_+_+e_ooo_'=_++_o_o_oeoo+_+_Q_+_+_e_eoo*o_+_=_++_+o+e_++_o_+_+_+++4+o samples to 61, improving the statistical reliabilityof the neutron probe calibration. Samples from the FY-1993 calibration also providedadditional moisture content measurements from the drier portionof the soil moisturerange. Field methods, laboratorymethods, and calibration results are presentedin the following sections. Field Methods Four neutron probeaccess holes(DF, DF2,DF3, andDF4) werehandaugered through the surficialsedimentsto theunderlying basalt.The neutron access holeswerelocatednorthof the SDA nearborehole W10 (Figure2). BoreholeDF wasaugeredon January13, 1993 to a toud depth(TD) of 5.5 ft, borehole DF2 wasaugered January14, 1993toa TD ofT.0 ft, boreholeDl:3 wasaugered February 4, 1993toa TD of 4.4 ft, andborehole DF4 wasaugered March2, 1993_Lo a "I'Dof 4.2 ft. The complete procedure used to install the access tubes and collect soil samples is attached in Appendix A. A summaryof the process is provided below. The neutron access holes were hand augered to each sampling depth using a 2-in. outside diameter (OD) sand head auger. The hole was sampled at 1-ft intervals using a 2-in. OD stainless steel core sampler containing four 1.5-in. OD aluminium sample rings. The sample rings were placed in the core sampler in the following configuration: one 1-in. (length) ring at the top, two 2in. (length) rings in the middle, and one 1-in. (length) ring at the bottom. The two 2-in. (length) rings in the middle of the configuration were carefully trimmed, capped, taped, and bagged for laboratory determination of moisture content. The 1-in. (length) ring samples were bagged for lithologic logging. Each borehole was augered to refusal, whore basalt was encountered. A 1.5 in., carbon steel, seamless, Schedule 40 [1.61-in. inside diameter 0D), 1.900-in. OD] pipe was then pushed to total depth, leaving 2 ft of pipe extending above land surface. Soil removed fromthe borehole during augering was pushed down the annular space (between the pipe and the hole) in the upper portion of the hole, where sampling activities had widened the hole, and the soil was tamped down. A Campbell Pacific Northwest neutron probe (Model 503 DR Hydroprobe)was then used to log the borehole at the sampled depths. A fast neutronsource (50 mCi Americium-241/Be) from the neutron probe was lowered down the 1.5-in. diameter tube. The source emits fast neutrons which are slowed by hydrogen nuclei in the soil matrix. The resultant thermalized (or slowed) neutrons are then counted by a detector in the Hydroprobe. Three measurements were taken at each sampled depth with the neuu'o_,probe. A standard count was also taken at each site priorto logging the hole with the neutron probe. The neutron probe access tubes for hole DF, DF3, and DF4 were removed and the holes were backf'tlledwith soil. The neutron probe access tube in DF2 (TD of 7 ft) was left in the hole to "OIA__loq_Joq _u t,_IOP_ 'fAG '_IO '_IO soIoq uo91uq.q_ _oid uoanou jo uo.ne_sy-I "i[ejnOl4°_ " VGS _OM provide a monitoring lo...

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-

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.

Waste disposal sites with volatile organic compounds (VOCs) frequently contain contaminants that are present in both the ground water and vadose zone. Vertical sampling is useful where transport of VOCs in the vadose zone may effect ground water and where steep vertical gradients in chemical concentrations are anticipated. Designs for combination ground water and gas sampling wells place the tubing inside the casing with the sample port penetrating the casing for sampling. This physically interferes with pump or sampler placement. This paper describes a well design that combines a ground water well with gas sampling ports by attaching the gas sampling tubing and ports to the exterior of the casing. Placement of the tubing on the exterior of the casing allows exact definition of gas port depth, reduces physical interference between the various monitoring equipment, and allows simultaneous remediation and monitoring in a single well. The usefulness and versatility of this design was demonstrated at the Idaho National Engineering and Environmental Laboratory (INEEL) with the installation of seven wells with 53 gas ports, in a geologic formation consisting of deep basalt with sedimentary interbeds at depths from 7.2 to 178 m below land surface. The INEEL combination well design is easy to construct, install, and operate.