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Tracer Research Corporation <br /> i <br /> SHALLOW SOIL GAS INVESTIGATION - METHODOLOGY <br /> Shallow soil gas investigation refers to a method developed by TRC for investigating <br /> t <br /> underground contamination from volatile organic chemicals (VOCs) such as industrial <br /> solvents, cleaning fluids and petroleum products by looking for their vapors in the shallow <br /> soil gas. .The method involves pumping a small amount of soil gas out of ground <br /> through a hollow probe driven into the ground and analyzing the gas for the presence of <br /> l ' <br /> volatile contaminants. The presence of VOCs in shallow soil gas indicates the observed <br /> i <br /> compounds may either be in the vadose zone near the probe or in groundwater below the <br /> a <br /> probe. The soil gas technology is most effective in mapping low molecular weight <br /> halogenated solvent chemicals and petroleum hydrocarbons possessing high vapor pressures <br /> and low aqueous solubilities. These compounds readily partition out of the groundwater and <br /> into the soil gas as a result of their high gas/liquid partitioning coefficients. Once in the soil <br /> gas, VOCs diffuse vertically and horizontally through the soil to the ground surface where <br /> they dissipate into the atmosphere. The contamination acts as a source and the above <br /> ground atmosphere acts as a sink,and typically a concentration gradient develops between <br /> the two. The concentration gradient in soil gas between the source and ground surface may <br /> f be locally distorted by hydrologic and geologic anomalies (e.g. clays, perched water); <br /> however, soil gas mapping generally remains effective. because distribution of the <br /> contamination is usually broader in areal extent than the local geologic barriers and is <br /> The presence of geologic obstructions on a small scale tends <br /> defined using a large database. p g g <br /> to create anomalies in the soil gas-groundwater correlation, but generally does not obscure <br /> I the broader areal picture of the contaminant distribution. <br /> Soil gas contaminant mapping helps to reduce the time and cost required to delineate <br /> underground contamination by volatile contaminants. The soil gas investigation does this <br /> by outlining the general areal extent of contamination. Conventional bore holes or <br /> f observation wells are used to verify both the presence and extent of the subsurface <br /> 2 <br />