Shallow aquifers and shales contain elevated organic carbon and reduced iron and uranium species, both of which profoundly affect the behavior of uranium in these environments. Our group has investigated mechanisms of uranium mobilization in both systems in order to evaluate and mitigate risk associated with release of this toxic element to the environment. This work has shown that organic-enriched sediments are common and at contaminated sites across the upper Colorado Basin, the regional locus of our field studies. Uranium(VI) diffuses into these sediments, is reduced to uranium(IV), and complexed to organic functional groups, facilitating accumulation to hundreds of mg/kg concentrations. This chemical form of uranium is sensitive to oxidation under late-summer dry-season conditions. Evapotranspiration facilitates upward transport of uranium under these conditions, effectively coupling the unsaturated and saturated zones. The diversity of mechanisms by which uranium can accumulate and be mobilized helps to explain why uranium plumes are so persistent in this region. Organic matter and redox processes also mediate attenuation and release of uranium in shale reservoirs. Hydraulic fracturing fluids contain oxygen and acid, which promote solubilization and oxidation of iron and uranium. Natural organic matter dramatically accelerates Fe(II) oxidation, particularly under acidic conditions. Large amounts of oxidized uranium can be released, but this pool is attenuated by uptake on Fe(III)(oxy)-hydroxides. Reaction rates are strongly impacted by pH, and therefore the acid-buffering capacity of the matrix (i.e., carbonate content) is a master variable. These examples emphasize the importance of organic matter and redox to uranium behavior in natural systems.
About the Speaker: John Bargar, SLAC National Accelerator Laboratory,
John Bargar was born in Delaware, Ohio and has a BSc in Geology from the Ohio State University and a PhD in Geological and Environmental Sciences from Stanford University. He was a National Research Council Postdoctoral Fellow at the US Geological Survey Menlo Park, and subsequently joined the Stanford Synchrotron Radiation Lightsource. He is presently a senior staff scientist and leads projects investigating biogeochemical-hydrological controls over groundwater quality in alluvial systems, fluid-shale geochemistry in unconventional reservoirs, and forensic signatures for uranium and plutonium. John has co-authored > 150 articles in these and related topical areas. He oversaw the commissioning and user science program of the SSRL environmental and radionuclide science beam line (11-2) and the design, implementation, and commissioning of the SSRL hard X-ray microprobe system at beam line 2-3. John’s research interests include structure-reactivity relationships of natural biogenic nano-minerals and their interfacial chemistry in low temperature aqueous fluids. His group also has extensively investigated biogeochemical-hydrological controls over uranium behavior in contaminated alluvial systems (molecular to regional scale) using bench, spectroscopic, and field-based observational approaches. He has used similar approaches to investigate fluid-shale interactions in unconventional reservoirs.
Host: Hang Deng