There were a total of 82 detonations on Pahute Mesa; 64 were located in the central or eastern part of the mesa and 18 were located in western Pahute Mesa. The detonations were in a variety of rock types ranging from confining units of zeolitized volcanic rocks to fractured lava flow and welded tuff aquifers. Pahute Mesa is a large plateau highland formed from the suc­cessive eruption of overlapping ash-flow sheets and local silicic lavas from at least six large collapse calderas (Figs 26.3 and 26.7). Three of the calderas are partly to completely covered by volcanic rocks from younger caldera cycles. The down gradient connectivity of the different rock types at the detonation depth strongly affects the local release and rate of groundwater transport of radionuclides from the underground tests. Transport of radio­nuclides is locally aided by multiple sets of north-northeast trending basin — range faults and may be aided or impeded by offsets of rock units along the basin-range faults or across volcanic structure (ring-fracture zones bounding zones of caldera collapse).

Groundwater flow beneath Pahute Mesa is controlled by underflow from the DVRFS and local recharge at the higher elevations of the eastern mesa areas. Flow is predominantly from higher topography on the northeast to lower topography on the southwest. Local diversions in directions of groundwater flow occur near basin-range faults (Blankennagel and Weir, 1976) and from juxtaposition of confining units and aquifer units across the basin-range faults and caldera structure (SNJV, 2009a). The resurgent dome of Timber Mountain south of Pahute Mesa (Fig. 26.7) diverts groundwater flow to the east or west from a combination of reduced permeability of volcanic rocks associated with intrusion of a granitic body beneath the resurgent dome and/or local recharge at higher elevations of Timber Moun­tain. Groundwater flow west of Timber Mountain follows the western ring — fracture zone of the Timber Mountain caldera and local basin-range faults,

26.6

Generalized geologic map of the Pahute Mesa corrective action units showing the domain area for numerical models of groundwater flow and radionuclide transport at sites of underground testing. Stiple = Quaternary playa deposits; white = Quaternary/Tertiary alluvium; light gray = Miocene volcanic rocks; cross-hatch = Quaternary/Pliocene basaltic rocks; diagonal line = Mesozoic granitic rocks; dark gray = Precambrian and Paleozoic sedimentary rocks. Dashed line is the PM-OV hydrostratigraphic framework model boundary. Solid line is the Nevada National Security Site boundary. Double-dashed line is the caldera structural margins. Dots show the location of 82 underground detonations in the Pahute Mesa corrective action units (as well as those in the Rainier Mesa CAU).

moving south and southwest to discharge areas of Oasis Valley (USDOE, 1997; Grauch et al., 1999). A smaller component of flow may be diverted around the eastern flanks of Timber Mountain, following the Fortymile Wash drainage beneath eastern Jackass Flats and reaching discharge areas of the Armagosa Valley (Fig. 26.2 ; SNJV, 2009a). A component of flow in western Pahute Mesa may be in carbonate rocks in the vicinity of the Black Mountain caldera west of and outside the Amargosa Desert rift zone. Here
groundwater flow remains west of the Purse fault, a probable hydrologic barrier, but merges with the recharge water from eastern Pahute Mesa near the juncture of the multiple coalesced calderas on the southwest edge of Pahute Mesa (Blankennagel and Weir, 1976; SNJV, 2009a).