Journal of Contaminant Hydrology, 38(1-3), 47-68, 1999.
Characterization and prediction of subsurface pneumatic response at Yucca Mountain, Nevada
C. Fredrik Ahlers, Stefan Finsterle, and Gudmundur S. Bodvarsson
Lawrence Berkeley National Laboratory, Earth Sciences Division
University of California, Berkeley, CA 94720
Abstract.
Yucca Mountain, Nevada is being investigated as the proposed site for geologic disposal
of the United States' high level nuclear waste. A massive effort to collect data for
characterization of the thermo-hydrologic behavior of the unsaturated zone is being
conducted at the site. Several boreholes have been instrumented by the United States
Geological Survey and the Nye County Nuclear Waste Repository Project Office for passive
pneumatic monitoring of the subsurface. One-, two- and three-dimensional numerical models
are used to simulate the observed subsurface pressure variations. The data are inverted
using these models in order to characterize the unsaturated system and estimate the
pneumatic diffusivity of important geologic features. Blind predictions of subsurface
response and subsequent comparison to recorded data have built confidence in the models
of Yucca Mountain. Inversions show that the pneumatic response of the system is dominated
by the Paintbrush non-welded unit (PTn). Faults are shown to be fast pathways for gas flow
but affect subsurface response only on a relatively local scale. Estimates of the diffusivity
of the tuffs that comprise Yucca Mountain range from a low of 0.008 m2/s in the non-welded
pre-Yucca bedded tuff layer to a high of 4 m2/s in the densely welded, fractured Topopah
Spring tuff. Estimates of diffusivity in fault zones at Yucca Mountain range between
0.03 m2/s and 1500 m2/s with the highest being in the Drill Hole Wash
fault where it intersects the Topopah Spring tuff.
