Uranium Accumulated in Anoxic Sediments Threatens Groundwater Quality at Contaminated Department of Energy Sites
Anoxic, organic-rich sediments in the subsurface retain enough uranium to sustain a groundwater plume for centuries.
Sediment cores sampled at “high resolution” for the first time (~10-cm depth intervals) from wells on a uranium contaminated floodplain near Rifle, Colorado, revealed that uranium has accumulated exclusively within organic-enriched sulfidic sediments. Molecular investigations of uranium and sulfur at this Department of Energy site indicated that uranium was present in a noncrystalline reduced (tetravalent) form and that even the interior parts of these sediment bodies are oxidized on an annual basis.
Release of uranium from anoxic, organic-enriched sediment bodies, defined through these detailed, centimeter-scaled investigations, could sustain a contaminant groundwater plume for centuries. Similar types of sulfidic, organic-enriched sediment bodies exist in other uranium-contaminated aquifers in the upper Colorado River Basin, meaning that these findings could offer regionally important explanations to uranium behavior. These new results highlight the need for better understanding of the vulnerability of anoxic, organic-rich sediments in this region to climate perturbations.
Uranium mobility is regulated by its chemical state; the reduced form, U(IV), is much less soluble than the oxidized U(VI). Consequently, oxidation of anoxic sediments could allow uranium to enter the aquifer at the Rifle site with a long-term impact on groundwater quality. The co-occurrence of uranium, sulfur, and organic carbon in the Rifle subsurface suggests that sulfate reduction coupled to microbial carbon oxidation is an important regulator of uranium retention in this floodplain. Sulfur was only found to accumulate in groundwater-saturated, fine-grained materials with an elevated organic carbon content, supporting the conclusion that reducing conditions, induced by the low permeability and microbial oxygen consumption, promote sulfide formation and uranium retention. The co-existence of multiple sulfur species (sulfate, elemental sulfur, mackinawite, greigite, and pyrite) throughout the reduced zone, suggests redox cycling of these materials, implying oxidative release of uranium occurs. Uranium was found to be associated with both organic carbon and sulfur, respectively. Therefore, the study concluded that uranium reduction and retention in these sediments resulted from abiotic reduction by iron sulfides, potentially enhanced by organic matter shuttling electrons, as well as via biotic reduction through respiratory and enzymatic activity coupled to organic matter decomposition.
SLAC National Accelerator Laboratory
This work was supported as part of the SLAC Scientific Focus Area (SFA), which is funded by the Subsurface Biogeochemical Research (SBR) program of the Office of Biological and Environmental Research, within the U.S. Department of Energy Office of Science, under subcontract DE-AC02-76SF00515. Logistical support was provided by the Rifle field research program of the Lawrence Berkeley National Laboratory, through SBR funding to the Sustainable Systems SFA under contract DE-AC02-05CH11231. Portions of the work were performed at the Stanford Synchrotron Radiation Lightsource at the SLAC National Accelerator Laboratory.
Janot, N., et al. “Physico-chemical heterogeneity of organic-rich sediments in the Rifle Aquifer, CO: Impact on uranium biogeochemistry.” Environmental Science & Technology 50(1), 46–53 (2016). [DOI:10.1021/acs.est.5b03208].