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r <br /> 47 PAF <br /> �y <br /> Tracor p,aeeroh Corporation <br /> J <br /> SHM-LOW SOIL GAS INVESTIGATION . METHODOLOGY <br /> Shallow soil gas investigation refers to a method developed by such <br /> investigating <br /> mdusm� <br /> underground contamination from volatile organic chemicals <br /> o icmi� for(their vapors in the shallow <br /> solvents, cleaning fluids and petroleum product y <br /> in a small <br /> amount of soil gas out of the ground <br /> soil gas, The method involves pump g the far the presence of <br /> through a hollow probe driven into the ground and analyzing g <br /> volatile contaminants. The presence of VOCs in shallow soil gas indicates the observed <br /> compounds may either be in the vadose zone near the probe or in oWn�fl���larbelow <br /> wetght <br /> probe The sail gas technology is most effective in mapping vapor pressures <br /> halogenated solvent chemicals and petroleum hydrocarbons possessing highP P <br /> he groundwater and <br /> and low aqueous solubilities. These compounds readily Paan ocoeffic en out oftnts Once in the oil <br /> into the soil gas as a result of their high gas/liquid partitioning <br /> gas, VOCs diffuse vertically and horizontally through the son t�ha rosoued surface where <br /> the above <br /> they dissipate into the atmosphere. The contamination ac adient develops between <br /> ground atmosphere acts as a sink, and typically a concentration gr <br /> the two, The concentration gradient in soil gas between the source and groud surface <br /> ace may <br /> }, <br /> + geologic an (e.g. clays, perched <br /> be locally distorted by hydrologic and g <br /> however, soil gas mapping generally remains effective because distributiau of the <br /> barriers and is <br /> contamination is usually broader in areal extent than the local geologic <br /> defined using a large database. The presence of geologic obstructions on a small scale tends <br /> to create anomalies in the soil gas-groundwater correlation,but generally does not obscure <br /> the broader areal picture of the contaminant distribution. <br /> Soil gas contaminant mapping helps to reduce the time and cast required to delineate <br /> underground contamination by volatile contaminants. The soil gas ational bore holes nvestigation does is <br /> or <br /> by outlining the general areal extent of contamination. Con <br /> observation wells are used to verify both the presence and extent of the subsurface <br /> 2 <br />