title: Physico-Chemical Heterogeneity of Organic-Rich Sediments in the Rifle Aquifer, CO: Impact on Uranium Biogeochemistry.
abstract: The Rifle alluvial aquifer along the Colorado River in west central Colorado contains fine-grained, diffusion-limited sediment lenses that are substantially enriched in organic carbon and sulfides, as well as uranium, from previous milling operations. These naturally reduced zones (NRZs) coincide spatially with a persistent uranium groundwater plume. There is concern that uranium release from NRZs is contributing to plume persistence or will do so in the future. To better define the physical extent, heterogeneity and biogeochemistry of these NRZs, we investigated sediment cores from five neighboring wells. The main NRZ body exhibited uranium concentrations up to 100 mg/kg U as U(IV) and contains ca. 286 g of U in total. Uranium accumulated only in areas where organic carbon and reduced sulfur (as iron sulfides) were present, emphasizing the importance of sulfate-reducing conditions to uranium retention and the essential role of organic matter. NRZs further exhibited centimeter-scale variations in both redox status and particle size. Mackinawite, greigite, pyrite and sulfate coexist in the sediments, indicating that dynamic redox cycling occurs within NRZs and that their internal portions can be seasonally oxidized. We show that oxidative U(VI) release to the aquifer has the potential to sustain a groundwater contaminant plume for centuries. NRZs, known to exist in other uranium-contaminated aquifers, may be regionally important to uranium persistence.
authors.affiliation: Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States.
authors.forename: Noémie
authors.initials: N
authors.lastname: Janot
authors.affiliation: Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States.|Department of Environmental Earth System Science, Stanford University , Stanford, California 94305, United States.
authors.forename: Juan S
authors.initials: JS
authors.lastname: Lezama Pacheco
authors.affiliation: Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States.
authors.forename: Don Q
authors.initials: DQ
authors.lastname: Pham
authors.affiliation: Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States.
authors.forename: Timothy M
authors.initials: TM
authors.lastname: O'Brien
authors.affiliation: Department of Environmental Earth System Science, Stanford University , Stanford, California 94305, United States.
authors.forename: Debra
authors.initials: D
authors.lastname: Hausladen
authors.affiliation: Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States.
authors.forename: Vincent
authors.initials: V
authors.lastname: Noël
authors.affiliation: Université de Lorraine, Vandœuvre-lès-Nancy, F-54501, France.
authors.forename: Florent
authors.initials: F
authors.lastname: Lallier
authors.affiliation: Department of Geological Sciences, Stanford University , Stanford, California 94305, United States.
authors.forename: Kate
authors.initials: K
authors.lastname: Maher
authors.affiliation: Department of Environmental Earth System Science, Stanford University , Stanford, California 94305, United States.
authors.forename: Scott
authors.initials: S
authors.lastname: Fendorf
authors.affiliation: Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United State s.
authors.forename: Kenneth H
authors.initials: KH
authors.lastname: Williams
authors.affiliation: Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United State s.
authors.forename: Philip E
authors.initials: PE
authors.lastname: Long
authors.affiliation: Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States.
authors.forename: John R
authors.initials: JR
authors.lastname: Bargar
mesh_terms: D002244:Carbon / Q000032:analysis; D003116:Color; D003120:Colorado; D019015:Geologic Sediments / Q000737:chemistry*; D060587:Groundwater / Q000737:chemistry*; D009930:Organic Chemicals / Q000032:analysis*; D010084:Oxidation-Reduction; D010316:Particle Size; D013455:Sulfur / Q000032:analysis; D014501:Uranium / Q000032:analysis / Q000737:chemistry*; D014875:Water Pollutants, Radioactive / Q000032:analysis*; D056928:X-Ray Absorption Spectroscopy
chemical_list: D009930:Organic Chemicals; D014875:Water Pollutants, Radioactive; D014501:Uranium; D013455:Sulfur; D002244:Carbon