Rainier Mesa/Shoshone Mountain CAU

A total of 68 underground detonations were conducted in tunnels con­structed in unsaturated zeolitized volcanic rocks of Rainier Mesa and Shos­hone Mountain (tunnel beds); all were located well above the regional groundwater table. Two detonations were in vertical shafts near the water table. The Rainier Mesa and Shoshone Mountain sites form plateau high­lands that demarcate the approximate eastern edge of the thick accumula­tions of volcanic rocks formed within the Amargosa Desert rift zone (Figs

26.3 and 26.6). The migration pathway of radionuclides released during testing beneath Rainier Mesa is generally downward through the unsatu­rated zone, complicated by local zones of perched water; travel time to the regional water table may be substantial. Additionally, there are local losses of radionuclides from drainage into tunnels constructed to host the

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26.5 Generalized geologic map of the Rainier Mesa-Shoshone Mountain (RM-SM) corrective action unit showing the domain area for numerical models of groundwater flow and radionuclide transport at sites of underground testing. 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 RM-SM hydrostratigraphic framework model boundary. Solid line is Nevada National Security Site boundary. Double-dash line is caldera structural margin (buried). Dots show the location of 68 underground detonations in the Rainier Mesa-Shoshone Mountain corrective action unit (as well as those in western Yucca Flat and eastern Pahute Mesa).

underground tests and from discharges from the tunnels into unlined drain­age ponds.

Groundwater beneath Rainier Mesa and Shoshone Mountain is recharged primarily by infiltration through the thick unsaturated zone beneath vol­canic highlands; downward infiltration through the zeolitized tunnel beds is locally aided by flow in zones of discontinuous fractures forming local perched water (Russell et al, 1987). The amount of underflow beneath the mesa from regional groundwater flow of the DVRFS is poorly constrained and the recharge mound beneath the mesa highlands may not be well con­nected to the regional groundwater flow system because of local juxtaposi­tion of clastic sedimentary confining units associated with thrust faults at the eastern edge of Rainier Mesa (Fenelon et al., 2008).

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