Uranium storage mechanisms in wet-dry redox cycled sediments

Biogeochemical redox processes that govern radionuclide mobility in sediments are highly sensitive to forcing by the water cycle. For example, episodic draining and intrusion of oxidants into reduced zones during dry seasons can create biogeochemical seasonal hotspots of enhanced and changed microbial activity, affect the redox status of minerals, initiate changes in sediment gas and water transport, and stimulate the release of organic carbon, iron, and sulfur by oxidation of solid reduced species to aqueous oxic species. In the Upper Colorado River Basin, water-saturation of organic-enriched sediments locally promotes reducing conditions, denoted 'Naturally Reduced Zones' (NRZs), that accumulate strongly U(IV)(sol center dot) Subsequently, fluctuating hydrological conditions introduce oxidants, which may reach internal portions of these sediments and reverse their role to become secondary sources of U-aq. Knowledge of the impact of hydrological variability on the alternating import and export of contaminants, including U, is required to predict contaminant mobility and short- and long-term impacts on water quality. In this study, we tracked U, Fe, and S oxidation states and speciation to characterize the variability in redox processes and related U-sol solubility within shallow fine-grained NRZs at the legacy U ore processing site at Shiprock, NM. Previous studies have reported U speciation and behavior in permanently saturated fine-grained NRZ sediments. This is the first report of U behavior in fine-grained NRZ-like sediments that experience repeated redox cycling due to seasonal fluctuations in moisture content. Our results support previous observations that reducing conditions are needed to accumulate U-sol in sediments, but they counter the expectation that U-sol predominantly accumulates as U(IV)(sol); our data reveal that Us01 may accumulate as U(V1)(sol) in roughly equal proportion to U(IV)sol. Surprisingly high abundances of U(VI)(sol) confined in transiently saturated fine-grained NRZ-like sediments suggest that redox cycling is needed to promote its accumulation. We propose a new process model, where redox oscillations driven by annual water table fluctuations, accompanied by strong evapotranspiration in low permeability sediments, promote conversion of U(IV)(sol) to relatively immobile U(VI)(sol), which suggests that U-sol is accumulating in a form that is resistant to redox perturbations. This observation contradicts the common idea that U(IV)(sol) accumulated in reducing conditions is systematically re-oxidized, solubilized and transported away in groundwater. (C) 2019 Elsevier Ltd. All rights reserved.