Uranium(IV) accumulates in reducing sediments in fluvial aquifers that have been contaminated as a result of U ore processing, such as Rifle, CO and Riverton, WY. These reduced sediments may act as reservoirs of U that can be released (as soluble U(VI)) into groundwater during seasonal fluctuations in water table height that drive redox cycling in the sediments. To assess these reactions under aquifer-relevant conditions, we employed reactors in which U-contaminated sediments from Riverton were exposed to the oxidants O2 and NO3- under diffusive flow, which allowed examination of processes controlling oxidative U release from sediments into groundwater. During the course of the 85-day experimental period (similar in temporal extent to redox cycling in fluvial aquifers), very little U was released from sediments into the reactor groundwater despite the continual supply of 250 μM O2 or 1 mM NO3-. Instead, reducing conditions were established within the reactors, as indicated by the production of aqueous sulfide and iron. The establishment of reducing conditions implies that ample organic carbon must have been available to deplete O2 and NO3- and then to stimulate anaerobic respiration within the reactor sediments, thereby stabilizing U(IV). Seasonal redox cycling of U may vary among contaminated sites depending upon organic carbon abundance and hydrologic conditions. We suggest that a major criterion for assessing whether sedimentary U(IV) will be mobile at contaminated sites should be the sediment’s ability to resist changes to its redox potential upon exposure to oxidizing conditions, particularly imparted by its organic matter content and composition.

Oxidative uranium release from anoxic sediments under diffusion-limited conditions

255th National Meeting (2018)

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