J. Fabryka-Martin, Los Alamos National Laboratory

M Caffee, G. Nimz, J. Southon, Lawrence Livermore National Lab

S. Wightman, W. Murphy, M. Wickham, Hydor Geo Chem, Inc., Tucson, AZ

P. Sharma, Purdue University

The 36Cl/Cl ratios for Cl extracted from soil and ream-bit cuttings are being used to provide information on characteristics of water movement through the unsaturated zone at Yucca Mountain. Deep penetration of the bomb-pulse signal in alluvium provides evidence for the variable effectiveness of alluvium and associated vegetation in attenuating infiltration. possibly as a result of different runoff characteristics of the watersheds. Fast transport of water via fractures through the Tiva Canyon welded unit is indicated by detection of elevated levels of 36Cl in the underlying Paintbrush nonwelded unit in 3 of 5 boreholes. Preliminary interpretation of 36Cl data for one deep borehole indicates average residence times of 5 to 7 x 10^5 yr in the deeper Topopah Spring welded units. and 1 to 3 x 10^5 yr in the underlying Calico Hills nonwelded unit.