“…Conventional macroscale continuum approaches (Nitao & Buscheck, 1991;Peters & Klavetter, 1988;Pruess, 1985;Wang & Narasimhan, 1985, 1992 are unlikely to capture the full process spectrum under such conditions because heterogeneous fracture and matrix permeabilities are averaged over larger-scale volumes. Multiple field studies at locations with thick vadose zones have shown evidence for non-volume averaged behavior (i.e., preferential flow bypassing much of the flow system volume; Pruess, 1998b), which highlights the importance of preferential flow processes for recharge dynamics (e.g., at Yucca Mountain) (Fabryka-Martin et al, 1996;Yang et al, 1995). Nativ et al (1995) calculated infiltration velocities along preferential pathways in the Avdat fractured chalk, which were found to be two to three orders of magnitude faster than the calculated matrix infiltration rates.…”