Systematic Hydrological Characterization of the Topopah Spring Lower Lithophysal Unit

Paul Cook and Yvonne Tsang

Contact: Paul Cook, 510/486-6110, PJCook@lbl.gov

Reserach Objectives
Over 80% of the proposed repository for the permanent disposal of high-level radioactive nuclear waste will be situated in the lower lithophysal unit of the Topopah Spring welded tuff, Yucca Mountain, Nevada . Within the Exploratory Studies Facility at Yucca Mountain, a 5 m diameter drift (tunnel), called the East-West Cross Drift, traverses this lower lithophysal unit. The welded tuff is intersected by many submeter fractures and interspersed with lithophysal cavities ranging from 15 to 100 cm in diameter. The size and spacing of both these features (fractures and cavities) varies appreciably . This indicates that hydrological characteristics at one particular location may not be representative of the entire lower lithophysal unit. Therefore, systematic testing at regular intervals, unbiased by the knowledge of specific features, is in progress, with the objective of gaining a greater understanding of the hydrological characteristics and associated heterogeneity of this potential repository unit.

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