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Firestone Store#3573 404 North El Dorado Street, Stockton, California October 25, 1995 VES Quarterly Report and Request for Closure-4589 Page 7 of 14 The absence of a TPH-G concentration spike following the shutdown period indicates that no VOC vapors accumulated under standard temperature and pressure conditions as will exist following the closure of this site. 3.3.2 Benzene The Benzene Concentration Graph on Page 8 was created using Harvard Graphics—Best Fit software on the data points. All soil vapor analytical results for benzene (nondetect to 0.42-ppm) remained low throughout the entire SVES. operating lifetime. There was no discernable trend of either increasing or decreasing benzene concentrations during the operational period. This includes the analysis of the soil vapor sample collected following immediately the one month shutdown period. This indicates that no elevation in benzene concentration occurred due to dormant, equilibrium subsurface conditions. Based on site history, the gasoline release at this facility is believed to be relatively old (- 15-years). Benzene is highly volatile and readily biodegradable. Given this situation, natural processes may have substantially reduced the benzene levels at this site prior to implementation of the SVES. This may explain why the benzene concentration trend throughout the entire SVES operation is relatively constant. • 3.4 Estimated Hydrocarbon Mass Removed The total mass of hydrocarbons removed during the six month operation of the SVES is represented by the TPH-G results. The mass has been estimated using two methods The first method is based on the quantity of carbon expended during operation of the system. The second method is based on the off-vapor analytical results generated over the duration of system operation. 3 4.1 Mass Removed based on Adsorptive Carbon Usage Seven 200-pound canisters (1400-pounds total) of granular activated carbon (GAQ were expended during operation of the SVES Carbon manufacturers specify the gasoline adsorption efficiency of GAC at 20-percent. This means that each '100-pounds of GAC has the capacity to adsorb approximately 20- pounds of gasoline compounds. Therefore, the 1400-pounds of expended carbon (determined by VOC breakthrough) equates to a maximum of 280- pounds of gasoline extracted and treated by the SVES. It should be noted that the adsorption efficiency of GAC can be reduced by temperature, moisture content, and air flow characteristics. The actual mass of gasoline extracted is believed to be less than 280-pounds. RYAN-MURPHY, INCORPORATED Dearer Bora=WeMW Constriction Mit Lm Aggdks