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VVZO INC <br /> 30 SOIL GAS SAMPLING METHODS AND PROCEDURES <br /> 31 Methods Available <br /> ' Two similar, but distinct methods are available to be used b WZI for obtaining soil as <br /> Y g 9 <br /> ' samples from the subsurface These include (1) sampling directly through the probe rods <br /> using an expendable or retractable drive point and (2) sampling through small-diameter <br /> ' tubing run down the probe rods (post-run tubing system [PRT]) and connected to the probe <br /> tip in conjunction with ether a retractable or expendable drive point The retractable drive <br /> point is useful in that independent samples can be obtained from different depths within <br /> the same borehole (i e , vertical profiling), however, the tool's short sampling interval <br /> (approximately 6 inches) limits its usefulness to relatively permeable, coarse-grained <br /> lithologic materials The retractable drive point is also limited in contaminated source <br /> ' areas where drag down (inclusion of soil in the sampling devise) may cause cross <br /> i <br /> contamination of samples <br /> When considering the first two methods, the PRT system is often preferred because it <br /> eliminates potential problems with rod leakage at threaded connections, multiple drive <br /> locations and decontamination issues, and also allows for smaller soil gas purge volumes <br /> However, sometimes the first method (i e , sampling through the probe rods from an j <br /> exposed formation) must be used in source areas to minimize drag down and possible <br /> cross contamination or to overcome smearing of the borehole wall that can occur in finer- <br /> grained IEthologic materials Smearing can significantly reduce the permeability of the <br /> formation, which, in turn, can make it difficult to obtain a sal-gas sample With an open <br /> probe system, small brushes can be inserted down through the probes on narrow <br /> extension rods to the exposed borehole, and the formation "developed" by brushing the <br /> ' borehole walls This borehole development is very effective and allows soil-gas sampling <br /> to be performed where it might otherwise be impractical <br /> APPENDIX Ill <br /> 12270020 025 4 <br /> 1 <br />