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To investigate the potential for flow through altered tuff of the nonwelded unit of the Paintbrush Group (PTn) at Yucca Mountain, Nevada, we carried out in situ field experiments using water released directly into the matrix and along a minor subvertical normal fault at Alcove 4 in the Exploratory Studies Facility. During the experiments, changes in moisture content were monitored within the test bed, and a slot excavated below the test bed was visually inspected for seepage. For water released into a fault, the water-intake rate gradually fell from ~200 ml min-1 to 50 ml min-1 over a period of 41 hours of cumulative release time spread over 17 days of testing. Resistance changes recorded by electrical resistivity probes in matrix rock on either side of the fault interval suggest that flow in the fault spread out laterally in the fault plane. The time for the wetting front to travel 1 m in the fault was initially ~0.15 days. Subsequent tests in the fault showed the response time to be dependent on wetting history, with a faster response observed in a wetter system. Data analysis using the numerical code TOUGH2/ITOUGH2 reveals that a simple decline in permeability caused by simulated clay swelling can explain the experimental observations.Our field tests suggest that the dry porous PTn matrix is capable of attenuating episodic percolation fluxes in localized areas (such as around faults) where fast flow would be expected to dominate. Once wetted, the matrix is able to retain the moisture over a period of months. As saturation increases in the matrix, less water imbibes along the fault and more water travels farther along the fault. From this observation, we infer that a sequence of infiltration events separated by periods of up to a few months could convey water over increasing distances along the fault.