MATRIX PERMEABILITIES OF FAULTED NONWELDED TUFFS

Nonwelded tuff units at Yucca Mountain, the proposed site for a high-level radioactive waste repository, play a prominent role in determining percolation rates through the portion of the unsaturated zone that lies above the potential repository horizon. Lateral flow at lithologic contacts within the nonwelded Paintbrush tuff (PTn) unit has been suggested as a possible mechanism for reduction of flow toward repository drifts. However, primary heterogeneity or secondary discontinuities (e.g., fractures and faults) could lead to preferential flow paths through the PTn and into the welded Topopah Spring Tuff (the repository horizon) that lies below, thus disrupting the potential for capillary or permeability barriers to divert water away from the repository.

Nonwelded tuffs are poorly exposed at Yucca Mountain; hence, work at analog sites for the PTn was initiated at the basal Bishop Tuff units near Bishop, CA. The Bishop Tuff includes matrix-supported, massive ignimbrites and clast-supported, bedded deposits; prior work has established the Bishop Tuff as a credible PTn analog. This study focuses on an innovative technique for measuring changes in matrix permeability near faults, which may lead to enhancement of vertical flow and an associated disruption of lateral flow.

Sample extraction from nonwelded tuffs continues to be a challenge. However, a new small-drillhole minipermeameter probe provides a means to eliminate sample extraction as a necessity for permeability measurement. Prior fieldwork in friable sandstone and saprolitic soils established the new probe as an effective field device. Advantages of this approach include (1) minimization of weathering on measured permeability, and (2) provision of a superior sealing mechanism around the gas injection zone. Finally, the new probe shows promise for data collection of permeability measurements within nonwelded tuffs at spatially refined positions and multiple orientations within the matrix of the deformation zone surrounding faults. The influence of primary lithology, texture, and faults on fluid flow through the nonwelded Bishop Tuff will be interpreted using data obtained from gas minipermeameter and water permeameter tests.

This abstract is an independent product of the CNWRA and does not necessarily reflect the views or regulatory position of the NRC.