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 [1]. 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 investigated uranium, sulfur, and iron biogeochemisty in contaminated floodplains at Grand Junction, Naturita, and Rifle (CO), Riverton (WY), and Shiprock (NM) to develop a perspective on regional drivers of uranium mobility. We are accomplishing this project by combining bulk U, S, and C chemical analyses of the sediments (XRF and TOC measurements) with U, Fe, and S edge X-ray absorption spectroscopy (XAS) to characterize speciation, and chemical analyses of the pore-waters. We find that U is retained at all locations in fine-grained, organic-rich sulfidic sediments, referred to as naturally reduced zones (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. These observations suggest that reduction processes, fuelled by the high organic matter content and facilitated by 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, suggesting that biological reduction of uranium is important. We are combining XAS with bicarbonate extractions and U isotopic signatures (δ238/235U) to better understand the processes of accumulation and distinguish the forms of U determining to evaluate the capacity of the sediments to prevent U mobilization. These results suggest that organic-rich reduced sediments play important roles governing iron and sulfur biogeochemistry as well as uranium mobility regionally within the upper CRB and other locations where organic sediments occur. [1] Hazen T et al. (2011) Complexity of groundwater contaminants at DOE sites. (Lawrence Berkeley National Laboratory), p 58

Regional importance of organic-rich sediments to uranium mobility in the upper Colorado River Basin

251st National Meeting (2016)

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