STRONTIUM CONCENTRATIONS AND ISOTOPIC COMPOSITIONS OF GROUND WATER DOWNGRADIENT FROM YUCCA MOUNTAIN, NEVADA

Hydrochemical studies of ground water at Yucca Mountain (YM), Nevada, site of a proposed high-level nuclear waste repository, include determinations of Sr concentrations and isotopic compositions of samples from the Nye County Early Warning Drilling Program (EWDP) boreholes downgradient from the site. Ground water transitions from fracture and matrix flow through the Tertiary volcanic rocks to predominantly matrix flow in the alluvium. Data for samples from EWDP Phase I and II boreholes, located about 17 km south of YM, are evaluated according to location, depth to water, and Sr concentration and isotopic composition. Ground water near southern Crater Flat has Sr isotopic ratios that are similar to those determined for Pleistocene-age carbonate spring deposits in the same area. Ground water in this area has high Sr concentrations (about 500 mg/L) and ⁸⁷Sr/⁸⁶Sr values (0.7123 to 0.7133) compared to ground water from EWDP boreholes to the east. North of the spring deposits, a water sample from borehole USW VH-2 in west-central Crater Flat has similar Sr isotopic and concentration values, suggesting a north-to-south flow path in this area. In general, depth to the water table increases from the spring deposits on the west toward Fortymile Wash on the east side of YM. Ground-water samples from the EWDP Phase III boreholes in the Fortymile Wash alluvial aquifer show lateral and vertical variation in Sr ratios (0.7096 to 0.7114) and concentrations (1.5 to 80 mg/L). These variations indicate complex, isolated flow paths within the alluvial aquifer. There is limited evidence for systematic variation among ⁸⁷Sr/⁸⁶Sr ratios of the EWDP water samples from different depths, indicating that mixing among zones is not sufficient to homogenize chemically distinct waters within the aquifer system. Despite the low Sr concentrations (about 2 mg/L) in deeper ground-water samples from a borehole in Fortymile Wash penetrating Tertiary volcanic rock, the ⁸⁷Sr/⁸⁶Sr ratio of the water is preserved and does not reflect the ⁸⁷Sr/⁸⁶Sr ratio of the rock, which indicates that Sr isotopes are a semi-conservative tracer. The Sr concentrations and ⁸⁷Sr/⁸⁶Sr ratios of all the EWDP water samples indicate the presence of discrete hydrochemical domains in the alluvial aquifer system, which are maintained southward from YM by fracture and (or) matrix flow channelization.