Radionuclide transport pathways off of the Nevada National Security Site

Regional groundwater flow in the eastern NNSS is southward through the carbonate aquifer beneath the basins and testing areas of Yucca and French­man Flats (Fig. 26.2). Groundwater flow directions change to the southwest in southern Frenchman Flat influenced by increased underflow from east of the NNSS, and following en echelon faults of the southwest trending, right slip Rock Valley fault system (USDOE, 1997; O’Leary, 2000; Belcher et al., 2004). The eastern carbonate flow system of the NNSS drains either into the Alkali Flat-Furnace Creek Ranch or Ash Meadows discharge areas of the southern Amargosa Valley and Death Valley located to the southwest of the NNSS (Winograd and Thordarson, 1975; Fenelon et al., 2010; Belcher and Sweetkind, 2010).

Radionuclides from underground testing in Yucca Flat, as noted previ­ously, remain mostly in the alluvial and volcanic rocks. Where local condi­tions allow migration through these rocks, radionuclides are expected to move vertically downward and feed into the carbonate aquifer in the central and southern part of the basin, most likely along sets of north-south trend­ing faults. Particle track studies for selected test locations in Yucca Flat show flow south beneath Yucca Flat, CP Basin and southwestward along the Rock Valley fault system, discharging into the Alkali Flat-Furnace Creek Ranch system (USDOE, 1997). Alternatively, flow may diverge southward across the Rock Valley fault system and terminate in the Ash Meadows discharge area (Fenelon et al., 2010; see Fig. 26.2). For either case, groundwater from Yucca Flat is expected to travel a minimum of 40 km from sites of underground testing before crossing the southern boundary of the NNSS.

Modeling studies of radionuclide transport in Frenchman Flat show that significant quantities of radionuclides are unlikely to reach the regional carbonate aquifer within 1,000 years. Two underground tests in the north part of the basin are located near the eastern edge of the NNSS (Fig. 26.4); radionuclide transport in the fractured volcanic aquifers from these two tests may cross the southeast boundary into Federally controlled land adja­cent to the NNSS within 1,000 years (NNES, 2010a).

Preliminary estimates of the travel times through the unsaturated zone to the regional groundwater table for radionuclides from the underground

tests in the tunnel beds of Rainier Mesa and Shoshone Mountain exceed hundreds of years; radionuclide concentrations in groundwater beneath the Mesa are expected to be low. Travel time estimates to the regional ground­water table for the two tests conducted in vertical shafts in southwest Rainier Mesa are much shorter than for the tunnel bed detonations. There are multiple permissive directions of groundwater flow from Rainier Mesa: northward, southwestward beneath Pahute Mesa, or southward (Fenelon et al., 2008). Southward migration of radionuclides from the Mesa areas is toward and beneath Fortymile Canyon and Jackass Flat entering into the Alkali Flat-Furnace Creek flow system (Fig. 26.2). Minimum distances of radionuclide migration from the Rainier Mesa and Shoshone Mountain underground tests to the south boundary of the NNSS are greater than 45 km for Shoshone Mountain and greater than 60 km for underground testing at Rainier Mesa.

Regional groundwater flow from testing areas of western and central Pahute Mesa is dominantly off the mesa highlands moving generally south — westward off the NNSS toward surface springs in Oasis Valley of the Oasis Valley flow system (Figs 26.2 and 26.7). Analysis of groundwater from an exploratory well located immediately outside of the NNSS boundary south of Pahute Mesa show small concentrations of tritium from underground testing, the only confirmed occurrence of local test-produced radionuclides outside of the boundaries of the NNSS.

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