The Hydrogeologic Character of the Lower Tuff Confining Unit and the Oak Springs Butte Confining Unit in the Tuff Pile Area of Central Yucca Flat
The lower tuff confining unit (LTCU) in the Yucca Flat Corrective Action Unit (CAU) consists of a monotonous sequence of pervasively zeolitized volcanic tuff (i.e., mostly bedded with lesser nonwelded to poorly welded tuff; not fractured) (Bechtel Nevada, 2006). The LTCU is an important confining unit beneath Yucca Flat because it separates the alluvial and volcanic aquifers, where many underground nuclear tests were conducted, from the regional lower carbonate aquifer. Recent sub-CAU-scale modeling by Los Alamos National Laboratory in the Tuff Pile area of Yucca Flat (Boryta, et al., in review) includes postulated low-porosity, high-permeability zones (i.e., fractured welded-tuff aquifers) within the LTCU. This scenario indicates that such postulated low-porosity, high-permeability zones could provide fast-path lateral conduits to faults, and eventually to the lower carbonate aquifer. A fractured and faulted lower carbonate aquifer is postulated to provide a flow path(s) for underground test-derived contaminants to potential offsite receptors. The ramifications of such a scenario are obvious for groundwater flow and contaminant migration beneath Yucca Flat. This paper describes the reasoning for not including postulated low-porosity, high-permeability zones within the LTCU in the Tuff Pile area or within the LTCU in the Yucca Flat CAU-scale model. Both observational and analytical data clearly indicate that the LTCU in the Tuff Pile area consists of pervasively zeolitic, nonwelded to poorly welded tuffs that are classified as tuff confining units (i.e., high-porosity, low-permeability). The position regarding the LTCU in the Tuff Pile area is summarized as follows: • The LTCU in the Tuff Pile area consists of a monotonous sequence of predominantly zeolitic nonwelded to poorly welded tuffs, and thus is accurately characterized hydrogeologically as a tuff confining unit (aquitard) in the Yucca Flat-Climax Mine hydrostratigraphic framework model (Bechtel Nevada, 2006). • No welded-tuff (or lava-flow aquifers), referred to as low-porosity, high-permeability zones in Boryta et al. (in review), are present within the LTCU in the Tuff Pile area. • Fractures within the LTCU are poorly developed, a characteristic of zeolitic tuffs; and fracture distributions are independent of stratigraphic and lithologic units (Prothro, 2008). • Groundwater flow and radionuclide transport will not be affected by laterally extensive zones of significantly higher permeability within the LTCU in the Tuff Pile area. Although not the primary focus of this report, the hydrogeologic character of the Oak Spring Butte confining unit (OSBCU), located directly below the LTCU, is also discussed. The OSBCU is lithologically more diverse, and does include nonwelded to partially welded ash-flow tuffs. However, these older ash-flow tuffs are poorly welded and altered (zeolitic to quartzofeldspathic), and consequently, would tend to have properties similar to a tuff confining unit rather than a welded-tuff aquifer.
- Research Organization:
- National Security Technologies, LLC (United States)
- Sponsoring Organization:
- USDOE; USDOE National Nuclear Security Administration (NA)
- DOE Contract Number:
- DE-AC52-06NA25946
- OSTI ID:
- 985874
- Report Number(s):
- DOE/NV/25946-544; TRN: US201024%%34
- Country of Publication:
- United States
- Language:
- English
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