Liquid-release tests in unsaturated fractured welded tuffs: I. Field investigations

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 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 percent 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. The data collected during these field tests has provided useful information for developing an understanding of liquid flow in unsaturated fractured welded tuff. It has also provided a basis for quantitative comparisons with numerical simulations of liquid-release experiments, as demonstrated in a companion paper [J. Hydrol. 256 (2002)].