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- # gg- h� response for benzene. Similarly, the column entitled "Total Detected petroleum Hydrocarbons" or "Total Volatile Hydrocarbons" .represents the sum of all V-sec responses, proportioned to that for benzene. E The Photovac is operated in backflush mode to prevent contamina- tion of the analytical Column with high concentrations of f interfering compounds. Blanks are run to ensure that the system is free of contamination. As necessary, the instrument is re- Calibrated by injecting standards and by running ambient air blanks approximately every two hours through the day. This re- calibration ensures that the system is operating consistently and that 'parametrie changes caused by temperature changes through the day are accounted for. These data, along with multiple standard runs, ensure system reproducibility, :A ��. 2.2.2 SVCA Results The SVCA chromatograms are. in Appendix A; the results of the .23 and 24 November survey are summarized in Table 3. Because ground ` �. water in this area was reported between .47 and 50 feet in the `' well logs, the soil vapor. probes were driven until refusal was encountered at 15.5 to 23.5 feet (see Table .3 E ) . No significant hydrocarbon contamination was detected at any of the vapor j sampling les locations. The heads e samples, �' p yielded low-to- LJ moderate hydrocarbon concentrations. Weekly gauging of the . monitoring wells has shown no evidence of floating r g Product E _E To verify the absence of hydrocarbons in shallow soils, EA i conducted a shallow (3 to 5.5 feet below land surface) SVCA on 22 . z December-1987 -These results are a ' presented in Table 4. Sampling locations were chosen inside of the former product :line trenches kf� a At points between the tank area and the pump islands ;(VI r j -V11, Figure 4) . No significant contamination was detected in { the shallow soil vapor samples. { L,f EXXON9:B4I, 10 .r IM11111 Ills ME r