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WO B. Permanent and temporary probes using direct-push techniques Permanent sampling probes can be installed with a direct-push rig (e.g., Geoprobel). Slam bar techniques should not be used. Using a single direct-push point that is sampled for soil vapor and then immediately pushed to the next depth interval for further sampling is prohibited for human health risk assessment since this method does not allow for multiple samples to be obtained from a given interval over a specified time period (e.g., months). Human health risk assessment purposes require an individual, permanent soil vapor sampling probe for each depth interval sampled. A multilevel direct-push probe installation for human health risk assessment simply consists of a series of single-level probes located within a short lateral distance of each other(i.e.,5 2 ft). These probes can be sampled multiple times to develop time series plots of soil vapor VOC and fixed gas (e.g., 02, CO2) concentrations and to investigate biodegradation as a function of depth. Soil vapor surveys using direct push techniques (non-permanent installations) are sometimes useful in determining the depth and location of subsequent, permanently installed sampling probes for human health risk assessment. A drawback to using direct-push probes for multilevel sampling is that a larger footprint is needed for installation compared to auger techniques. Also, direct-push installations can fail if the subsurface contains cobbles or is rigid. Furthermore, it may be difficult or impossible to obtain sediment samples for chemical and physical analyses using direct- push techniques, and it is difficult to install sand packs and seals through direct-push rods. However, direct-push probes can be installed more quickly than augered installations, and disruption of the subsurface is minimized, resulting in shorter required re-equilibration times prior to soil vapor sampling. Direct-push installations are optimal when shallow-depth (e.g., 5 ft) soil vapor samples are required, or when soil vapor samples are analyzed at an on-site, mobile laboratory, thus enabling real-time, on-site adjustments to a sampling program. 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. Permanent probes must be installed so that the vapor sampling screen is situated 2 to 3 ft above the highest historically 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). Temporary probes should be installed approximately 1 ft above the existing water table or at any other designated depth in the vadose zone. Direct-push techniques involve using a truck-mounted hydraulic ram to push hollow metal rods equipped with a drive point to a designated depth. There are two common methods of installation: 1) permanent implants (tubular screen probes), and 2) the Post Run Tubing System(PRT). ETC Soil Vapor Sampling Technical Toolkit 8 Version 1.4,September 7,2005