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rill01 A' <br /> 1 - -W-,_,ovai I r_ Eam a <br /> '... Tracer perra®mrnh Carparmtinrs I j <br /> I }I <br /> i <br /> SHALLOW SOIL GAS INVEESTIGATION - METHODOLOGY <br /> Shallow soil gas investigation refers to a method developed by TRC for investigating <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 the ground <br /> through a hollow probe driven into the ground and analyzing the gas for the presence of <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 groundwater below the <br /> ` probe. The soil gas technology is most effective in mapping low molecular weight <br /> halogenated solvent chemicals and petroleum hydrocarbons possessin,high vapor pressures <br /> and low aqueous solubilities. These compounds readily partition out of the groundwater <br /> and into the soil gas as a result of their high gas f liquid partitioning coefficients. Once in <br /> t" the soil gas,VOCs diffuse vertically and horizontally through the soil to the ground surface <br /> where they dissipate into the atmosphere. The contamination acts as a source and the <br /> above ground atmosphere acts as a sink, and typically a concentration gradient develops <br /> between the two. The concentration gradient in soil gas between the source and ground <br /> ta surface may be locally distorted by hydrologic and geologic an (e.g. clays, perched <br /> 1.# <br /> water); however, soil gas mapping generally remains effective because distribution of the <br /> i contamination is usually broader in areal extent than the local geologic barriers and is <br /> r defined using a large data base. The presence of geologic obstructions on a small scale <br /> l b tends to create anomalies in the soil gas-groundwater correlation,but generally does not <br /> y obscure the broader areal picture of the contaminant distribution. <br /> t <br /> ` EQUIPMENT <br /> Tracer Research Corporation utilized a one ton Ford analytical field van which was <br /> t <br /> equipped with one gas chromatograph and two Spectra Physics SP4270 computing <br /> ( integrators. In addition, the van has two built-in Gasoline powered generators which provide <br /> the-electrical-power(110.v..olts.-.AC)_to._operate_ail af_the.-gas chromatographic instruments <br /> and field equipment. A specialized hydraulic mechanism consisting of two cylinders and <br /> 2 <br />