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sampling probes, and a sand pack to be emplaced around each probe. Also, augered probes can be installed in some lithologies where direct-push methods can fail (e.g., calcified sediments). However, augered installations can be more expensive and time- consuming than direct-push methods, and a longer re-equilibration time prior to soil vapor sampling is required to account for the introduction of oxygen into the sediment during the drilling process(see Section III Q. Prior to commencing drilling, it is critical to review the site groundwater elevation data to determine the highest historic groundwater elevation in the vicinity of the soil vapor sampling probe. For a single-level installation, the soil vapor sampling probe must be installed so that the vapor sampling screen is situated 2 to 3 ft above the highest measured groundwater elevation. This will reduce the likelihood that the probe will be submerged during periods of high groundwater elevation, and will enable the probe to sample that portion of the subsurface with potentially highest soil vapor VOC concentrations (i.e., a conservative sample). For multilevel probes,the deepest probe is situated 2 to 3 ft above the highest measured groundwater elevation, with shallower probes situated at defined intervals above the deepest probe (e.g., 5 ft intervals). For example, at a site where the highest measured groundwater elevation is 18 ft, probes could be emplaced at depths of 15, 10, and 5 ft below grade. A conventional drill rig equipped with a hollow-stem auger can be used for permanent soil vapor probe installation. Rotosonic and mud or air rotary drilling methods are not recommended since they can influence soil vapor sample results and/or alter the physical properties of the subsurface adjacent to the sampling probe. However, if Rotosonic and mud or air rotary drilling methods are necessary due to stratigraphic limitations (e.g., cobbles), sufficient time must be allowed between installation and sampling for equilibration to occur. During drilling, soil cores should be collected for lithologic and stratigraphic description, and for evaluation of soil porosity and moisture content for vapor transport modeling. Sediment samples should be collected and preserved for off-site chemical and physical analyses. Sampling interval selection is site specific, based on stratigraphic heterogeneity. Chemical analyses are chosen based on the site contaminants, but typically would include TPHg and BTEX for gasoline release sites. Physical analyses must include total porosity, dry bulk density, and air- and water-filled porosity sine these physical data are required for vapor transport modeling using the Johnson and Ettinger analytical model. Air permeability and total organic carbon (TOC) may be included (if necessary) for transient modeling purposes. Sediment samples should be collected in stainless steel liners and capped with Teflon sheeting and plastic end caps and placed in re-sealable plastic bags. The liners should then be stored in iced coolers and transported to a certified laboratory under chain-of-custody documentation. After the borehole is drilled to its maximum depth, the deepest soil vapor sampling probe is installed (Figure 1). Each sampling probe should consist of a 3/8-inch diameter by 6 inch long stainless steel screen with stainless steel end caps(PVC screen and end caps are also acceptable, as are ceramic aquarium"bubbler"devices). A Swageloo type fitting is ETC Soil Vapor Sampling Technical Toolkit 5 Version 1.4, September 7,2005