CHEMISTRY OF UNSATURATED ZONE GASES SAMPLED IN OPEN BOREHOLES AT THE CREST OF YUCCA MOUNTAIN, NEVADA - DATA AND BASIC CONCEPTS OF CHEMICAL AND PHYSICAL PROCESSES IN THE MOUNTAIN

Boreholes open to the unsaturated zone at the crest of Yucca Mountain, Nevada, were variously sampled for CO2 (including C-13 and C-14), CH4 , N-2, O-2, Ar, CFC-11, CFC-12, and CFC-113 from 1986 to 1993. Air ent ers the mountain in outcrops, principally on the eastern slope, is enr iched in CO2 by mixing with soil gas, and is advected to the mountain crest, where it returns to the atmosphere. The CFC data indicate that travel times of the advecting gas in the shallow Tiva Canyon hydrogeol ogic unit are less than or equal to 5 years. The C-14 activities are p ostbomb to depths of 100 m, indicating little retardation of (CO2)-C-1 4 in the shallow flow systems. The C-14 activities from 168 to 404 m i n the Topopah Spring hydrogeologic unit are 85-90 pMC at borehole USW- UZ6. The CFC data show that the drilling of USW-UZ6 in 1984 has altere d the natural system by providing a conduit through the Paintbrush Non welded unit, allowing flow from Topopah Spring outcrops in Solitario C anyon on the west to USW-UZ6, upward in the borehole through the Paint brush, to the shallow Tiva Canyon flow systems, and out of the mountai n.