HR: 09:00h
AN: H41J-05 INVITED    [Abstracts]
TI: Chromium isotope variation along a contaminated groundwater plume: a coupled Cr(VI)- reduction, advective mixing perspective
AU: * Bullen, T
EM: tdbullen@usgs.gov
AF: U.S. Geological Survey, MS 420, 345 Middlefield Rd., Menlo Park, CA 94025, United States
AU: Izbicki, J
EM: jaizbick@usgs.gov
AF: U.S. Geological Survey, Water Science Center, San Diego, CA 92101,
AB: Chromium (Cr) is a common contaminant in groundwater, used in electroplating, leather tanning, wood preservation, and as an anti-corrosion agent. Cr occurs in two oxidation states in groundwater: Cr(VI) is highly soluble and mobile, and is a carcinogen; Cr(III) is generally insoluble, immobile and less toxic than Cr(VI). Reduction of Cr(VI) to Cr(III) is thus a central issue in approaches to Cr(VI) contaminant remediation in aquifers. Aqueous Cr(VI) occurs mainly as the chromate (CrO₂^2-) and bichromate (HCrO₂^-) oxyanions, while Cr(III) is mainly "hexaquo" Cr(H₂O)6³⁺. Cr has four naturally-occurring stable isotopes: ⁵⁰Cr, ⁵²Cr, ⁵³Cr and ⁵⁴Cr. When Cr(VI) is reduced to Cr(III), the strong Cr-O bond must be broken, resulting in isotopic selection. Ellis et al. (2002) demonstrated that for reduction of Cr(VI) on magnetite and in natural sediment slurries, the change of isotopic composition of the remnant Cr(VI) pool was described by a Rayleigh fractionation model having fractionation factor ε_{Cr(VI)-Cr(III)} = 3.4‰. We attempted to use Cr isotopes as a monitor of Cr(VI) reduction at a field site in Hinkley, California (USA) where groundwater contaminated with Cr(VI) has been under assessment for remediation. Groundwater containing up to 5 ppm Cr(VI) has migrated down-gradient from the contamination source through the fluvial to alluvial sediments to form a well-defined plume. Uncontaminated groundwater in the aquifer immediately adjacent to the plume has naturally-occurring Cr(VI) of 4 ppb or less (CH2M-Hill). In early 2006, colleagues from CH2M-Hill collected 17 samples of groundwater from within and adjacent to the plume. On a plot of δ⁵³Cr vs. log Cr(VI), the data array is strikingly linear and differs markedly from the trend predicted for reduction of Cr(VI) in the contaminated water. There appear to be two groups of data: four samples with δ⁵³Cr >+2‰ and Cr(VI) <4 ppb, and 13 samples with δ⁵³Cr <+2‰ and Cr(VI) >15 ppb. Simple mixing lines between the groundwater samples having <4 ppb Cr(VI), taken to be representative of regional groundwater, and the contaminated water do not pass through the remainder of the data, discounting a simple advective mixing scenario. We hypothesize a more likely scenario that involves both Cr(VI) reduction and advective mixing. As the plume initially expands downgradient, Cr(VI) in water at the leading edge encounters reductant in the aquifer resulting in limited Cr(VI) reduction. As a result of reduction, δ⁵³Cr of Cr(VI) remaining in solution at the leading edge increases along the "reduction" trend from 0 to ~+2‰. Inevitable mixing of this water at the leading edge with regional groundwater results in a suitable mixing end-member to combine with Cr(VI) within the plume in order to explain the bulk of the remaining data. Neither Cr(VI) reduction nor advective mixing of plume and regional groundwaters can explain the data on their own, implying an interplay of at least these two processes during plume evolution. Ellis, A.S., Johnson, T.M. and Bullen, T.D. 2002, Science, 295, 2060-2062.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1831 Groundwater quality
SC: Hydrology [H]
MN: 2007 Fall Meeting