Ardyth M. Simmons

Isotopic and Geochemical Tracers for U(VI) Reduction and U Mobility at an in Situ Recovery U Mine

In situ recovery (ISR) uranium (U) mining mobilizes U in its oxidized hexavalent form (U(VI)) by oxidative dissolution of U from the roll-front U deposits. Postmining natural attenuation of residual U(VI) at ISR mines is a potential remediation strategy. Detection and monitoring of naturally occurring reducing subsurface environments are important for successful implementation of this remediation scheme. We used the isotopic tracers (238)U/(235)U (δ(238)U), (234)U/(238)U activity ratio, and (34)S/(32)S (δ(34)S), and geochemical measurements of U ore and groundwater collected from 32 wells located within, upgradient, and downgradient of a roll-front U deposit to detect U(VI) reduction and U mobility at an ISR mining site at Rosita, TX, USA. The δ(238)U in Rosita groundwater varies from +0.61‰ to -2.49‰, with a trend toward lower δ(238)U in downgradient wells. The concurrent decrease in U(VI) concentration and δ(238)U with an ε of 0.48‰ ± 0.08‰ is indicative of naturally occurring reducing environments conducive to U(VI) reduction. Additionally, characteristic (234)U/(238)U activity ratio and δ(34)S values may also be used to trace the mobility of the ore zone groundwater after mining has ended. These results support the use of U isotope-based detection of natural attenuation of U(VI) at Rosita and other similar ISR mining sites.

Simulating infiltration tests in fractured basalt at the Box Canyon Site, Idaho

Simulating Infiltration Tests in Fractured Basalt at the Box Canyon Site, Idaho. Andre J.A. Unger *,a , Boris Faybishenko a , Gudmundur S. Bodvarsson a , and Ardyth M. Simmons b a Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA b Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico, USA 87545 Corresponding author (AJAUnger@lbl.gov) Phone: (510) 495-2823 Fax: (510) 486-5686

Isotopic Evidence for Reductive Immobilization of Uranium Across a Roll-Front Mineral Deposit

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.

Stratigraphy of the PB-1 well, nopal I Uranium deposit, Sierra Peña Blanca, Chihuahua, Mexico

The Nopal I site in the Peña Blanca uranium district has a number of geologic and hydrologic similarities to the proposed high-level radioactive waste repository at Yucca Mountain, making it a useful analogue to evaluate process models for radionuclide transport. The PB-1 well was drilled in 2003 at the Nopal I uranium deposit as part of a DOE-sponsored natural analogue study to constrain processes affecting radionuclide transport. The well penetrates through the Tertiary volcanic section down to Cretaceous limestone and intersects the regional aquifer system. The well, drilled along the margin of the Nopal I ore body, was continuously cored to a depth of 250 m, thus providing an opportunity to document the local stratigraphy. Detailed observations of these units were afforded through petrographic description and rockproperty measurements of the core, together with geophysical logs of the well. The uppermost unit encountered in the PB-1 well is the Nopal Formation, a densely welded, crystal-rich, rhyolitic ashflow tuff. This cored section is highly altered and devitrified, with kaolinite, quartz, chlorite, and montmorillonite replacing feldspars and much of the groundmass. Breccia zones within the tuff contain fracture fillings of hematite, limonite, goethite, jarosite, and opal. A zone of intense clay alteration, encountered in the depth interval 17.45-22.30 m, was interpreted to represent the basal vitrophyre of this unit. Underlying the Nopal Formation is the Coloradas Formation, which consists of a welded lithic-rich rhyolitic ash-flow tuff. The cored section of this unit has undergone devitrification and oxidation, and has a similar alteration mineralogy to that observed in the Nopal tuff. A sharp contact between the Coloradas tuff and the underlying Pozos Formation was observed at a depth of 136.38 m. The Pozos Formation consists of poorly sorted conglomerate containing clasts of subangular to subrounded fragments of volcanic rocks, limestone, and chert. Three thin (2-6 m) intervals of intercalated pumiceous tuffs are present within this unit. The contact between the Pozos Formation and the underlying Cretaceous limestone basement was encountered at a depth of 244.40 m. The water table is located at a depth of ~223 m. Several zones with elevated radioactivity in the PB-1 core occur above the current water table. These zones may be associated with changes in redox conditions that could have resulted in the precipitation of uraninite from downward-flowing waters transporting U from the overlying Nopal deposit. All of the intersected units have low (typically submillidarcy) matrix permeability, thus fluid flow in this area is dominated by fracture flow. These stratigraphic and rock-property observations can be used to constrain flow and transport models for the Peña Blanca natural analogue.

Geochronology and Fluid-Rock Interaction Associated with the Nopal I Uranium Deposit, Pena Blanca, Mexico

The objectives of this report are: (1) Establish chronology of uranium minerals; (2) Characterize fluids; and (3) Relate ages to geological tectonic events.

Simulating infiltration in unsaturated basalt for the Large-Scale Aquifer Pumping and Infiltration Test at INEEL

The Large-Scale Aquifer Pumping and Infiltration Test (LPIT) conducted at the Idaho National Engineering and Environmental Laboratory was modeled using TOUGH2 to simulate the highly transient water infiltration and perched-water conditions in the fractured basalt and sedimentary interbeds existing at the site. The fracture and matrix continua of the basalt were represented using a dual-permeability approach. Six perched-water hydrographs, measured during the infiltration test, were used for calibration with iTOUGH2 to estimate six parameters that controlled unsaturated flow in the fractured basalt and the dense clay sedimentary interbed underlying the basalt flow. These parameters included the interfacial area between the basalt fracture and matrix continua, the basalt fracture continuum permeability, the basalt matrix continuum permeability, the interbed matrix continuum permeability, and the interbed van Genuchten capillary pressure parameters. The intent of the calibration was to obtain large-scale properties of the lithological units affecting the field-scale LPIT test. Finally, we tested the applicability of the dual-permeability conceptual model for representing transient variably saturated flow in fractured basalt. Parameters obtained from the calibration were within the range of hydrogeological parameters measured from cores obtained in the field, substantiating the physical relevance of the calibration effort.