HYDROLOGIC CHARACTERISTICS OF FAULTS AT YUCCA MOUNTAIN, NEVADA

Yucca Mountain comprises a series of north-trending ridges composed of tuffs within the southwest Nevada volcanic field, 120 km northwest of Las Vegas, Nevada. These ridges are formed of east-dipping blocks of interbedded welded and nonwelded tuff that are offset along steep, mostly west-dipping faults that have tens to hundreds of meters of vertical separation. Yucca Mountain is currently under study as a potential site for underground storage of high-level radioactive waste, with the principle goal being the safe isolation of the waste from the accessible environment. To this end, an understanding of the behavior of ground-water flow through the mountain in the unsaturated zone and beneath the mountain in the saturated zone is critical. The percolation of water through the mountain and into the ground-water flow system beneath the potential repository site is predicated on: (1) the amount of water available at the surface as a result of the climatic conditions, (2) the hydrogeologic characteristics of the volcanic strata that compose the mountain. and (3) the hydrogeologic characteristics of the structures, particularly fault zones and fracture networks, that disrupt these strata. This paper addresses the hydrogeologic characteristics of the fault zones at Yucca Mountain, focusing primarily on the central part of the mountain where the potential repository block is located.