Permeable reactive barriers (PRBs) provide a means for passive remediation of ground water for both organic and inorganic contaminants. This USGS-and BLM-funded collaborative project focuses on identifying chemical reaction mechanisms resulting in uranium sequestration at the Fry Canyon, Utah, permeable reactive barrier (PRB) field demonstration project. Questions of central importance are: 1) What reactions result in U sequestrationin field-scale PRB deployments?; 2) What is the long-term stability of the sequestered U with respect to remobilization?; and 3) How do biogeochemical processes occurring within the PRB affect U removal and PRB performance? Initial results indicate that U is sequestered dominantly as U(IV) by the zero valent iron (ZVI) PRB and that secondary Fe(II) minerals have filled ZVI grain pores and cemented grains, which reduce porosity and reactivity to U limiting PRB longevity and performance.

This research is providing the scientific basis to evaluate and the long-term behavior of sequestered uranium and to optimize barrier design and operation.

Collaborator: C. Fuller

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A groundwater plume containing uranium from purposeful discharges of wastewater to cribs, trenches, and ponds, along with some accidental leaks and spills associated with nuclear fuel fabrication activities has persisted beneath the Hanford Site 300 Area for many years. The uranium plume is just upstream of the city of Richland municipal water supply intake on the Columbia River. Despite the cessation of uranium releases and the removal of shallow vadose zone source materials, the concentration of the uranium plume has not decreased as predicted.

Phosphate-based remediation strategies may provide a means for remediating this large and tenacious vadose-zone subsurface uranium contamination problem. The basis of this approach is the relatively low solubility of U(VI)-phosphate phases such as autunite, Ca(UO₂)₂(PO₄)₂x(H₂O), in comparison to other forms of U(VI). In order for this approach to be effective, however, existing U(VI)-bearing solids must dissolve/reprecipitate or transform into U(VI)-phosphates. The extent to which such reactions may occur, as well as their rates, are unknown at present. The collaboration with the SLAC SFA is helping to address critical knowledge needs required to evaluate this remediation technology, including: 1) quantifying the rates of U(VI) immobilization via formation of uranium-phosphate phases in the presence of various micro-environments, 2) establishing the identity of the uranium-phosphate phase(s) formed and therein, the long-term stability of uranium, and 3) evaluating the optimum infiltration rate for polyphosphate stabilization. We are investigating the mechanisms of uranyl-phosphate formation/ transformations, the roles of geochemical micro-environments, and the roles of wet-dry cycling on the formation, identity, and stability of uranium-phosphate phases from laboratory- and field-phosphate amended sediments.

This work is advancing validation and development of this important remediation technology.

Collaborators: D. Wellman and E. Pierce

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Plutonium is a toxic and long-lived radionuclide that occurs as a contaminant of concern at several DOE sites, including Hanford, Los Alamos National Laboratory, Idaho National Laboratory, Oak Ridge National Laboratory, and the Nevada test site, and is of potential concern in the long-term performance of nuclear waste repositories. The need to understand the biogeochemical factors controlling Pu mobility in the environment is driven not only by regulatory requirements, but also by the powerful negative public perception of Pu as a severely acute health concern. Current plans to mitigate shallow Pu-tainted soils generally involve excavation and removal of the most contaminated surface layer as has been performed at the Rocky Flats Environmental Technology Site. This approach explicitly leaves a Pu inventory in the subsurface (i.e., below excavation level), to be treated by monitored natural attenuation. A significant management challenge is created by this strategy, namely, that the fate of Pu must be known without untenable uncertainty, which requires knowledge of the biogeochemical processes operating at the molecular through meter scales.

This component of the SSRL-SFA will support the characterizations of the fundamental biogeochemical processes that control the fate and transport of Pu in the environment. These collaborative efforts will address the following major research questions:

• What is the molecular-scale structure of nanoparticulate biogenic PuO₂ and Pu nano clusters formed under environmentally relevant conditions? To what extent does PuO₂ acquire structural impurities from ground water? How do particle size and structural impurities moderate its stability?
• What is the fate of Pu during microbially-driven dissimilatory Fe reduction and reoxidation? What are the products following Fe reduction and following subsequent reoxidation? Can presorbed Pu or PuO₂ be reduced or mobilized during these processes? Is Pu reduced to Pu(III) and incorporated into Fe(II,III) (hydr-)oxides?
• What are the molecular structures of Pu surface complexes on natural Fe- and Mn-oxides?
• What are the molecular structures of Pu-associated subsurface sediment from DOE contaminated sites?
• How does Pu partition between organic complexing ligands, mineral surfaces, and PuO2 in aerobic biofilm-mineral assemblages? What are the dominant oxidation states?

These investigations will significantly expand our understanding of the critical biogeochemical processes that impact long-term monitoring, remediation, and stewardship of sites with subsurface plutonium contamination.

Collaborator: H. Boukhalfa

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Uranium released from waste at the Yucca Mountain repository will eventually come into contact with alluvium of the surrounding host rock. Sorption onto this geomedia will be one of the most important mechanisms that retards U(VI) migration. The mechanisms of these processes are therefore of significance to repository performance and risk assessment. Previous work has shown that U sorbed onto alluvium from southern Nevada exhibits a wide range of desorption rates, which suggests a range of sorption site affinities and mechanisms on these chemically and physically heterogeneous sediments. Significantly, even after a few thousand hours of desorption under continuous flow conditions, which maximizes the driving force for desorption, a substantial fraction (ca 10-40%) of the uranium remains. Sorption parameters used to predict subsurface U(VI) transport are typically derived from batch sorption experiments that do not effectively interrogate these slow desorption processes. This failure leads to significant underestimation of the retardation capacity of the alluvium, and in turn, to overly conservative and excessively costly waste isolation and remediation strategies. The lack of a mechanistic molecular-scale understanding of uranium adsorption mechanisms, and in particular those responsible for slow desorption processes, is therefore an important barrier to the development of quantitative and defensible transport models. Addressing these issues is a principal objective of an ongoing Los Alamos National Laboratory (LANL) project funded by the DOE OCRWM Office of Science, Technology and International (OSTI) Program. Although this project is focused on nuclear waste isolation at the proposed Yucca Mountain repository site, it clearly has implications for remediation strategies at many uranium-contaminated DOE sites or mining sites. The SSRL environmental remediation science SFA program will assist with the characterization of U(VI) adsorption processes in sediments from Yucca Mountain.

This work will contribute to the development of defensible quantitative models for sorption processes in Yucca Mountain alluvium and, consequently, to overall cost savings for waste isolation and remediation.

Collaborator: P. Reimus

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SR-based techniques

SR-based techniques provide powerful tools for assisting in the characterization and remediation of contaminated DOE sites and in assessing risk from future contaminant migration at nuclear waste repositories. However, the need to travel to a SR source and the technical complexity of data acquisitionand analysis methods present significant barriers to utilization of SR capabilities in fundamental and applied field-scale research. SLAC SFA staff will bring SR capabilities and staff expertise to bear on subsurface contamination problems at a number ofcontaminated field sites around the U.S. through collaborations with researchers at LANL, PNNL, and the US Geological Survey.