cesses as evaporation and geothermal heating are also recorded by the isotopes. Figure 1 shows ground waters from the Yucca Mountain area compared to Craig 's (1961) meteoric water line. The ground-water isotopic contents occupy a narrow range overlapping and parallel to the meteoric water line. Both the Ash Meadows flow system and the ground waters of the Alkali Flat/Furnace Creek system are indistinguishable in terms of δ18O and δD. The δ18O and δD values of the Ash Meadows flow system are controlled primarily by the isotopic composition of winter-spring precipitation over recharge areas in the Spring Mountains. The similar values of the Alkali Flat/ Furnace Creek ground-water system suggest a similar control on isotopic content. Note that precipitation collected at stations on the Nevada Test Site (north and east of Yucca Mountain) has higher δD

Figure 1 Plot of δD vs δ18O for ground waters of the Ash Meadows and Alkali Flat/Furnace Creek (Alk./Furn.) flow systems. Also shown are precipitation on the Nevada Test Site, surface waters and the meteoric water line (“MWL”) of Craig (1961). “LMWL” is the local meteoric water line. Data and additional discussion in Claasen (1985, 1986); Benson and Klieforth (1989); Benson and McKinley (1985); Benson et al. (1983); Ingraham et al. (1990); and Lyles et al. (1990). Analytical uncertainty is ±0.2‰ for δ18O and ±2.5‰ for δD.

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