HYDROCHEMICAL FACIES IN GROUND WATER NEAR YUCCA MOUNTAIN, NEVADA

Yucca Mountain, Nevada is being studied as a potential geologic repository for high-level nuclear waste. As part of these studies, the chemical and isotopic composition of ground water in the vicinity of Yucca Mountain is being used to identify hydrochemical facies and to delineate potential flow paths from the repository. Five hydrochemical facies can be defined on the basis of dissolved ions and isotopes: 1) Yucca Mountain, 2) Fortymile Wash, 3) Bare Mountain, 4) Amargosa River, and 5) Eastern Amargosa Valley. Ground-water compositions in both the Yucca Mountain and Fortymile Wash facies are dominated by sodium and bicarbonate, but water from the Fortymile Wash facies contains substantially larger calcium and magnesium concentrations. Ground water from the Bare Mountain facies has a composition similar to that of the regional Paleozoic carbonate aquifer with large calcium and magnesium concentrations (relative to sodium) and large bicarbonate concentrations. Ground water from the Amargosa River facies is characterized by large total dissolved solids and large sulfate and chloride concentrations relative to bicarbonate. Water from the Eastern Amargosa Valley facies is similar to that of the Amargosa River facies but has substantially larger chloride concentrations. The isotopic data are not unique in all five facies but are independent evidence of the processes associated with the development of these facies. Ground water in the Yucca Mountain facies has relatively low d⁸⁷Sr values and has the largest ²³⁴U/²³⁸U ratios found in the area. The d⁸⁷Sr values increase slightly in the Fortymile Wash facies relative to Yucca Mountain, but the ²³⁴U/²³⁸U ratios decrease to the south. The Bare Mountain facies has d⁸⁷Sr values consistent with water from the regional Paleozoic carbonate aquifer discharging at Ash Meadows and larger than values from the Yucca Mountain or Fortymile Wash facies. Within the Amargosa River facies, ground water near the Funeral Range has elevated d⁸⁷Sr values resulting from flow through alluvium derived from Precambrian rocks to the southwest. Differences in d⁸⁷Sr values in the Amargosa River and Eastern Amargosa Valley facies appear to be homogenized in the saline waters at Franklin Lake Playa.