HR: 1340h
AN: H13G-1668    [Abstracts]
TI: Sulfur Isotopes as Indicators of Bacterial Sulfate Reduction Processes Influencing Field Scale Uranium Bioremediation
AU: * Druhan, J L
EM: jennydruhan@berkeley.edu
AF: University of California, Berkeley, Department of Earth and Planetary Science, 307 McCone Hall MC4767, Berkeley, CA 94720,
AU: Conrad, M E
EM: msconrad@lbl.gov
AF: Lawrence Berkeley National Lab, Earth Sciences Division, 1 Cyclotron Rd MS 90-1116, Berkeley, CA 94720,
AU: Williams, K H
EM: khwilliams@lbl.gov
AF: Lawrence Berkeley National Lab, Earth Sciences Division, 1 Cyclotron Rd MS 90-1116, Berkeley, CA 94720,
AU: N'Guessan, L
EM: nguesa@pltpath.umass.edu
AF: University of Massachusetts, Department of Microbiology, 203 Morrill Science Center IVN, Amherst, MA 01003,
AU: Long, P E
EM: philip.long@pnl.gov
AF: Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352,
AU: Hubbard, S S
EM: sshubard@lbl.gov
AF: Lawrence Berkeley National Lab, Earth Sciences Division, 1 Cyclotron Rd MS 90-1116, Berkeley, CA 94720,
AB: An in-situ acetate amendment at a DOE Uranium Mill Tailings Remedial Action (UMTRA) site near Rifle, CO demonstrated successful reduction of aqueous U(VI), to less soluble U(IV) through stimulated microbial activity. U(VI) reduction rates were highest during iron reduction and decreased with the onset of sulfate reduction. However, sustained U(IV) attenuation was observed following subsequent termination of the acetate amendment. These findings illustrate the importance of the transition between iron and sulfate reducing conditions in stimulating bioreduction of uranium. The sulfur isotope compositions of sulfate and sulfide were measured through this transition in order to explore the utility of these data in tracking the extent of microbial sulfate reduction and to assess the stability of sulfide precipitates. Samples for isotopic analyses and aqueous measurements of sulfate, ferrous iron, U(VI) and acetate were collected in one background well and three monitoring wells down-gradient of the acetate injection. Results show an increase of up to 7‰ in the δ³⁴S of sulfate at the onset of sulfate reduction, followed by a return to background δ³⁴S values of -8‰ following cessation of the acetate amendment. The δ³⁴S values of sulfide increased from roughly -20‰ at the onset of sulfate reduction to a maximum of -0.8‰ during peak sulfate removal, followed by a gradual return to values of roughly -28‰ upon cessation of the acetate amendment. These data present a unique perspective on the processes governing the bioreduction experiment in that the sulfate isotopes are a function of both transport and mixing processes, whereas the sulfide isotopes represent biogenic sulfide that is rapidly removed from the aqueous phase. Thus a comparable enrichment in sulfate isotopic data noted in the closest and furthest wells from the injection gallery suggest bioreduction in both of these locations, while a larger increase in sulfide isotopic values in the closest well indicates greater rates of sulfate reduction closer to the injection gallery. In addition, a steady decline in δ³⁴S of sulfide concurrent with increased sulfide concentrations following cessation of acetate amendment suggests that this increase is not a result of reoxidation of precipitated FeS species. FeS precipitates formed during the height of sulfate reduction therefore appear to be stable on a timeframe of months following acetate amendment and may support the stability and long-term sequestration of precipitated U(IV).
DE: 0418 Bioremediation
DE: 0471 Oxidation/reduction reactions (4851)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1834 Human impacts
SC: Hydrology [H]
MN: 2007 Fall Meeting