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the VES has been in operation and the rate at which vapors were withdrawn: removed HC = mean PID value x . 43 x hours x cu ft/min x 60 min/hr I The total volume of removed hydrocarbons is thus the sum of the volumes for each time interval . The volume calculated in this way is 19 , 314 cu ft of hydrocarbon vapors . 1 4 . 5 Current Rate of Hydrocarbon Extraction 1 The current rate of extraction can be determined by multiplying the the PID reading by the rate of vapor withdrawal and the PID/hydrocarbon ratio: . 000450 x 65 cu ft/min x . 43 = . 0125 cu ft/min 5. 4 CONCLUSIONS & RECOMMENDATIONS Comparison of hydrocarbon vapors measured at the VES inlet with the concentration of vapors measured in actual gas samples indicates that actual concentrations are roughly 43 percent of those measured at the inlet. If it can be assumed that this ratio has remained constant throughout the 2-year operation of the VES, the initial concentration in the inlet stream when operation started was approximately 1075 ppm, and the present concentration is approximately 100 ppm. This value cannot be directly related to the TPH concentration remaining in place with present data. Prior field experience with vapor extraction has shown that vapor extraction systems become less efficient as hydrocarbon concentrations decline. When concentrations decline below 100 ppm, most of the remaining hydrocarbons are long-chain, heavy molecules that do not volatilize readily. Further operation of the system is of little benefit This pattern is evident in Figure 3 , which shows that the rate of decline of vapor concentrations has decreased considerably over time and is now almost flat. Continued operation of the VES would probably not be cost-effective and is unlikely to result in significant further reductions In hydrocarbon contamination. GeoAudit recommends that representatives of Genova Bakery request a meeting with San Joaquin PHS/EHD to discuss termination of the operation of the VES. The meeting could also be used to discuss the overall status of the remediation effort prior to proceeding. I • 6 . 0 LIMITATIONS ` Our professional services were performed using that degree of care and skill ordinarily exercised by environmental consultants 4