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199 `'i's R 20� sufficient volumes to exceed the roil binding C,'Par-"Y, free-, liquid will reach ground water, at which point it will. float And may higin to vaporize and nolubilj-.c. The SVCA technique takers advantage of thin, and through the colxj!ction and Analynia of 3ojj vapor permits a rapid, cost-effective delineation of the extent of contamination. On 18 November 1987, CA collected and analyzed soil vapor from ton locations and vapor from the two monitoring wells located on OR the tank field periphery (Figure 3) . 2.1.1 SVCA t,-1th0d-" Before each trample: was collected, a vacuum pump was used to purge previously collected vapor from the probe to ensure that the sample wan not contaninated. 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 in 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 sarnple of the soil gas may not be obtained, and anomalously low hydrocarbon readings may result. In 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 la assumd to be representative. In addition to obtaining soil gas 58MPICS, well headspace ,a&mPles were also analyzed for hydrocarbon contents. Before a sample was taken, the well was capped with a rubber stopper to prevent atmospheric air from entering it. A vacuum PUMP was used to Lod sections Of purge the well and draw soil gas through the acreen the well casing. Again, anomalously low results may be obtained if the well is i.—.properly screened (e.g., the top of the wall CHV16:81A