The Science

Two new studies shed light on an important and previously underappreciated biogeochemical redox ‘engine’ believed to mediate groundwater quality in floodplains within the upper Colorado River Basin (CRB). Sediments enriched in organic carbon were found to be common within saturated zones and capillary fringes, to be highly redox active, and to strongly accumulate sulfide and uranium. The research showed that uranium was present as U(IV) complexed to organic matter and likely to mineral surfaces. The stability and predominance of these complexes is controlled by the abundance of organic and mineral surface functional groups, and the intensity of oxidative cycling.

The Impact

Complexation of U(IV) by sediment organic matter drives accumulation of uranium. However, redox cycling provides a mechanism by which U(IV), nutrients, and other contaminants can be seasonally transformed and released to groundwater. These new findings provide biogeochemical processes models needed to predict the behavior of redox-active species in floodplains in the upper CRB.

Summary

Uranium contamination stubbornly persists as a challenging and costly water quality concern at former uranium ore processing sites across the upper CRB. Plumes at these sites are not self-attenuating via natural flushing by groundwater as originally expected. Recent studies at the Rifle, CO legacy site suggest that organic-enriched anoxic sediments locally create conditions that promote reduction of U(VI) to relatively immobile U(IV), causing it to accumulate. Organic-enriched sediments at Rifle accumulate uranium under persistently saturated and anoxic conditions. However, incursion of oxidants into reduced sediments, if it were to occur, could transform contaminants, allowing organic-enriched sediments to act as secondary sources of uranium. Oxidant incursions do take place during periods of changing water tables, which occur throughout the year in the upper CRB. If organic-enriched sediments were regionally common in the upper CRB, and if they were exposed to varying redox conditions, then they could help to maintain the longevity of U plumes regionally. Cyclic redox variability would also have major implications for mobility of carbon, nitrogen, and metal contaminants in groundwater and surface waters.

To investigate these issues, Noël et al. (2017a,b) examined the occurrence and distribution of reduced and oxidized iron, sulfur, and uranium species in sediment cores spanning dry/oxic to wet/reduced conditions at three different sites across the upper CRB. The research used detailed molecular characterization involving chemical extractions, X-ray absorption spectroscopy (XAS), Mössbauer spectroscopy, and X-ray microspectroscopy. This work demonstrates that anoxic organic-enriched sediments occur at all sites, strongly accumulate sulfides and uranium, and are exposed to strong seasonal redox cycles. Uranium was found to be present as U(IV) complexed to sediment-associated organic carbon and possibly to mineral surfaces. This finding is significant because complexed U(IV) is relatively susceptible to oxidative mobilization. Sediment particle size, organic carbon content, and pore saturation control redox conditions in sediments and thus strongly influence the biogeochemistry of iron, sulfur, and uranium. These findings help to illuminate the mechanistic linkages between hydrology, sediment texture, and biogeochemistry. They further provide enhanced contextual and conceptual underpinnings to support reactive transport modeling of uranium, other contaminants, and nutrients in redox variable floodplains, a subject of importance to Biological and Environmental Research (BER) research missions.

Principal Investigator

John Bargar
SLAC National Accelerator Laboratory
bargar@slac.stanford.edu

Program Manager

Paul Bayer
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
paul.bayer@science.doe.gov

Funding

Funding was provided by the Subsurface Biogeochemistry Research (SBR) program of the Office of Biological and Environmental Research (BER), within the U.S. Department of Energy (DOE) Office of Science, to the SLAC Scientific Focus Area (SFA) program under contract DE-AC02-76SF00515. Use of the Stanford Synchrotron Radiation Lightsource (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; located at Pacific Northwest National Laboratory (PNNL)], a DOE Office of Science user facility sponsored by 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, Colo., sites was supported by the DOE Office of Legacy Management.

References