Unsaturated Flow Through Fractured and Nonwelded Tuff

The nonwelded Paintbrush Tuff (PTn) unit, which overlies the proposed high-level radioactive waste repository at Yucca Mountain is believed to play a prominent role in spatially and temporally dampening episodic percolation pulses moving downward to the repository horizon. Numerical model simulations show that a porous and permeable nonwelded tuff matrix (PTn) attenuates rapid and transient fracture flow from the welded tuff unit above (Tiva Canyon) resulting in nearly steady state unsaturated flow regime through the fractured tuffs of YM. Discovery of bomb-pulse chlorine-36 below the PTn and the dilute chemical composition of the perched water below the proposed repository horizon, however, suggest that a significant fraction of episodic water pulses follow fast pathways through the PTn. Preferential flow pathways could be induced by structural heterogeneity and discontinuities such as faults and fractures that develop even in poorly welded tuff units. Because the nonwelded tuffs are poorly exposed at YM, we are studying unsaturated fluid flow through the fractured and faulted nonwelded Bishop Tuff exposed near Bishop, CA. The basal Bishop Tuff includes matrix-supported, massive ignimbrites and clast-supported, bedded deposits similar to those of the PTn. Geologic investigations show that the fractures in the non- and poorly welded sequences are due to extension and are coated by calcite and silica. Faults exhibit fine-grained gouge. Preliminary results of in situ hydrologic testing and geologic mapping suggest that faulted and fractured basal facies of the Bishop Tuff form a suitable outcrop analog for the PTn. Detailed mapping of faults and fractures, combined with in situ hydrologic tests (infiltrometer and permeameter tests and dye tracing), demonstrate the influence of these features on the flow of water through unsaturated fractured and nonwelded tuff. For example, vertical fractures are found to cause a two-fold increase in the vertical-to-horizontal anisotropic bulk permeability ratio. Field tests revealed that fractures were extensively filled or coated, and generally acted to constrain or as a barrier to fluid flow. Laboratory experiments are underway to quantify the role of faults, fractures and tuff facies variability on unsaturated flow through nonwelded Bishop Tuff. [Work performed by CNWRA under contract NRC-02-97-009 does not necessarily reflect the views or positions of NRC].