title: Isotopic Evidence for Reductive Immobilization of Uranium Across a Roll-Front Mineral Deposit.
abstract: We use uranium (U) isotope ratios to detect and quantify the extent of natural U reduction in groundwater across a roll front redox gradient. Our study was conducted at the Smith Ranch-Highland in situ recovery (ISR) U mine in eastern Wyoming, USA, where economic U deposits occur in the Paleocene Fort Union formation. To evaluate the fate of aqueous U in and adjacent to the ore body, we investigated the chemical composition and isotope ratios of groundwater samples from the roll-front type ore body and surrounding monitoring wells of a previously mined area. The (238)U/(235)U of groundwater varies by approximately 3‰ and is correlated with U concentrations. Fluid samples down-gradient of the ore zone are the most depleted in (238)U and have the lowest U concentrations. Activity ratios of (234)U/(238)U are ∼5.5 up-gradient of the ore zone, ∼1.0 in the ore zone, and between 2.3 and 3.7 in the down-gradient monitoring wells. High-precision measurements of (234)U/(238)U and (238)U/(235)U allow for development of a conceptual model that evaluates both the migration of U from the ore body and the extent of natural attenuation due to reduction. We find that the premining migration of U down-gradient of the delineated ore body is minimal along eight transects due to reduction in or adjacent to the ore body, whereas two other transects show little or no sign of reduction in the down-gradient region. These results suggest that characterization of U isotopic ratios at the mine planning stage, in conjunction with routine geochemical analyses, can be used to identify where more or less postmining remediation will be necessary.
authors.affiliation: Department of Earth and Planetary Science University of California , Berkeley, California 94709, United States.|Energy Geosciences Division, E.O. Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.
authors.forename: Shaun T
authors.initials: ST
authors.lastname: Brown
authors.affiliation: Department of Earth and Planetary Science University of California , Berkeley, California 94709, United States.|Energy Geosciences Division, E.O. Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.
authors.forename: Anirban
authors.initials: A
authors.lastname: Basu
authors.affiliation: Energy Geosciences Division, E.O. Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.
authors.forename: John N
authors.initials: JN
authors.lastname: Christensen
authors.affiliation: Earth and Environmental Sciences Division, Los Alamos National Laboratory , Los Alamos, New Mexico 87454, United States.
authors.forename: Paul
authors.initials: P
authors.lastname: Reimus
authors.affiliation: Earth and Environmental Sciences Division, Los Alamos National Laboratory , Los Alamos, New Mexico 87454, United States.
authors.forename: Jeffrey
authors.initials: J
authors.lastname: Heikoop
authors.affiliation: Earth and Environmental Sciences Division, Los Alamos National Laboratory , Los Alamos, New Mexico 87454, United States.
authors.forename: Ardyth
authors.initials: A
authors.lastname: Simmons
authors.affiliation: Earth and Environmental Sciences Division, Los Alamos National Laboratory , Los Alamos, New Mexico 87454, United States.
authors.forename: Giday
authors.initials: G
authors.lastname: Woldegabriel
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 Geological Sciences, Stanford University , Stanford, California 94305, United States.
authors.forename: Karrie
authors.initials: K
authors.lastname: Weaver
authors.affiliation: Power Resources Inc. , Smith Ranch-Highland Operation, 762 Ross Road, Douglas, Wyoming 82633, United States.
authors.forename: James
authors.initials: J
authors.lastname: Clay
authors.affiliation: Department of Earth and Planetary Science University of California , Berkeley, California 94709, United States.|Energy Geosciences Division, E.O. Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.
authors.forename: Donald J
authors.initials: DJ
authors.lastname: DePaolo
mesh_terms: D060587:Groundwater / Q000737:chemistry; D008903:Minerals; D008906:Mining; D014501:Uranium; D014875:Water Pollutants, Radioactive
chemical_list: D008903:Minerals; D014875:Water Pollutants, Radioactive; D014501:Uranium