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A coupled field‐scale aquifer pumping and water infiltration test was conducted at the Idaho National Engineering and Environmental Laboratory in order to evaluate subsurface water and contaminant transport processes in a heterogeneous flow system. The test included an aquifer pumping test to determine the storage properties of the aquifer and the state of confinement of the aquifer (∼190 m below land surface), and a vadose zone infiltration test to determine vertical moisture and radioactive tracer migration rates. Pump test results indicated that the Snake River Plain Aquifer was locally unconfined with a transmissivity ranging from 5.57 × 105 to 9.29 × 104 m2day. Moisture monitoring with neutron probes indicated that infiltrating water was initially transported vertically through the upper basalt layer of the vadose zone, primarily through fractures and rubble zones, at an average rate of 5 m/day (based on vertical distance traveled and first arrival of water at the monitoring points). Analysis of breakthrough curves for a conservative tracer allowed estimation of the arrival of the peak concentration and yielded an average velocity of 1 m/day. The migration velocities from the neutron probe and tracer tests are in good agreement given the scale of the test and difference in analysis techniques. None of the data sets showed a correlation between migration velocity (arrival time) and distance from the point source, but they strongly indicate preferential flow through discrete fractures. Upon reaching the first continuous sedimentary interbed layer in the basalt formation, water flow was diverted laterally along the interbed surface where it spread outward in primarily three areas corresponding to topographic lows on the interbed surface, and slowly infiltrated into the interbed. The nonpredictable movement of water and tracer through specific fractures underlying the site suggests that a priori prediction of trans‐missive fractures in this media is not possible. Results do suggest that the continuous sedimentary interbed layers, in general, impede vertical water flow and contaminant migration.

moisture contents. Flint et al. (2001) described the evolution of the conceptual flow model for Yucca Moun-Conceptual flow models provide a framework for predictive modeltain, an arid site under consideration as a high-level ing of contaminant transport. This study tests the assumptions of steady-state flow and a unit hydraulic gradient in a 177-m-thick vadose waste repository, during 15 yr. The primary driver bezone beneath a mixed waste site, using a network of advanced tensiom-hind the evolution of the conceptual flow model was eters. The conceptual flow model at the waste site, located on the the accumulation of site-specific data from neutron log-Idaho National Engineering Laboratory (INEEL), describes moisture ging of boreholes, bomb-pulse isotopes, perched water movement through a geologically complex site comprising basalt flows analyses, and thermal analyses.intercalated with sedimentary interbeds. The presence of sedimentary Collecting site-specific temporal data in deep vadose interbeds is expected to dampen and store much of the episodic zone systems present unique challenges because of the recharge, resulting in near steady-state conditions and unit gradient difficulty and expense of installing and maintaining inflow. Thirty advanced tensiometers in 18 wells provided field water strumentation for extended time periods (years). Howpotential data at depths ranging from 6.7 to 73.5 m below land surface ever, the in situ measurement of water or matric poten-(bls), beneath and adjacent to the waste site. Measured water potentials from February 2000 through August 2002 ranged from near tials provides data needed to characterize flow processes, saturation (Ϫ30 cm of water) to about Ϫ400 cm of water. Above 17 m, track infiltration or drainage, and estimate deep percothe observed long-term drying trends were presumed to be a response lation. to the cumulative effect of lower than average annual precipitation Thermocouple psychrometers, heat-dissipation senfor the last 3 yr (2000-2002). Below 17 m, steady-state conditions sors, and tensiometers measure components of the total were observed at more than one-half of the monitored locations. Howenergy potential of water. The total energy potential of ever, long-term drying and wetting trends were also observed at 9 of water is the sum of the contributions of the gravitational, the 25 monitored locations below 17 m, in contrast to the steady-state pressure, and osmotic potentials (Hillel, 1980). Thermoflow assumptions in the conceptual model. Long-term water potential couple psychrometers measure water (matric and oschanges ranged from about 20 to 200 cm of water. It is hypothesized motic) potential in the Ϫ2000 to Ϫ80 000 cm of water that these drying trends are related to areas of focused infiltration, such as drainage ditches, and are a response to decreased runoff from Abbreviations: bls, below land surface; INEEL, Idaho National Engi-

Data from vadose-zone monitoring provide direct measures of soil water pressures, temperatures, and water fluxes. Vadose-zone monitoring can document waste site responses to changes in meteoric inputs of precipitation (rain and snowmelt), to assess the impacts of waterline leaks, or support ex-tank leak detection during retrieval operations. Since most flux rates in the vadose zone are relatively low and changes generally occur slowly, results will not be instantaneous. Meaningful data sets will require an extended monitoring period (several years or more). Based on current observations, however, data from the tensiometers indicate that drainage is occurring within the two monitored Tank Farms at the Hanford Site. Similar drainage conditions are expected at other Tank Farms at Hanford, where surfaces are coarse-textured and bare. As multiple years of data are collected, hydrologic monitoring systems with water fluxmeters will be able to provide a direct measure of annual recharge within Tank Farms and other waste sites, thus providing an early warning to the potential of future groundwater contamination. v

Soil water pressures, measured over space and time, are needed to predict the direction of water flow and chemical transport in the vadose zone. Advanced tensiometers (ATs), which utilize a water‐filled porous cup connected directly to a pressure transducer, can be installed at almost any location and depth using standard drilling techniques such as auger drilling, but these methods can significantly disturb the site. For sites where minimal disturbance is desired, alternate approaches for tensiometer placement have been sought. To test installation techniques and performance longevity, advanced tensiometers were placed into the ground at a test site near Richland, WA using two different installation methods, auger drilling and a drive‐cone push technique. The tensiometers were subsequently monitored for nearly 2 yr without refilling or recalibration. The data indicated that tensiometers placed by the auger technique took several months to equilibrate, while the cone push units came to equilibrium within 24 h following their installation. Soil water pressures always remained above −90 cm pressure head (−90 mbar) at depths >90 cm. At the greatest depth (730 cm), positive then negative pressures were observed as the water table was lowered and the soil drained. The results suggest that for our test conditions (coarse sandy soil, no vegetation), soil water pressures stay well within the tensiometer range and unit gradient conditions persist, indicating a draining profile. Advanced tensiometers, placed either by auger or cone penetrometer, provide a robust and reliable method for long‐term monitoring of soil water pressure profiles.

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Approaches for estimating liquid flux in the shallow (0–2 m) vadose zone are hindered by the high degree of spatial and temporal variability present near the land surface. It is hypothesized that high‐frequency variations in flux will be damped with depth. This study was conducted to estimate deep liquid flux using the Darcian approach at a waste disposal site in south‐central Idaho that is underlain by a complex sequence of unsaturated basalt flows intercalated with thin sedimentary layers. Flux is estimated by combining in situ water potential measurements from sedimentary interbeds located at depths of 34 and 73 m below land surface (bls) with laboratory estimates for the unsaturated hydraulic conductivity. Tensiometer data at seven locations indicated nearly constant conditions for 30 mo, while nine of the other 10 sites showed small gradual trends. Assumption of a unit hydraulic gradient led to flux estimates ranging from 0.2 to 10000 cm yr−1 Estimates in the 34‐m interbed ranged across four orders of magnitude while flux estimates for the 73‐m interbed ranged three orders of magnitude. While the tensiometer data appear to reflect in situ conditions and are a sensitive indicator of hydrologic conditions in the deep vadose zone, the laboratory‐developed hydraulic properties introduce a high degree of uncertainty, potentially affecting predictions by orders of magnitude. There is a need to develop techniques for assessing flux rates for the range of applicable field conditions to improve the confidence in deep flux estimates.