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

Yucca Mountain, Nevada is being investigated as the proposed site for geolo gic disposal of the United States' high level nuclear waste. A massive effo rt to collect data for characterization of the thermo-hydrologic behavior o f the unsaturated zone is being conducted at the site. Several boreholes ha ve been instrumented by the United States Geological Survey and the Nye Cou nty Nuclear Waste Repository Project Office for passive pneumatic monitorin g 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. Bli nd predictions of subsurface response and subsequent comparison to recorded data have built confidence in the models of Yucca Mountain. Inversions sho w 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 bu t 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 m(2)/s in the non-welded pre-Yucca bedded tuff layer to a high of 4 m(2)/s in the densely welded, fractured Topopah Spring tuff. Estimates o f diffusivity in fault zones at Yucca Mountain range between 0.03 m(2)/s an d 1500 m(2)/s with the highest being in the Drill Hole Wash fault where it intersects the Topopah Spring tuff. (C) 1999 Elsevier Science B.V. All righ ts reserved.