IFRC site in which bicarbonate was injected to de-sorb U(VI) from aquifer solids. Samples from the CU-03 well, 1 m downgradient from the bicarbonate injection gallery, showed an initial concentration of 150 ng/mL U(VI), followed by a desorption-induced increase to 300 ng/mL for ten days, then a resorption-induced decrease to 70 ng/mL over two weeks, and a return to 150 ng/mL over two weeks.

Thirty-three samples, taken over 67 days and covering all four phases of the system response, were analyzed. No significant shift in 238U/235U was observed; the standard deviation of all the 238U/235U measurements was 0.06 per mil, and no significant trends appeared in the data. This is encouraging, as it indicates that the 238U/235U variations observed during U(VI) reduction induced by acetate injection solely reflect the reduction process. This simplifies interpretation of previously observed reduction induced shifts. It is not clear why sorption-related 238U/235U shifts reported in recent laboratory experiments did not appear; experiments with sediments from this site are planned to address that issue.