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F gg Ig ISM_ ME1 2.2.1 SVCA Sampling Before each sample was collected, a vacuum um was used tri pump pu cge preciously collected vapor from the probe to ensure that the sample was not contaminated. The vacuum 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 that there is a resistance to soil gas movement and a vacuum is being created between the tip of the probe and the pump. If this vacuum is maintained through the pumping period a representative sample of the soil p gas may not be obtained, and anomalously low hydrocarbon readings may l result. ID most situations, initial vacuums are released and the vacuum '.pressure readings drop. A low reading indicates that there is a free flow of soil gas from the vadose zone through the probe, and after purging the sample is assumed to be _ X representative. In addition to obtaining soil gas samples, if there were y monitoring wells .on the site, well headspace samples were also i analyzed for hydrocarbon contents. Before a sample was taken, the well was capped with a rubber stopper- . Aper to prevent atmospheric air .from entering it. A vacuum pump was used to # purge the well and .draw soil gas through the screened sections of the well i casing. Again, anomalously low results may be obtained if the 3 well is improperly screened (e.g., the top of the well screening + is below the to of the water table) , because soil gas will not be properly. transported. F - -- The soil vapor-and -monitoring well headspace .samples.were - collected through, a septum with a microsyringe and injected into the gas chromatograph. A..Photovac ,1OS50 gas_.chroma us lfor the 23 and 24 November surve tog�aph y; an HNu 421 for the shalwas ow survey performed on 22 December. 3j ,M- 1 EXXON9:84L 8 �a n }y