ESTIMATES OF PAST SEEPAGE FROM CALCITE DISTRIBUTION IN THE EAST-WEST CROSS DRIFT, YUCCA MOUNTAIN, NEVADA
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|
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 95th 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.