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Tracer Research Carporatton <br /> i <br /> SHALLOW SOIL GAS MESTIGATION a 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 /> C. 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 possessing 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/liquid partitioning coefficients. Once in <br /> _. 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 /> surface may be locally distorted by hydrologic and geologic anomalies (e.g. clays, perched <br /> water}; however, soil gas mapping aenerally remains effective because distribution of the <br /> contamination is usually broader in areal extent than the Iocal geologic barriers and is <br /> defined using a large data base. The presence of geologic obstructions on a small scale <br /> tends to create anomalies in the soil gas-groundwater correlation, but generally does not <br /> obscure the broader areal picture of the contaminant distribution. <br /> EQiUIPMEIVT <br /> Tracer Research Corporation utilized a one ton Ford analytical field van which was <br /> equippedwith one gas chromatograph and two Spectra Physics SP4270 computing <br /> integratorsAn additioncthe van has-two built-in-gasolinee powered"aenerators which provide <br /> the electrical power (110 volts AC) to operate all of the gas chromatographic instruments <br /> ,� and field equipment. A specialized hydraulic mechanism consisting of two cylinders and <br />