Date of this Version
CHEMICAL COMPOSITION OF GROUND WATER IN THE YUCCA MOUNTAIN AREA, NEVADA, 1971-84 By L.V. Benson and P. W. McKinley. UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY Open-File Report 85-484. Denver, Colorado 1985
Fifteen test wells in the Yucca Mountain area of southern Nevada have been sampled for chemical analysis at least once during 1971-84. Samples were obtained by pumping water from the entire well bore (composite sample), and, in three instances, by pumping from one or more isolated intervals within a well bore. Sodium was the most abundant cation, and bicarbonate was the most abundant anion in all water samples. Samples from the deep carbonate aquifer penetrated by well UE-25p#l contained higher relative concentrations of calcium and magnesium than did samples from overlying volcanic tuffs. Concentrations of the stable isotopes of oxygen and hydrogen were relatively negative (light) and had deuterium-excess values ranging from +5 to +10. The distribution of uncorrected radiocarbon ages of water from volcanic tuffs sampled within 1 kilometer of the exploratory block on Yucca Mountain ranged from 12,000 to 18,500 years before present. Variation in the concentrations of inorganic constituents and of stable and radioactive isotopes indicates a significant degree of lateral and vertical chemical inhomogeneity in ground water of the Yucca Mountain area.
Part of Yucca Mountain, the exploratory block, in southern Nevada (fig. 1) is being investigated as a potential repository for the disposal of high-level nuclear wastes by the U.S. Department of Energy. The site is underlain by partially altered volcanic tuffs (Caporuscio and others, 1982) that probably extend to depths greater than 3,000 m. If approved by the Federal government, the repository probably would be excavated within the unsaturated zone, 150 to 300 m above the water table. There is concern that radionuclides, once leached from the stored wastes, would eventually reach the saturated zone, where they would be transported in the ground-water system from the repository site to the accessible environment. In order to understand the types and magnitudes of chemical processes that affect the potential movement of radionuclide species, compositional characterization of groundwater samples from the Yucca Mountain area has been ongoing since 1971. This report presents the chemical analyses of ground-water samples collected between 1971 and 1984, and incorporates selected data from earlier reports (Claassen, 1973; Benson and others, 1983). Drilling, sampling, and analytical procedures are those described in Benson and others (1983). The investigation is being conducted by the U.S. Geological Survey in cooperation with the U.S. Department of Energy as part of the Nevada Nuclear Waste Storage Investigations.