RELATIVE HUMIDITY, PRESSURE, AND TEMPERATURE VALUES FROM THE GHOST DANCE FAULT ALCOVE 7, YUCCA MOUNTAIN, NEVADA

Yucca Mountain, Nevada is being studied as a potential repository for nuclear waste disposal. Data for relative humidity, pressure, and temperature were collected from Alcove 7. Alcove 7 is located at the Yucca Mountain Project underground Exploratory Studies Facility (ESF) station 50+64, in the Topopah Spring Formation middle nonlithophysal and lower lithophysal units. Alcove 7 was constructed to a horizontal depth of 201 m and penetrated the Ghost Dance Fault (GDF) at 167 m. To evaluate the role of the GDF in the flow and transport of fluids through Yucca Mountain, bulkheads were installed in Alcove 7 at 64 m and 132 m. Monitoring stations were installed at 4, 57, 72, 126, and 144 m. For the period September 25, 2001 to June 10, 2002 the relative humidity at all stations was drier than the range of the thermocouple psychrometers (95 to 100%) and therefore not measured. Pressure monitoring indicated that the transmission of atmospheric pressure fluctuations across both bulkheads showed minimal dampening (< 5%) and short time lags (on the order of hours). Large temperature fluctuations (19.5 to 27.5 degrees Celsius) at station 4 m reflect the transmission of seasonal temperature fluctuations down the ESF by the ventilation system. Ventilation-driven diurnal temperature fluctuations at station 4 m were as large as 1.2 degrees. Temperatures behind the bulkheads indicated no discernible annual fluctuation and diurnal fluctuations of less than 0.05 degrees. Abrupt temperature decreases (as much as 1.0 degree) behind the bulkheads were correlated with the pressure recovery period following a barometric low-pressure event. One hypothesis is that the system operates similarly to a household evaporative cooler. The rising barometric pressure in front of the bulkhead could force warm, dry ESF air through the fractures around the bulkhead, which would evaporate water, cool the air, and result in the abrupt temperature decreases behind the bulkhead.