TRACE AND MINOR ELEMENTS IN SATURATED-ZONE WATER NEAR YUCCA MOUNTAIN, NEVADA

Identification of ground-water flow paths in the saturated zone downgradient of the proposed high-level radioactive waste repository at Yucca Mountain, Nevada, is important for predicting travel times, dispersion, and potential exposure to radionuclides that may be released in the future. Current interpretations of hydrochemical facies and related flow paths are based on major ions and isotopes, however concentrations of trace and minor elements also exhibit areal distributions that help identify flow from beneath Yucca Mountain. Concentrations of boron, uranium, and lithium are smaller in ground water from boreholes in the Yucca Mountain and Fortymile Wash areas relative to water in adjacent areas. This signature extends south into the Amargosa Farms area and the distribution is similar to that of the major ions sodium, chloride, sulfate, and bicarbonate. Concentrations of barium and strontium also are smaller in ground water from boreholes at Yucca Mountain, upper Fortymile Wash, and directly south of Yucca Mountain, but this signature does not extend south in Fortymile Wash.

Concentrations of fluoride as well as the trace metals iron, zinc, manganese, and aluminum are relatively larger in water from a number of boreholes in the volcanic aquifer at Yucca Mountain and the alluvial aquifer directly south of Yucca Mountain than in adjacent areas. However, the larger fluoride and trace metal concentrations generally do not extend southeast in Fortymile Wash. Nitrate concentrations also are larger in water from several boreholes at Yucca Mountain and in Fortymile Wash. This nitrate signature extends south in Fortymile Wash into the Amargosa Farms area.

Although variations in concentrations of trace and minor elements are small, persistent differences among groups of wells help verify and refine interpretations of ground-water flow paths based on major-ion chemistry and identify potential flow paths where major-ion chemistry is not conclusive. Although some of the trace and minor element distributions are consistent with the conclusion that flow beneath Yucca Mountain is southeast toward Fortymile Wash and then south toward the Amargosa Farms area, many trace and minor element distributions may indicate more southerly flow than has been interpreted from the major-ion data.