title: Uranium 238U/235U isotope ratios as indicators of reduction: results from an in situ biostimulation experiment at Rifle, Colorado, U.S.A.
abstract: The attenuation of groundwater contamination via chemical reaction is traditionally evaluated by monitoring contaminant concentration through time. However, this method can be confounded by common transport processes (e.g., dilution, sorption). Isotopic techniques bypass the limits of concentration methods, and so may provide improved accuracy in determining the extent of reaction. We apply measurements of 238U/235U to a U bioremediation field experiment at the Rifle Integrated Field Research Challenge Site in Rifle, Colorado. An array of monitoring and injection wells was installed on a 100 m2 plot where U(VI) contamination was present in the groundwater. Acetate-amended groundwater was injected along an up-gradient gallery to encourage the growth of dissimilatory metal reducing bacteria (e.g., Geobacter species). During amendment, U concentration dropped by an order of magnitude in the experiment plot. We measured 238U/235U in samples from one monitoring well by MC-ICP-MS using a double isotope tracer method. A significant approximately 1.00 per thousand decrease in 238U/235U occurred in the groundwater as U(VI) concentration decreased. The relationship between 238U/235U and concentration corresponds approximately to a Rayleigh distillation curve with an effective fractionation factor (alpha) of 1.00046. We attribute the observed U isotope fractionation to a nuclear field shift effect during enzymatic reduction of U(VI)(aq) to U(IV)(s).
authors.affiliation: Department of Geology, University of Illinois at Urbana-Champaign, 253 Natural History Building, 1301 West Green Street, Urbana, Illinois 61801, USA. cbopp2@illinois.edu
authors.forename: Charles John
authors.initials: CJ
authors.lastname: Bopp
authors.forename: Craig C
authors.initials: CC
authors.lastname: Lundstrom
authors.forename: Thomas M
authors.initials: TM
authors.lastname: Johnson
authors.forename: Robert A
authors.initials: RA
authors.lastname: Sanford
authors.forename: Philip E
authors.initials: PE
authors.lastname: Long
authors.forename: Kenneth H
authors.initials: KH
authors.lastname: Williams
mesh_terms: D001673:Biodegradation, Environmental; D003120:Colorado; D011834:Radiation Monitoring / Q000379:methods*; D011849:Radioactive Tracers; D014501:Uranium / Q000032:analysis*; D014875:Water Pollutants, Radioactive / Q000032:analysis*
chemical_list: D011849:Radioactive Tracers; D014875:Water Pollutants, Radioactive; D014501:Uranium