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 238 U/ 235 U 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 m 2 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 238 U/ 235 U in samples from one monitoring well by MC-ICP-MS using a double isotope tracer method. A significant ∼1.00 decrease in 238 U/ 235 U occurred in the groundwater as U(VI) concentration decreased. The relationship between 238 U/ 235 U and concentration corresponds approximately to a Rayleigh distillation curve with an effective fractionation factor (α) 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) .
ASJC Scopus subject areas
- Environmental Chemistry