264 Geochemical Approaches to Earth Processes

Wednesday, 21 October 2009
1:30 PM - 5:30 PM
Portland Ballroom 252

238U/235U ISOTOPE RATIOS AS TRACERS OF URANIUM REDUCTION: IN SITU EXPERIMENTS AT RIFLE, CO

Paper number: 264-8
Program: Technical Sessions
Time: 3:15 PM – 3:30 PM

Authors

Abstract

Assessing the efficacy of groundwater remediation by measuring temporal changes in contaminant concentration can be complicated by issues of dilution and sorption. To avoid such problems, isotopic ratio measurements of redox active metals have been used to quantify reduction and extent of immobilization where one redox state is less mobile than another. Here we present new 238U/235U results for groundwater samples from Rifle, CO, in a near-surface aquifer contaminated by earlier uranium milling operations. The U.S. Department of Energy removed the tailings pile and the upper few meters of alluvial sediment. However, residual uranium contamination remains in the groundwater. This contaminated alluvial aquifer provides an excellent natural laboratory for studying in situ bioremediation of uranium (e.g. Anderson, et al., 2003; AEM).

The Winchester Experiment conducted at the Rifle Integrated Field Research Challenge Site investigated uranium reduction accompanying the injection of acetate-amended groundwater. This carbon-rich groundwater stimulates the growth of native iron-reducing bacteria, such as Geobacter sp., which reduce mobile U(VI) to immobile U(IV), enabling the in situ sequestration of aqueous uranium.

We have analyzed a set of groundwater samples from the injection experiment, as well as natural background samples for 238U/235U by MC-ICP-MS using double spike methods. We observe that 238U/235U decreases steadily with time as injection and uranium reduction proceeds. As uranium concentrations decrease from ≈200μg/L to ≈20μg/L, we observe a systematic change to lower 238U/235U of ≈1.00‰. This change to lower 238U/235U is complimentary to that observed in both reduced sediments (Weyer, et al, 2008; GCA) and reduced uranium ore deposits (Bopp, et al, 2009; Geology): of opposite sense because we test the fluid rather than the sediments, and similar magnitude. Thus, measurements of 238U/235U provide a method for assessing extent of uranium reduction, which is free of the problems that hamper concentration measurements. This study provides the first demonstration of 238U/235U ratios as a tracer of the reduction and immobilization of uranium at the field scale.