In situ flow experiments were conducted at Alcove 4 in the Exploratory Studies Facility at Yucca Mountain to investigate flow dynamics in a small normal fault located in zeolitically and argillically altered tuffs of the nonwelded rocks of the Paintbrush group. Water was released at constant head into a packed-off interval straddling the fault within a horizontal borehole. Changes in water-intake rate into the interval were monitored along with saturation in the formation using boreholes installed with electrical resistivity probes. For water released into the fault the water-intake rate gradually fell from ~200 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. ¿ 2001 American Geophysical Union