To investigate unsaturated flow through a fault located within fractured welded tuff, we performed in situ field experiments in the Exploratory Studies Facility at Yucca Mountain, Nevada. This experiment involved the release of approximately 82000 L water for a period of 17 mo directly into a near-vertical fault under both constant positive head (at about 0.04 m) and decreasing fluxes. As water was released into the fault, changes in moisture content were monitored in the formation while a large cavity excavated below the test bed was visually inspected for seepage. We observed that water (introduced along the fault) maintained the fault as the primary vertical flowpath, while the adjacent fractured rock served to move water laterally and vertically. Further, unlike primary flowpaths along the fault, flow was not persistent along the secondary flowpaths. While this field experiment provided preliminary insights about the flow field associated with a fault, it also demonstrated the need to investigate the role of infill material and secondary fractures in diverting flow from gravity-driven fast flow toward flowpaths in which lateral flow may occur.

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