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i <br /> Tracar Remaarch Corporation <br /> t <br /> w <br /> r BACKGROUND rM THE METHODOLOGY <br /> f The presence of volatile organic chemicals (VOCs) in shallow <br /> soil gas indicates the observed compounds <br /> vadose zone near the may either be in the <br /> �! probe or in groundwater below thepro be. The <br /> soil gas ter_hnologY is most effective in <br /> [� mapping low molecular <br /> ! weight halogenated solvent chemicals and petroleum hydrocarbons <br /> '1 possessing ,high vapor pressures and low aqueous solubilities. <br /> These compounds readily partition out of the groundwater and into <br /> the sail gas as a. result oftheir high_ e <br /> 9 gas/liquid partitioning <br /> coefficients. Once in the soil gas, VOCs diffuse vertically and <br /> r horizontally through the soil to the ground surface where .they <br /> 4 ' dissipate into - <br /> the atmosphere: The contamination acts as a <br /> -; source and the above ground atmosphere acts as a. <br /> sink., and <br /> typically a concentration gradient develops between the two. The <br /> \; concentration gradient in soil gas between the. source and ground <br /> surface <br /> may be locally distorted by hydrologic and geologic <br /> ' anomalies (e.g. clays, perched water) ; howev <br /> er, sail gas mapping <br /> . . . ... <br /> i <br /> generally ' remains effective because <br /> distribution ',. of _ <br /> tine <br /> contamination is usual; broader in areal <br /> y. extent than the local I <br /> geologic barriers and is de <br /> fined' <br /> u5in a <br /> 9 large data base. The , <br /> presence of geologic obstructions Lewan a small scale tends to <br /> - create anomalies in the soil gas-groundwater correlation, but - <br /> generally does not obscure the . broader ,-areal picture of the . <br /> contaminant distribution. <br /> 1 c. <br /> aax. <br />