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2 . SOIL VAPOR CONTAMINANT ASSESSMENT 2 . 1 SVCA SAMPLING On 30 June 1988, EA conducted an SVCA at an undeveloped lot ( Chevron property SS 9-0265 ) . Soil gas samples were collected and measured at 17 locations on a grid (Figure 4 ) . The sampling points were spaced 50-75 feet apart. Sample depths ranged from 3 to 13 feet; most of the points were at 5.5 feet. Vertical profiles were made at the northern, west- ern, and eastern boundaries of the site. 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 vacuum reading on the purging apparatus was recorded. This vacuum is related to the soil ' s gas permeability: a high reading suggests 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 gas may not be obtained, and an anomalously low hydrocarbon reading may result. In most situations, the initial vacuum is released and the vacuum reading drops. 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 representative. The samples were collected through a septum with a microsyringe and injected into an HNU 421 chromatograph for analysis. The HNU 421 is a laboratory-size, temperature-programmable gas chromato- graph equipped with a flame ionization detector (FID) . The hydrogen-air flame ionizes compounds, generating an energy increase in the detector, which appears as an electrical signal. The gases are separated on an analytical column, sensed by the detector, integrated, and reported as individual compounds on c40/90265 5