Improved Tension Infiltrometer for Measuring Low Fluid Flow Rates in Unsaturated Fractured Rock

similar fractured rock formations elsewhere. One possible means for obtaining such estimates is the use of The search for a permanent storage facility for the geological distension infiltrometers. posal of high-level nuclear waste has motivated extensive research Tension infiltrometers have long been used in soil hyduring the past several decades to characterize and predict fluid flow into and through unsaturated fractured rock. Tension infiltrometer drology to measure field soil infiltration rates and/or to experiments are extremely useful to investigate infiltration into fracestimate various infiltration parameters such as the sorture networks, but are difficult to perform using commercially availptivity and the saturated hydraulic conductivity (Perable equipment developed mostly for soils. Our objective was to develop roux and White, 1988). Others have used tension infila tension infiltrometer suited for accurate measurements of infiltration trometers to characterize flow through soil macropores into fractured rock at very low flow rates and for long equilibration at or near saturation (e.g., Wilson and Luxmoore, 1988; times. We constructed several prototype instruments from porous Lin and McInnes, 1995; Jarvis and Messing, 1995; Mostainless-steel membrane, stainless-steel casing, acrylic tubing, several hanty et al., 1996). Unfortunately, tension infiltration temperature-compensated pressure transducers, solenoid valves, and experiments are much more problematic to conduct in a data logger for automated control and data acquisition. An autofractured rock than in most soils. Extremely low infiltramated refill system was also developed to facilitate long unattended tion rates are expected in fractured rocks, particularly equilibration periods typical in infiltration experiments on unsaturated fractured rock. Results show that the improved design reduces temperat high tensions, thus making accurate measurements ature effects on the infiltration rate, allows for much longer periods of difficult. Because of low rock permeabilities and long unattended operation (auto-refill), and reduces evaporation from the equilibration times, various environmental factors norinfiltrometer. The estimated upper flow-rate limit of our new infiltrommally neglected during tests with soils may not be neglieter is about 1 mm d 1, based on the conductance of the porous steel gible for rock formations. membrane (11 mm d 1). We were able to make measurements of the Recent attempts to use tension infiltrometers to meafluid flux as low as 10 mm yr 1 at a pressure head of about 110 cm. sure water flow properties of welded tuff layers exposed along the Exploratory Study Facility (ESF) at Yucca Mountain (a tunnel constructed to provide access along P to use the unsaturated zone of Yucca Mounthe eastern edge of the potential repository area) showed tain, Nevada as a permanent storage facility for the that traditional soil infiltrometer designs are inadequate geological disposal of high-level nuclear waste has motito accurately describe flow in fractured rock media (Hudvated extensive research aimed at understanding fluid son et al., 2000, unpublished data). Many fractures, meaflow processes into and through unsaturated fractured sured in micrometers, are still fully functional at infilrock formations (e.g., NRC, 2001; Bodvarsson et al., 2003; trometer supply tensions above 100 cm of water. These Wu et al., 2004). Yucca Mountain, located in a semiarid refractures are most likely responsible for the recharge gion of southern Nevada, is characterized by alternating rates of 5 to 10 mm yr 1 (1.6 10 10 to 3.2 10 10 m welded and nonwelded volcanic tuff layers. Generally, s 1) estimated to occur at Yucca Mountain (e.g., Flint nonwelded units have relatively high matrix porosities et al., 2002). Tensions needed to fully characterize the and permeabilities and lower fracture densities, whereas flow regime of these fractures are unattainable using curthe welded layers, such as those that may host the envirent soil infiltrometer technology. Furthermore, because sioned repository, have lower porosities and higher fracof extremely low infiltration rates and long equilibrature densities. One major challenge is to estimate the untion times at especially higher tensions, evaporative saturated hydraulic properties of the major hydrologic fluxes that inevitably occur between the disk and the units of Yucca Mountain (e.g., Flint et al., 2001) or ring (normally neglected during tests in soils) may become important error sources when measurements are made on fractured rock. As well, temperature fluctuaP. Castiglione, Land Resources and Environmental Science Dep., tions causing pressure variations at the rock surface Montana State Univ., 334 Leon Johnson Hall, Bozeman, MT 59717; P.J. Shouse and M.Th. van Genuchten, USDA-ARS, George E. Brown, may reduce the accuracy of the measurements. Finally, Jr. Salinity Lab., 450 W. Big Springs Road, Riverside, CA 92507; B. the need to periodically refill the reservoir tower, which Mohanty, Dep. of Biological and Agricultural Engineering, 301 requires an operator and induces severe disturbances Scoates Hall, Texas A&M Univ., College Station, TX 77843-2117; D. to the infiltration process, can become an important Hudson, U.S. Geological Survey, MS 423, 1180 Town Center Drive, Las Vegas, NV 89134. Received 17 Sept. 2004. *Corresponding author practical problem. Hence, the primary objective of this ([email protected]). study was to develop a tension infiltrometer that overcomes some of these limitations and can be used to Published in Vadose Zone Journal 4:885–890 (2005). measure infiltration rates at levels thought to occur at Notes doi:10.2136/vzj2004.0135 the Yucca Mountain site. © Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: ESF, Exploratory Study Facility. 885 Published online August 16, 2005