Hydrogeological setting of the Nevada National Security Site (NNSS)

The NNSS is located in the Great Basin portion of the basin-range physi­ographic province of the southwestern United States (Hunt, 1967; Stewart, 1980), approximately 150 km east of the Sierra Nevada mountain range containing the highest point in the contiguous United States (Mount Whitney, 4421 m) and about 40 km northeast of Death Valley, the lowest

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image285"point in North America (86 meters below sea level) (see Fig. 26.1). There are multiple definitions of the Great Basin based on hydrographic, physi­ographic, and floristic criteria (Grayson, 1993), but the most useful defini­tion for this chapter is the hydrographic definition. The Great Basin is an area centered about the state of Nevada, and including parts of the states of California, Utah, Oregon, and Idaho of the western United States that are internally drained. Precipitation in the Great Basin has no ocean outlet

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26.1 Shaded relief map of Southern Nevada, and adjacent areas of California in the southwestern United States. The solid line denotes the boundary of the Nevada National Security Site (NNSS; formerly the Nevada Test Site). The dashed line is the boundary of the Death Valley regional flow system (DVRFS) after Belcher and Sweetkind (2010). Death Valley and the Amargosa Desert are major discharge areas for the DVRFS.

and surface drainage flows into ephemeral streams that empty into saline lakes or dissipate through combined evaporation, transpiration and/or infiltration. Groundwater flow is an important component of the regional water budget and the NNSS is located in the central part of the Death Valley regional flow system (DVRFS; Winograd and Thordarson, 1975; D’Agnese et al., 1997; Belcher et al., 2004), a large internally drained area of Nevada (Fig. 26.1). Recharge in the DVRFS occurs primarily at higher elevation mountain ranges in the north, east and southern parts of the flow system. Discharge areas are distributed in the lower elevations of the Amar — gosa Desert and ultimately Death Valley (Fig. 26.1). The climate of the region is arid and is controlled largely by the rain shadow of the Sierra Nevada mountain range to the west with local variations controlled by elevation.

The hydrology of the NNSS is controlled primarily by three hydrologic and geological features. The first is the underflow of groundwater in the DVRFS and the location of areas of significant local recharge in the NNSS at the higher elevation mountain ranges and mesas of the site. The second feature is the physical properties and spatial distribution of diverse assem­blages of rock lithologies that form the aquifers and aquitards for the groundwater flow system. These rocks comprise three major lithologically and temporally distinct groups including Paleozoic carbonates and clastic sedimentary rocks, Miocene volcanic rocks erupted from multiple coalesced caldera centers, and thick alluvium deposited in fault-controlled basins. The third feature is the location and nature of major structural and tectonic features, including regional thrust belts formed in late Paleozoic and Meso­zoic time, major structures associated with caldera collapse and resurgence, and Miocene and younger extensional and strike-slip faults that formed the alluvial basins of the eastern and southern areas of the NNSS. This combi­nation of features control the volume, velocity and direction of groundwater flow and resulting transport of testing-introduced radionuclides.

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