IN SITU WATER-POTENTIAL VALUES OF THE NONWELDED UNITS OF THE PAINTBRUSH GROUP, YUCCA MOUNTAIN, NEVADA

Yucca Mountain, Nevada is being studied as a potential repository for nuclear waste disposal. Boreholes drilled vertically from test alcoves located in the underground Yucca Mountain Exploratory Studies Facility (ESF) were instrumented with heat-dissipation probes to measure the in situ water potentials of the nonwelded units of the Paintbrush Group. These units include the nonwelded units of the Tiva Canyon and Topopah Spring Tuffs, the Pah Canyon Tuff, and several bedded units. Borehole PTN#1, located in Alcove #3 at ESF station 7+60 m, penetrated the lower welded Tiva Canyon Tuff, the nonwelded units of the Paintbrush Group, and the upper welded units of the Topopah Spring Tuff. Borehole PTN#2, located in Alcove #4 at ESF station 10+27 m, penetrated the lower nonwelded units of the Paintbrush Group and the upper welded units of the Topopah Spring Tuff. The ESF ventilation system caused near-surface drying to a depth of 4.9 m in Alcove #3 borehole PTN#1. Ventilation effects were minimal in Alcove #4 borehole PTN#2. Interestingly, Alcove #3 was isolated from the ESF ventilation system by a bulkhead but Alcove #4 was not. Water-potential values from stations below the zone of ventilation effects ranged from -0.3 to -1.7 bars in the nonwelded units of the Paintbrush Group and -0.8 to -4.3 bars in the upper welded units of the Topopah Spring Tuff. Water-potential values in the upper welded units of the Topopah Spring Tuff were drier with depth. Water-potential values of the nonwelded units of the Paintbrush Group agree with the 0.0 to -2.0 bars water-potential values measured in the same units by thermocouple psychrometers installed in surface-based boreholes as part of the Surface-Based Monitoring Program. Several of the Alcove #4 borehole PTN#2 water-potential values from the upper welded units of the Topopah Spring Tuff were drier (-3.0 to -4.3 bars) than those measured in the same units in the surface-based boreholes (0.0 to -2.0 bars).