ESTIMATES OF PAST SEEPAGE FROM CALCITE DISTRIBUTION IN THE EAST-WEST CROSS DRIFT, 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. Excavations in this unit expose calcite with minor opal and fluorite that was deposited on the floors of lithophysal cavities and on the footwalls of fractures by meteoric water percolating through the unsaturated zone. Geochronologic studies show that the calcite and opal were deposited throughout most of the post-emplacement history of the 12.8-Ma ash-flow tuff. These low-temperature deposits are a long-term record of the distribution and volume of past seepage into underground openings and can be used to estimate future seepage into the proposed waste emplacement drifts.

The amount of secondary calcite has been estimated by continuous line surveys (60-cm wide) in the East-West Cross Drift (750 m to 2100 m distance). These line surveys covered approximately equal areas of each of three lithostratigraphic zones of the proposed repository (listed by increasing depth): upper lithophysal (Tptpul), middle nonlithophysal (Tptpmn), and lower lithophysal (Tptpll). Systematic differences in the abundance of calcite indicate that seepage decreases with stratigraphic depth. Analyses of CO₂ in cuttings from the same zones in a nearby borehole (USW SD-6) yield similar secondary calcite abundances and depth relations.

Lithostratigraphic	Secondary Mineral Abundance	Percent of 5-m Segments
Zone	(in area percent)	with Secondary Minerals
Tptpul	0.11	88
Tptpmn	0.059	72
Tptpll	0.026	49

Assuming that all seepage locations are revealed by the presence of calcite, the line survey calcite abundance data can be scaled to the upper half of the 5-m-diameter drift. Estimates of the water volumes required to deposit the amounts of calcite over 10 million years result in seepage volumes of 0 to 4 liters of water per year per 5-m drift segment (median to 95^th percentile). These seepage estimates are at the low end of the range of seepage rates from recent performance assessment (PA) calculations, but the fraction of 5-m drift segments with seepage is higher than indicated in the PA calculations.