Hexavalent Uranium Storage Mechanisms in Wet-Dry Cycled Sediments at Contaminated DOE Sites in the Western United States

The Science

Sediments enriched in organic carbon are known for their remarkable ability to accumulate uranium in its reduced form, U(IV), which is sparingly soluble in groundwater, and to slowly release this uranium when it re-oxidizes to the soluble and mobile form, hexavalent uranium [U(VI)]. These sediment-groundwater interactions are important to DOE because they contribute to prolonged uranium groundwater plumes and render them extremely difficult to remediate at contaminated DOE legacy ore processing sites in Colorado, Wyoming, New Mexico, and the intermountain West. The research team used X-ray absorption and Mössbauer spectroscopy, hydrological and pore water analyses, sediment extractions, and elemental and mineralogical correlations to show that a large fraction of uranium accumulated in organic-enriched sediments at the contaminated Shiprock, New Mexico, site is present as solid-associated hexavalent uranium. U(VI) has not previously been observed to accumulate in shallow sediments in this region. The team proposes a new biogeochemical-hydrological process model for uranium redox cycling in sediments under varying moisture conditions.

The Impact

This study overturns two widely held assumptions about uranium behavior in Western organic-enriched alluvial sediments, namely (1) that uranium accumulates as U(IV) because (2) U(VI) reacts so strongly with groundwater that it is released immediately when U(IV) is oxidized. The project shows that biogeochemical redox cycling coupled to annual water table fluctuations causes hexavalent uranium to accumulate in shallow contaminated sediments. This finding is widely relevant to DOE sites across the western United States; the presence of multiple accumulation mechanisms helps to explain why uranium is so strongly retained in shallow sediments and, by extension, to explain why groundwater plumes in this region are much longer lived than originally expected.

Summary

Uranium is a major groundwater quality problem at contaminated former ore processing and nuclear complex sites across the United States. In the intermountain West, which hosts most of the U.S. legacy ore-processing sites, uranium groundwater plumes are not dissipating through the natural flushing by groundwater as originally expected. At many of these sites, uranium accumulates within organic-enriched, sulfidic sediments as sparingly soluble U(IV). When water tables drop during summer drought, moisture drains away and air enters sediment pore spaces, allowing oxygen to access and oxidize U(IV) and transform it into highly mobile U(VI).  When this happens, organic-enriched sediments release uranium back to groundwater, contributing to  plume longevity. Thus, seasonal water table fluctuations force a cascade of coupled biogeochemical processes that seasonally transform and release uranium, nutrients, and other contaminants to groundwater.

It widely believed that that oxidation of sediment-hosted U(IV) will lead to mobilization of uranium as U(VI). This recent study, however, shows exactly the opposite behavior: that oxidation reactions driven by annual water table fluctuations cause U(VI) to become trapped in sediments. To investigate this issue, Noël et al. (2019) examined the occurrence, distribution, and stability of reduced and oxidized iron, sulfur, and uranium species in shallow sediments at the Shiprock, New Mexico, site affected by annual water table fluctuations. The research used detailed molecular characterization involving X-ray absorption spectroscopy (XAS), Mössbauer spectroscopy and X-ray microspectroscopy. The team found that, during the oxidation stage, sediment-hosted U(IV) is oxidized to sediment-hosted U(VI) faster than dissolved U(VI) can be transported away. Thus, within individual pores, dissolved U(VI) becomes more concentrated in solution over time, helped by low diffusion in fine-grained sediments and evapotranspiration. the researchers posit that U(VI) eventually precipitates in solid phases that are kinetically stable against dissolution. Overall, this study shows that strong wet-dry and biogeochemical redox cycling accumulates both U(IV) and U(VI) in low-permeability sediments. This behavior suggests, somewhat surprisingly, that low-permeability organic-enriched zones could provide long-term storage for U(VI), which has major environmental implications for floodplain water quality. This work corroborates previous observations that reducing conditions are needed to accumulate uranium in sediment solid-phases, but counters the expectation that it predominantly accumulates as U(IV).

Principal Investigator

John Bargar
SLAC National Accelerator Laboratory
[email protected]

Program Manager

Amy Swain
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]

Funding

Funding was provided by the Office of Biological and Environmental Research (BER), within the U.S. Department of Energy (DOE) Office of Science, Subsurface Biogeochemistry Research (SBR) activity to the SLAC SFA program under contract DE-AC02-76SF00515 to SLAC. Use of the Stanford Synchrotron Radiation Laboratory (SSRL) is supported by the Office of Basic Energy Sciences within the DOE Office of Science. A portion of the research was performed using the Environmental Molecular Sciences Laboratory (EMSL), a DOE Office of Science user facility (located at PNNL) sponsored by the BER.  Sample collection at the Rifle, Colorado, site was supported by the Lawrence Berkeley National Laboratory Watershed Function SFA, sponsored by the BER Climate and Environmental Sciences Division. Sample collection at the Naturita and Grand Junction, Colorado, sites was supported by the DOE Office of Legacy Management.

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