Uranium 238U/235U Isotope Ratios as Indicators of Reduction: Results from an in situ Biostimulation Experiment at Rifle, Colorado, U.S.A.

详细信息    查看全文

• 作者：Charles John Bopp ; IV ; Craig C. Lundstrom ; Thomas M. Johnson ; Robert A. Sanford ; Philip E. Long ; Kenneth H. Williams
• 刊名：Environmental Science & Technology
• 出版年：2010
• 出版时间：August 1, 2010
• 年：2010
• 卷：44
• 期：15
• 页码：5927-5933
• 全文大小：204K
• 年卷期：v.44,no.15(August 1, 2010)
• ISSN：1520-5851

文摘

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 1.00‰ 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 (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).