A total of 747 underground detonations were conducted in Yucca Flat (USDOE, 2000b), an extensional basin located north of Frenchman Flat in the eastern NNSS (Figs 26.3 and 26.5). Some 664 were in alluvium and volcanic rocks in the unsaturated zone; 76 were in saturated alluvium and volcanic rocks; four in carbonate rocks with two of the detonations in the unsaturated zone and two below the water table; three detonations were in granitic rock in a small Cretaceous stock at the north end of the Yucca Flat basin (Pohlman et al ;, 2007). The radiological source term for detonations in the unsaturated zone remains in the unsaturated zone with two excep­tions. Detonations near the water table may directly inject radionuclides

540 000 560 000 580 000 600 000

Yucca Flat/ Climax Mine CAU 97 (720 CASs)
N
Western _/ Pahute Mesa CAU 102 (18 CASs)
20 19
15
10
Rainier Mesa/ Shoshone Mountain CAU 99 18 (66 CASs)
1
30	11
Frenchman Flat CAU 98 (10 CASs)
27
23 22
Nevada National Security Site

26.2 Shaded relief map of the Nevada National Security Site showing the location of sites of underground testing of nuclear weapons. The 907 underground detonations are identified as corrective action sites, a subset of the number of underground detonations. Clusters of corrective action sites are grouped into corrective action units (CAUs) and the hydrology and geology of the four major CAUs are described in this chapter.

26.3

Generalized geologic map of the Frenchman Flat basin of the southeast Nevada National Security Site 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; dark gray = Precambrian and Paleozoic sedimentary rocks. Dashed line is the Frenchman Flat hydrostratigraphic framework model boundary. Solid line is the Nevada National Security Site boundary. Dots show the location of ten underground nuclear detonations in the Frenchman Flat corrective action unit.

26.4

Generalized geologic map of the Yucca Flat-Climax Mine (YF-CM) corrective action units showing the domain area for numerical models of groundwater flow and radionuclide transport at sites of underground testing. Stipple = Quaternary playa deposits; white = Quaternary/Tertiary alluvium; light gray = Miocene volcanic rocks; diagonal line = Mesozoic granitic rocks; dark gray =

Precambrian and Paleozoic sedimentary rocks. Dashed line is the YF-CM hydrostratigraphic framework model boundary. Solid line is the Nevada National Security Site boundary. Dots show the location of 747 underground detonations in the Yucca Flat-Climax Mine corrective action units.

into the water table. Underground detonations that created surface subsid­ence craters can accumulate surface runoff in the craters. Enhanced infiltra­tion in the crater bottoms moves downward in the collapse chimneys through the test cavity of underground detonations, and may transport radioactive contaminants to the saturated zone. Similarly, underground tests in the unsaturated and saturated zone of subsurface volcanic rocks may directly inject radionuclides into the underlying carbonate aquifer or radionuclides may move downward along local faults and fractures. Local flow of groundwater may transport radionuclides along faults driven by transient pressure gradients created by pressurization of low permeability zeolitized volcanic rocks during underground testing. The phenomenology of underground tests detonated in carbonate rocks is significantly different from tests conducted in other rocks types (Carle et al., 2008; SNJV, 2008). The thermal decomposition of carbonate rocks releases large quantities of CO2 gas that contributes to pressure and density-driven flow. Additionally, radionuclides released in saturated carbonate rocks may be transported directly in the regional groundwater flow system.

Groundwater flow along the length of the Yucca Flat basin is limited by restricted regional underflow from a combination of confining units bound­ing the basin on the north (granitic confining unit), on the northeast (lower clastic confining unit) and on the west (upper clastic confining unit) (Laczniak et al., 1996; Bechtel Nevada, 2006). Recharge in the basin interior is low from the arid climate and downward drainage to the LCA is addition­ally restricted by the presence of a thick and continuous tuff confining unit at the base of the volcanic section above the carbonate aquifer. Directions of groundwater flow in the alluvial and volcanic aquifers in the Yucca Flat basin are variable and these flow systems are incompletely coupled to the carbonate aquifer (Fenelon et al., 2010)