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Firestone Store#3573 400 North El Dorado Street, Stockton, Cahforma October 25, 1995 VES Quarterly Report and Request for Closure-45$9 Page 11 of 14 4.0 DISCUSSION Based on the VOC concentration trend, it is clear that SVES operation has substantially reduced the level of VOCs present in the subsurface environment at this site As discussed in this section, VOC reduction and site-specific conditions appear to support site closure The SVES operation is believed to have achieved the level of remediation possible from a technical efficiency standpoint During the initial startup phase of SVES operation (November - December 1994), soil vapor VOC concentrations decreased significantly During the remaining operational period (January - May 1995), including the shutdown period, soil vapor concentrations remained low and essentially constant The constant VOC concentrations over time represent an asymptotic state in which continued system operation did not reduce the concentration of VOCs in soil vapor The system was operated and monitored throughout this time period to establish and confirm that this ' asymptotic state had been achieved RMI anticipates that continued SVES operation at this time will produce no discernable reduction of soil vapor VOC concentrations ' RMI believes that a large percentage of the total subsurface hydrocarbon mass has been successfully removed As discussed previously, the hydrocarbon mass removed by the SVES operation is estimated to be between 195- and 280-pounds The total mass of the hydrocarbon release is unknown, therefore, a direct calculation of the mass remaining is not possible However, an estimate of the remaining hydrocarbon mass can be generated by applying the known concentrations of VOCs remaining in the soil vapor, the calculated concentrations of VOCs remaining in the soil pore water, and solid soil phases across an estimated volume of impacted subsurface material The volume of impacted subsurface material is believed to be defined by a vertical cylinder with a horizontal 20-foot radius and a depth of 30-feet based upon field observations This results in an estimated impacted soil volume of approximately 1400-cubic yards Applying the remaining VOC concentrations in the soil vapor, soil pore water, and solid soil phases to this volume yields a remaining VOC mass of approximately 5-pounds These calculations suggest that over 95-percent of the total volatile hydrocarbon mass existing in the subsurface at system startup has been removed by SVES operation at this site (Total Remaining TPH Mass Calculations, Appendix 7 9) The calculated remaining concentrations of TPH-G and benzene in pore water are 0 364- mg/L and 0 0024-mg/L, respectively The benzene level is below the EPA Maximum Contaminant Level (MCL) of 0 005-mg/L There is no EPA MCL for TPH-G 1 RYAN-MURPHY, INCORPORATED Dever FWw"o ental Construction Management Lw Angeks