Characterization and prediction of subsurface pneumatic response at Yucca Mountain, Nevada

Yucca Mountain, Nevada is being investigated as the proposed site for geologic disposal of the United Statesʹ 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.