A regional model for uranium redox and mobility

Subtitle: Redox variable sediments mediate uranium mobility in the upper Colorado River Basin.

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. Detailed molecular characterization using chemical extractions, X-ray absorption spectroscopy (XAS), Mössbauer spectroscopy and X-ray microspectroscopy 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. This work demonstrated 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 several BER SFA research activities.

 

Contacts (BER PM)

Roland Hirsch
DOE Office of Biological and Environmental Research, Climate and Environmental Sciences Division
roland.hirsch@science.doe.gov

(PI Contact)

John Bargar
SLAC National Accelerator Laboratory, Stanford Synchrotron radiation Lightsource
Bargar@slac.stanford.edu

Funding

Funding was provided by the DOE Office of Biological and Environmental Research, Subsurface Biogeochemistry Research (SBR) activity to the SLAC SFA program under contract DE-AC02-76SF00515 to SLAC. Use of SSRL is supported by the U.S. DOE, Office of Basic Energy Sciences. A portion of the research was performed using EMSL, a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research (BER) (located at PNNL).  Sample collection at the Rifle, CO site was supported by the LBNL Watershed Function SFA, sponsored by the BER Climate and Environmental Sciences division.  Sample collection at the Naturita and Grand Junction, CO sites was supported by the DOE Office of Legacy Management.

Publications

Noël, V.; Boye, K.; Dynes, J.; Lezama-Pacheco, J. S.; Bone S.; Janot, N.; Cardarelli, E.; Williams, K. H.; Bargar, J. R. Redox constraints over U(IV) mobility in the floodplains of Upper Colorado River basin. Environmental Science & Technology. 2017b, 51, 10954-10964.

Noël, V.; Boye, K.; Kukkadapu, R. K.; Bone, S.; Lezama-Pacheco, J. S.; Cardarelli, E.; Janot, N.; Fendorf, S.; Williams, K. H.; Bargar J. R. Understanding controls on redox processes in floodplain sediments of the Upper Colorado River Basin. Science of the Total Environment. 2017a, 603-604, 663-675.