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PI-Submitted Research Highlights for
Subsurface Biogeochemical Research Program

Uranium Accumulated in Anoxic Sediments Threatens Groundwater Quality at Contaminated DOE sites

John Bargar
SSRL, SLAC National Accelerator Laboratory

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Anoxic, organic rich sediments in the subsurface retains enough uranium to sustain groundwater plume for centuries

The Science                       
Sediment cores sampled at “high resolution” for the first time (~10 cm depth intervals) from wells on the uranium contaminated floodplain at the Rifle, CO, DOE site revealed that uranium has accumulated exclusively within organic-enriched sulfidic sediments. Molecular investigations of uranium and sulfur indicated that uranium was present in a non-crystalline reduced (tetravalent) form and that even the interior parts of these sediment bodies are oxidized on an annual basis.

The Impact
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 the present results could offer regionally important explanations to uranium behavior. These new results highlight the need to better understand the vulnerability of anoxic organic-rich sediments in this region to climate perturbations.

Summary
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 C 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, which implies oxidative release of uranium occurs. Uranium was found to be associated with both organic C and sulfur, respectively. Therefore, it was 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.

Contacts (BER PM)
Roland F. Hirsch, SC-23.2, roland.hirsch@science.doe.gov, (301) 903-9009

(PI Contact)
John Bargar
SSRL, SLAC National Accelerator Laboratory
bargar@slac.stanford.edu

Funding
This work was supported as part of the SLAC Scientific Focus Area (SFA), which is funded by the U.S. Department of Energy, Office of Biological and Environmental Research, Subsurface Biogeochemical Research (DOE-BER-SBR) program under subcontract DE-AC02-76SF00515. Logistical support was provided by the Rifle field research program of the Lawrence Berkeley National Laboratory, through funding from DOE-BER-SBR 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.

Publications
Janot, N. et al. (2016). Physico-chemical heterogeneity of organic-rich sediments in the Rifle aquifer, CO: Impact on uranium biogeochemistry. Environmental Science & Technology 50(1): 46-53. DOI: 10.1021/acs.est.5b03208

Citation:

Janot, N. et al. (2016). Physico-chemical heterogeneity of organic-rich sediments in the Rifle aquifer, CO: Impact on uranium biogeochemistry. Environmental Science & Technology 50(1): 46-53. DOI: 10.1021/acs.est.5b03208

Article (PDF):
Janot_EST_Rifle_NRZ_Biogeochemistry_2015_published.pdf

Additional information

Performer is an SFA, funded by DOE-BER, Climate and Environmental Sciences Division (CESD), Subsurface Biogeochemical Research (SBR) Activity.  Support for the Rifle, CO research site and K. Williams was provided by the LBNL Sustainable Systems SFA, which is also funded by DOE-BER-CESD-SBR. The Stanford Synchrotron Radiation Lightsource user facility at the SLAC National Accelerator Laboratory (SLC) was used to acquire X-ray spectroscopy and microprobe data.


Organic rich anoxic sediments below the groundwater table accumulate sulfur and uranium in their reduced forms (sulfide and U(IV) ). If these sediments are exposed to oxidants under drought conditions, which occur annually in fall/winter and increasingly frequent multi-year cycles, then sulfur and uranium can be oxidized and released back to groundwater. If the uranium flux from the NRZ is higher than an estimated 13 mg/m2/yr, then a plume extending to the Colorado River could be sustained for hundreds of years.

 

Image courtesy of SLAC National Laboratory, based on a modification of Figure 5 in Janot et al. (2016) ES&T.

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