Lawrence Berkeley National Laboratory

To better understand preferential flow in fractured rock, we carried out an in situ field experiment in the Exploratory Studies Facility at Yucca Mountain, Nevada. This experiment involved the release of ~;22 m3 of ponded water (at a pressure head of ~;0.04 m) over a period of 7 months, directly onto a 12 m2 infiltration plot located on a fractured welded tuff surface. As water was released, changes in moisture content were monitored along horizontal boreholes located in the formation ~;19-22 m below. Distinct flow zones, varying in flow velocity, wetted cross-sectional area, and extent of lateral movement, intercepted the monitoring boreholes. There was also evidence of water being diverted above the ceiling of a cavity in the immediate vicinity of the monitoring boreholes. Observations from this field experiment suggest that isolated conduits, each encompassing a large number of fractures, develop within the fractured rock formation to form preferential flow paths that persist if there is a continuous supply of water. In addition, in fractured welded tuffs the propensity for fracture-matrix interactions is significantly greater than that suggested by existing conceptual models, in which flow occurs along a section of fracture surfaces. An overriding conclusion is that field investigations at spatial scales of tens of meters provide data critical to the fundamental understanding of flow in fractured rock.