GEOHYDROLOGICAL MODELS AND EARTHQUAKE EFFECTS AT YUCCA-MOUNTAIN, NEVADA

Yucca Mountain, the proposed site for the high-level nuclear waste rep ository, is located just south of where the present water table begins a sharp rise in elevation. This large hydraulic gradient is a regiona l feature that extends for over 100 km. Yucca Mountain and its vicinit y are underlain by faulted and fractured tuffs with hydraulic conducti vities controlled by flow through the fractures. Close to and parallel with the region of large hydraulic gradient, and surrounding the core of the Timber Mountain Caldera, there is a 10- to 20-km-wide zone con taining few faults and thus, most likely, few open fractures. Conseque ntly, this zone should have a relatively low hydraulic conductivity, a nd this inference is supported by the available conductivity measureme nts in wells near the large hydraulic gradient. Also, slug injection t ests indicate significantly higher pressures for fracture opening in w ells located near the large hydraulic gradient compared to the opening pressures in wells further to the south, hence implying that lower ex tensional stresses prevail to the north with consequently fewer open f ractures there. Analytical and numerical modeling shows that such a bo undary between media of high and low conductivity can produce the obse rved, large hydraulic gradient, with the high conductivity medium havi ng a lower elevation than the water table. Further, as fractures can c lose due to tectonic activity, the conductivity of the Yucca Mountain tuffs can be reduced to a value near that for the hydraulic barrier du e to strain release by a moderate earthquake. Under these conditions, simulations show that the elevation of the steady-state water table co uld rise between 150 and 250 m at the repository site. This elevation rise is due to the projected shift in the location of the large hydrau lic gradient to the south in response to a moderate earthquake, near m agnitude 6, along one of the major normal faults adjacent to Yucca Mou ntain. As the proposed repository would only be 200-400 m above the pr esent water table, this predicted rise in the water table indicates a potential hazard involving water intrusion.