STRONTIUM ISOTOPES IN PORE WATER AND TRAVEL TIME IN THE UNSATURATED ZONE, YUCCA MOUNTAIN, NEVADA

The proposed radioactive waste repository at Yucca Mountain, Nevada, would be constructed in the high-silica rhyolite (Tptp) member of the Topopah Spring Tuff, a mostly welded ash-flow tuff within the ~500-m-thick unsaturated zone. Downward-percolating water may be transmitted by fracture flow in the welded Topopah Spring Tuff, but the overlying tuffs of the Paintbrush Group nonwelded hydrogeologic unit (Ptn) are highly porous and relatively unfractured, facilitating matrix flow. Based on carbon-14 measurements of carbon dioxide from packed-off intervals in borehole USW UZ-1 at the northern edge of the proposed repository block (Yang, App. Geochem., 2002), the percolation flux is about 2 millimeters per year; calculated travel times from the land surface to the top of the Tptp range from 4,700 to 6,800 years in three vertical boreholes located adjacent to the proposed repository block. These calculated travel times assume matrix flow in the PTn and that the hydrologic property measurements on core samples are characteristic of the bulk rock.

Strontium isotope compositions have been measured on pore water in core samples from surface-based boreholes and boreholes drilled from underground excavations in the Tptp. Strontium ratios (⁸⁷Sr/⁸⁶Sr) vary systematically with depth in the surface-based boreholes: ratios in pore water near the surface (0.7114 to 0.7124) reflect the range of ratios in soil carbonate (0.7112 to 0.7125) collected near the boreholes, but ratios in the Tptp (0.7122 to 0.7127) have a narrower range and are more radiogenic due to interaction with the volcanic rocks (primarily PTn) above the Tptp. These data can be fit to an advection-reaction model that reveals a relation between the flow velocity and the rate of dissolution (extraction of strontium) of the rocks. Constraints on the dissolution rate show that the strontium isotope record in pore water agrees with the long travel times from the land surface to the Tptp calculated from the carbon-14 data. Strontium isotope ratios on samples from underground boreholes in the Tptp (0.7121 to 0.7127) show a similarly narrow range, including those of pore water from a zone near the Sundance fault (0.71250 to 0.71263) that may be a preferential flow path. These results indicate that long travel times are characteristic of the unsaturated zone at Yucca Mountain.