IMPLICATIONS OF STRONTIUM ISOTOPIC COMPOSITIONS OF GROUND WATER FROM THE NYE COUNTY EARLY WARNING DRILLING PROGRAM BOREHOLES NEAR YUCCA MOUNTAIN, NEVADA

Strontium isotopic compositions of ground-water samples from the Nye County Early Warning Drilling Program (EWDP) boreholes were determined as part of the U.S. Geological Survey’s hydrochemical studies downgradient from Yucca Mountain, Nevada, site of a proposed high-level nuclear waste repository. The ⁸⁷Sr/⁸⁶Sr ratios are expressed here as δ⁸⁷Sr values, which are per mil deviations from the ratio of modern seawater (0.70920). The EWDP boreholes, because of their location, are particularly important in delineating flow paths downgradient from the proposed repository. Samples of ground water beneath Yucca Mountain, including below the repository block, are relatively uniform isotopically with a mean δ⁸⁷Sr value of +2.3± 0.9 (2 sigma). Three hydrochemical domains or facies of ground water are penetrated by the EWDP boreholes south and southwest of the proposed repository. Ground water near Pleistocene-age tufa deposits in southern Crater Flat has δ⁸⁷Sr values between +4.4 and +5.8, similar to those of the deposits. Samples from new boreholes in a small valley north of the Lathrop Wells basaltic cone have significantly smaller δ⁸⁷Sr values of +0.2, which contrast with much larger δ⁸⁷Sr values of +4.4 for water from a supply well at the cone. These differences indicate the presence of isolated flow paths that have not mixed in the saturated zone. The low δ⁸⁷Sr value of +0.2 could result from interaction of the ground water with rocks in the Crater Flat Group (Miocene age), which typically has larger Sr content and lower δ⁸⁷Sr values than ground water from rocks in the Paintbrush Group (Miocene). The mean δ⁸⁷Sr value of +2.0 for samples from the upper part of the saturated zone in Fortymile Wash is identical to the mean (+2.3 +/-0.9) for samples under Yucca Mountain, indicating that the transition of ground water from fractured volcanic rocks under Yucca Mountain to matrix-dominated alluvium has no immediate effect on the strontium isotopes. Some samples from Fortymile Wash boreholes have δ⁸⁷Sr values as small as +0.6, similar to the low values north of Lathrop Wells cone. The similarities in the mean δ⁸⁷Sr in ground–water samples from Yucca Mountain and Fortymile Wash support models in which ground-water beneath the proposed repository flows to the southeast towards Fortymile Wash rather than directly south.