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N1,111 1 ,.. calibrated with standards consisting of 10 ppm benzene and 10 ppm toluene in air. For the reading of each standard, the in.,trument stores the known concentration and the V-sec response to it. The ratio for the standard, V•-sec:ppm, is then used to quantify the concentrations of identifiable vapors in field samples according to their V-sec values. The concentrations of unidentifie, compounds are calculated in a similar manner. Although petroleum hydrocarbons produce variable instrumental responses, the assumption may be made that all of _ -wwthe hydrocarbon constituents have response-to-concentiration ratios approximately equivalent to that of benzene and-that all. -- quantifications ll ------quantifications may be based on the ratio for benzene. In the table of results, the column entitled "Peaks Prior to Benzene" represents tnc sum of the responses in V-sec for all peaks eluting prior to benzene, proportioned to the calibrated V-sec response for benzene. Similarly, the column entitled "Total Detected Petroleum Hydrocarbons" or "Total Volatile Hydrocarbons" represents the .;um of all V-sec responses, proportioned to that for benzene. The gas chromatograph is operated in backflush mode to prevent contamination of the analytical column with high concentrations of interfering cc�r.pounds. Blanks are run to ensure that the system is free of contamination. As necessary, the instrursent is re-calibrated by injecting standards and by running ambient air blanks approximately every two hours through the day. This re-- calibration ensuras that the system is operating consistently and that parametric changes caused by temperature changes through the day are accounted for. These data, along with multiple standard runs, ensure system reproducibility. 2.2.2. SVCA Results The highest valuer of hydrocarbon concentration vere obtained from the headspace of the mouitoring wells (Table 2) . These CHV18:81J 7