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I i. 2.2.1 SVCA Sam lin i Before each sample was collected, a vacuum pump was used to purge Previously collected vapor from the probe to ensure that the sample was not contaminated. The vactxum pressure reading on the Purging apparatus was recorded. This vacuum -. pressure is related to the soil's gas permeability and is useful in data interpreta- tion. High vacuum readings suggest g g that there is a resistance to - soil gas movement and a vacuum is being created between the tip ;f of the probe and the um ? 3 pump. f this vacuum is maintained through the pumping period a representative sample of the soil s not be obtained, and anomalously low hydrocarbon readings may result. In most situations, initial vacuums are released and the i vacuum pressure readings drop. A low reading indicates that i there is a free flow of soil gas from the vadose zone through the "�- probe, and after purging the sample is assumed to be i s - representative. In addition to obtainingsoil gas samples, if there were monitoring wells on the site; well headspace samples were .also :- } analyzed for hydrocarbon contents. Before a sample was taken, the well was capped with a rubber stopper to prevent atmospheric s air from entering it. A vacuum pump was used to purge the well ' ' and draw soilz gas through the screened sections of the well casing.. Again,` anomalously low results may be obtained if the - 6 J well is improperly screened (e.g., the to of p the well screen ng is below the top of the water:table) , because soil gas will not be properly transported. J The soil vapor and monitoring well headspace samples were y collected through a septum with a microsyringe and in 'erted ' the gas chromatograph. A Photovac 10850 gas chromatograph was to ' used. for the 23 and 24 November survey: an HNu 421 for the shallow survey performed on 22 December. s EXXON9:84I; 8 + t s