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.C,7 -? -- � sa'"..'tt '.��", R.' nr.£ .11 s% z �,.s�x� mss'-'- ��. %s E ' sufficient volume to exceed the zr,il binding capacity, free liquid will reach ground water, at which point it will float and may begin to vaporize and Bolubilize. The SVGA technir.b takes advantage of thin* and through the collection and analysis of Boil vapor perrd to a rapid, coat-affective delineation of the extent of contamination. On 18 November 1987, EA Collected and analyzed soil vapor frons ten locations and vapor from the two monitoring wells located on the tank field periphery (figure 3) . 3.1.1 SVG Methods Before each sample vas collected, a vacuum pump was used to purge previously collected vapor from the probe to ensure that the sample was not ca:,tatttinated. The vacuum pressure reading on the . purging apparatus was recorded. ::his 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 $oil 9&8 movement and a vacuum is being created between the tip of the probe and the pump. If this vacuum is maisitained through the Pumping period a representative aattple of the soil gas may not be obtained, and anemaloumiy low hydrocarbon readings may result. In most situations, initial vacuums are released and the vacuum pressure readings drop. A low raadEing indicates that there is a free flotr of soil gas from the vadose zone through the Probe, and after purging the sample is assumed to be - representative. In addition to obtaining soil gas samplen, well headspace .;�,amples were also analyzed for hydrocarbon contents._ -8efore--a-sample -teas- ta .en, the well was capped with a rubber stopper to prevent atmospheric air from entering it. A vacuum pump was ueed to purge the well slid draw soil gas through the Screened &actions of the well casing. Again, ano aalously lox results may be obtained if the well is improperly screened (e.g., the top of the well CXV15:8171