A Regional Model for Uranium Redox and Mobility

Anoxic organic-enriched sediments strongly accumulate uranium as U(IV), sulfide, and other reduced species. Seasonal reduction-oxidation (redox) cycling triggered by changing water tables can intermittently mobilize these species. [Courtesy Vincent Noël and John Bargar, Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory]
Uranium (U) contamination stubbornly persists as a challenging and costly water quality concern at former uranium ore processing sites across the Upper Colorado River Basin (UCRB). Plumes at these sites are not self-attenuating via natural flushing by groundwater as originally expected. Recent studies at the Rifle, Colo., legacy site suggest that organic-enriched anoxic sediments create conditions that promote reduction of U(VI) to relatively immobile U(IV), causing it to accumulate locally under persistently saturated and anoxic conditions. However, incursion of oxidants into reduced sediments could transform contaminants, allowing these sediments to act as secondary sources of uranium. Oxidant incursions take place during periods of changing water tables, which occur in UCRB throughout the year. If these sediments were regionally common in the UCRB and exposed to varying reduction-oxidation (redox) conditions, then they could contribute to maintaining the longevity of regional uranium plumes.

To investigate these issues, researchers examined the occurrence and distribution of reduced and oxidized iron (Fe), sulfur (S), and U species in sediment cores spanning dry and oxic to wet and reduced conditions at three different UCRB sites. Detailed molecular characterization involved 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 U, 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 Fe, S, and U biogeochemistry. 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 BER research missions. Cyclic redox variability has major implications for mobility of carbon (C), nitrogen (N), and metal contaminants in groundwater and surface waters. Redox-variable, organic-enriched sediments mediate the mobility of C, N, Fe, S, U, and metal contaminants regionally in the UCRB. Organic-enriched sediments were established to regionally mediate groundwater quality within the UCRB.

Noël, V., et al. “Understanding controls on redox processes in floodplain sediments of the Upper Colorado River Basin.” Sci. Total Environ. 603–604, 663–675 (2017). DOI:10.1016/j.scitotenv.2017.01.109.

Noël, V., et al. “Redox constraints over U(IV) mobility in the floodplains of Upper Colorado River Basin.” Environ. Sci. Technol. 51(19), 10954–10964 (2017). DOI:10.1021/acs.est.7b02203.

Instruments and Facilities Used: X-ray absorption spectroscopy and X-ray microprobe mapping at Stanford Synchrotron Radiation Lightsource (SSRL), SLAC National Accelerator Laboratory (SLAC), and Mössbauer spectroscopy at Environmental Molecular Sciences Laboratory (EMSL), Pacific Northwest National Laboratory (PNNL).