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Wetting-front movement, flow-field evolution, and drainage of fracture flow paths were evaluated within the Topopah Spring welded tuff at Yucca Mountain, Nevada. Equipment and techniques were developed for in situ quantification of formation intake rates, flow velocities, seepage rates, and volumes of fracture flow paths. Localized injections of tracer-laced liquid into a low-permeability zone (LPZ) and a high-permeability zone (HPZ) along a borehole were detected in two boreholes below the point of injection. For the LPZ tests, water did not seep into an excavated slot that defined the lower boundary of the test bed, and the liquid-intake rate under constant-head conditions was observed to steadily decrease by two orders of magnitude. In the HPZ, liquid-intake rates under constant-head conditions were significantly higher and did not exhibit a strong systematic decline.
HPZ tests were also conducted under a range of constant-flow conditions. Slot seepage rates showed intermittent responses and the percentage of injected water recovered in the slot increased as each test progressed. A maximum of 80% of the injected water was recovered during high-rate injection tests. The flow path volumes were found to increase during the course of each HPZ test.
Tracers injected in the HPZ were detected in the water samples collected in the slot. Analyses of the breakthrough curves show that flow and transport pathways are dynamic, rather than fixed, and related to the liquid release rates. Fractures act as the main conducting pathways, with minimal fracture-matrix interactions, under high injection rates. Fracture-matrix interactions, as well as matrix flow, can significantly influence flow and transport under low release rates. Observations of tracer concentrations rebounding in the seepage water following interruption of flow provide evidence of mass exchange between the fast-flowing fractures and other slow- and/or no-flow regions.
Numerical modeling was a key component of this research effort. Data from this experiment were used to make quantitative comparisons with numerical simulations of liquid-release experiments. The purpose of the modeling was to aid in experimental design, predict experimental results, and study the physical processes accompanying liquid flow through unsaturated fractured welded tuff. Model results suggest that a secondary fracture network with distinct characteristics from the network of larger mapped fractures may play a role in the behavior of liquid flow observed in the field test.