Using modern through mid-Pleistocene climate proxy data to bound future variations in infiltration at Yucca Mountain, Nevada

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 rock-property 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 ash-flow 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 were observed within this unit. The contact between the Pozos Formation and the underlying Cretaceous limestone basement was observed 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 are located 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.

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“…Preliminary models for fluid flow at Nopal I (Ghezzehei et al, 2006) can be updated using these observations. Sharpe, 2007…”

Section: Discussionmentioning

confidence: 99%

Stratigraphy of the PB-1 Well, Nopal I Uranium Deposit, Sierra Peña Blanca, Chihuahua, Mexico

Dobson

Fayek

Goodell

et al. 2008

International Geology Review

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“…Current rainfall at Nopal I (~300 mm/y) is significantly higher than at Yucca Mountain (177 mm/y; Sharpe, 2007). Rain at Nopal I is intercepted by a flat surface that is unlikely to produce significant runoff, which would not be the case on the rugged slopes of Yucca Mountain.…”

Section: Conclusion and Relevance To Yucca Mountainmentioning

confidence: 94%

Heterogeneous seepage at the Nopal I natural analogue site, Chihuahua, Mexico

et al. 2011

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An integrated field, laboratory, and modeling study of the Peña Blanca (Chihuahua, Mexico) natural analogue site is being conducted to evaluate processes that control the mobilization and transport of radionuclides from a uranium ore deposit. One component of this study is an evaluation of the potential for radionuclide transport through the unsaturated zone (UZ) via a seepage study in an adit at the Nopal I uranium mine, excavated 10 m below a mined level surface. Seasonal rainfall on the exposed level surface infiltrates into the fractured rhyolitic ash-flow tuff and seeps into the adit. An instrumented seepage collection system and local automated weather station permit direct correlation between local precipitation events and focused flow. In most locations, however, there is a 1-6 month time lag between major precipitation events and seepage within the adit, with longer residence times observed for the front adit area. Seepage data obtained from this study will be used to provide input to flow and transport models being developed for the Nopal I hydrogeologic system.

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“…Sharpe [2003] analysis is built on the extensive work by Forester et al [1999] and Thompson et al [1999], and adopts the approach used for the future climate analysis for the next 10,000 years at Yucca Mountain presented by Bechtel SAIC Company, LLC [2004a]. Sharpe [2007] describes the approach as applied to the climate of the next 500,000 years, and provides an extensive discussion of paleoclimate proxies in the Yucca Mountain area.…”

Section: The Sharpe Model For Climate Changementioning

confidence: 99%

Average infiltration at Yucca Mountain over the next million years

Stothoff

Walter

2013

Water Resources Research

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[1] An uncertainty distribution is developed to describe areal-average infiltration at Yucca Mountain averaged over the next million years. The distribution uses Infiltration Tabulator for Yucca Mountain (ITYM) model results to estimate uncertainty in net infiltration given decadal-average climate, driven with two independent estimates for potential future climatic sequences. Both future climatic sequences use orbital mechanics linked to paleoclimatic proxies to estimate future climate states, but one uses primarily biotic indicators and the other uses primarily abiotic indicators to link paleoclimate to orbital stage. Abiotic indicators suggest that present-day climate variation by latitude in the Great Basin is reasonably analogous to glacial-scale climate variation at a single location once orographic, lake effect, and insolation influences are accounted for. In the southern Sierra Nevada, abiotic indicators suggest that mean annual precipitation was 1.7-1.9 times larger than present and mean annual temperature was 5.6-7C cooler than present at the last glacial maximum. The influence of decadal to millennial-scale variability is estimated using the 8000 year Methuselah Walk tree-ring record, which suggests that even large inferred fluctuations over the Holocene only modestly increase 8000-year-average infiltration. Combining the future-climate projections with the infiltration model uncertainty distribution yields approximately lognormal uncertainty distributions for projected million-year arealaverage net infiltration, with a mean of 41 mm/yr and coefficient of variation of 0.81. Even if anthropogenic effects significantly alter climatic patterns relative to past climate cycles, the projected anthropogenic period is sufficiently brief that million-year averages are not likely to be substantially influenced.Citation: Stothoff, S. A., and G. R. Walter (2013), Average infiltration at Yucca Mountain over the next million years, Water Resour.

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Using modern through mid-Pleistocene climate proxy data to bound future variations in infiltration at Yucca Mountain, Nevada

Cited by 9 publications

References 116 publications

Stratigraphy of the PB-1 Well, Nopal I Uranium Deposit, Sierra Peña Blanca, Chihuahua, Mexico

Stratigraphy of the PB-1 Well, Nopal I Uranium Deposit, Sierra Peña Blanca, Chihuahua, Mexico

Heterogeneous seepage at the Nopal I natural analogue site, Chihuahua, Mexico

Average infiltration at Yucca Mountain over the next million years

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