HR: 0800h
AN: H31C-0515    [Abstracts]
TI: Chromium Isotopic Monitoring of HRC-Stimulated Bio-containment at the 100H Test Site, Hanford, Washington
AU: * Christensen, J N
EM: jnchristensen@lbl.gov
AF: Lawrence Berkeley National Lab, 1 Cyclotron Rd., Berkeley, CA 94720,
AU: Brown, S T
EM: stbrown@lbl.gov
AF: Lawrence Berkeley National Lab, 1 Cyclotron Rd., Berkeley, CA 94720,
AU: Brodie, E L
EM: elbrodie@lbl.gov
AF: Lawrence Berkeley National Lab, 1 Cyclotron Rd., Berkeley, CA 94720,
AU: Chakraborty, R
EM: rchakraborty@lbl.gov
AF: Lawrence Berkeley National Lab, 1 Cyclotron Rd., Berkeley, CA 94720,
AU: Conrad, M E
EM: msconrad@lbl.gov
AF: Lawrence Berkeley National Lab, 1 Cyclotron Rd., Berkeley, CA 94720,
AU: Long, P E
EM: philip.long@pnl.gov
AF: Pacific Northwest National Lab, Box 999, Richland, WA 99352,
AU: Faybishenko, B
EM: bafaybishenko@lbl.gov
AF: Lawrence Berkeley National Lab, 1 Cyclotron Rd., Berkeley, CA 94720,
AU: Hazen, T C
EM: tchazen@lbl.gov
AF: Lawrence Berkeley National Lab, 1 Cyclotron Rd., Berkeley, CA 94720,
AB: Hexavalent Cr (Cr(VI)) groundwater contamination is a common problem in the U.S. associated with industrial activity (e.g. electroplating, tanning, paints, anti-corrosion). In the particular case of the Hanford Site, Washington, chromate was used primarily to inhibit corrosion in nuclear reactor cooling systems. During the active operation of the Hanford Site, disposal of waste water bearing chromate, and accidental releases to the vadose zone resulted in significant groundwater contamination with local concentrations near the Columbia river reaching over 1000 ppb Cr(VI). In an effort to test an effective bio-containment strategy for groundwater Cr(VI), a site was selected between the 100D and 100H reactor areas with modest concentrations (~100 ppb Cr(VI) over the past two decades). A slow-release ¹³C labeled polylactate amendment (HRC_TM, Regenesis, Ltd.) was injected into groundwater within a sandy formation to stimulate bacterial activity in order to produce conditions that promote the reduction of dissolved Cr(VI) to insoluble Cr(III) complexes [1]. Since the injection of HRC_TM in August 2004, groundwater Cr(VI) concentration has been locally below 1 ppb, and reducing conditions have been maintained to at least the present time. The isotopic composition of Cr can be fractionated during reduction from Cr(VI) to Cr(III) and so has the potential to be used as a monitor of hexavalent Cr reduction [2, 3]. This would provide a direct signature of Cr(VI) reduction, discernable from simple attenuation by dilution. In order to explore the use of Cr isotopic measurements for evaluating processes of Cr(VI) reduction, we have analyzed a series of samples in space and time for Cr isotopic composition (δ⁵³Cr, permil deviation of sample ⁵³Cr/⁵²Cr from that of SRM970). Groundwater samples came from the HRC injection well, from multiple depths of three down-gradient wells, and from an up-gradient well. Samples from down-gradient wells have Cr that is isotopically fractionated relative to samples from the up-gradient well. Taken together, samples from a single sampling campaign yield an apparent fractionation of 2.2 ‰. Cr isotopic measurements of the latest samples (June, 2007) confirm continued reduction of Cr(VI) nearly three years after the introduction of HRC_TM, suggesting the long-term effectiveness of this bio-stimulated containment strategy. [1] http://esd.lbl.gov/ERT/hanford100h/ [2] Ellis, AS, Johnson, TM and Bohlen, TD (2002) Science. 295:2060-2062. [3] Johnson, TM and Bohlen, TD (2004) Reviews in Mineralogy and Geochemistry, Vol. 55, p.289-317.
DE: 0418 Bioremediation
DE: 0432 Contaminant and organic biogeochemistry (0792)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1829 Groundwater hydrology
SC: Hydrology [H]
MN: 2007 Fall Meeting