H31I-1544
Combining U speciation and U isotope fractionation to evaluate the importance of naturally reduced sediments in controlling the mobility of uranium in the upper Colorado River Basin

Wednesday, 16 December 2015
Poster Hall (Moscone South)
Vincent Noel1, Pierre Lefebvre2, Kristin Boye3, John Bargar4, Katharine Maher5, Juan Lezama-Pacheco4, Emily Cardarelli5, Sharon Bone1, William L Dam6 and Raymond H Johnson7, (1)SLAC National Accelerator Laboratory, Menlo Park, CA, United States, (2)Ecole Normale Supérieure Paris, Paris, France, (3)Stanford Earth Sciences, Stanford, CA, United States, (4)Stanford University, Los Altos Hills, CA, United States, (5)Stanford University, Stanford, CA, United States, (6)Department of Energy Grand Junction, Grand Junction, CO, United States, (7)Organization Not Listed, Washington, DC, United States
Abstract:
Long-term persistence of uranium (U) in groundwater at legacy ore-processing sites in the upper Colorado River Basin (CRB) is a major concern for DOE, stakeholders, and local property owners. In the past year, we have investigated U distributions in contaminated floodplains at Grand Junction, Naturita, and Rifle (CO), Riverton (WY), and Shiprock (NM). We find that U is retained at all locations in fine-grained, organic-rich sulfidic sediments, referred to as naturally reduced zones (NRZs). The retention mechanisms (e.g., complexation, precipitation or adsorption) and the processes responsible for U accumulation in NRZs will directly determine the capacity of the sediments to prevent U mobilization. However, these processes remain poorly understood at local and regional scales yet they are critical to management and remediation of these sites.

We have used U LIII/II-edge XANES to systematically characterize U oxidation states, and EXAFS and bicarbonate extractions to characterize U local structure and reactivity in order to distinguish the forms of U. We are measuring U isotopic signatures (δ238/235U) to better understand uranium sources and processes of accumulation in NRZs. We have found that high U concentrations correspond to reduced and relatively insoluble U forms, mainly non-crystalline U(IV), and co-occur with ferrous iron and sulfides. This suggests that reduction processes, fueled by the high organic matter content and constrained to the diffusion-limited environment in the fine-grained NRZs, are important for the retention of U in these sediments. We also observe a strong correlation between the U concentrations in the NRZs and the extent of isotopic fractionation, with up to +1.8 ‰ difference between uranium-enriched and low concentration zones. In some locations the δ238/235U values are within the range of values typical of the mine tailings, whereas at other sites the more positive δ238/235U values suggest that redox cycling and/or partial reduction of uranium are occurring. These results suggest that organic-rich reduced sediments play important roles governing uranium mobility in contaminated floodplains regionally within the upper CRB and other locations where organic sediments occur