Carbon isotopic data from test hole USW UZ-1, Yucca Mountain, Nevada

Rock-CO 2 -gas analyses in test hole USW UZ-1 at Yucca Mountain indicate that gas movement in the unsaturated zone is likely through a dry-fracture system with little porewater or caliche-calcite interaction. This is because near-surface δ 13 C values are of biogenic origin and have changed little throughout the total depth. Post-bomb 14 C activity is observed to the depth of about 12 m. An abrupt change in plotted 14 C/depth slope is seen at 61 m. The less steep upper segment corresponds to the zone with greater porosity and moisture content, and consequently more tortuosity, with an estimated traveltime of 1.27 cm/yr; the steeper sloped zone corresponding to the lower segment has smaller porosity and moisture content but larger fracture density for gas transport, with an estimated traveltime of 3.26 cm/yr.