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(effectively containing the groundwater plume on-site and preventing or reducing future migration of contaminants) and expose hydrocarbon -impacted soil to vapor extraction The relatively high yield of groundwater extraction (approximately 4 gpm maximum sustainable yield with a radius of influence in excess of 100 feet) will result in the ability to capture and remediate larger volumes of water over a relatively short period of time (three years) ' 7.1 Soil Vapor Extraction 7 1 1 Summary of Major Components for VES The extraction wells are connected to header lines which, in turn, are connected to a blower/thermal oxidizer system with the ability to change over to catalytic oxidation as soil vapor concentrations drop below 3,500 ppmv (equivalent to approximately 25% of the LEL) Soil vapor extraction testing performed by Alton in June 1995 detected average soil vapor concentrations of approximately ' 5,000 ppmv, indicating that a thermal oxidizer system capable of handling higher concentrations of influent soil vapor(assuming no dilution air) is necessary for the Site A vacuum is applied by the treatment unit blower to the header lines Soil vapor is then extracted by the blower from the area around each well through the perforations in the well pipe (negative pressure) Such treatment units are designed for soil vapor extraction and groundwater remediation 1 projects and other VOC destruction applications where combustion of the volatiles has been specified as the most cost-effective method These units operate automatically The proposed treatment unit for this Site has been designed to run on supplemental fuel (natural gas or propane) As remediation proceeds at the Site, and as concentrations in the process gas begin to decline, it may become more cost effective to utilize an alternative treatment technology such as activated carbon adsorption The decision to change the off-gas treatment system will be based on the results of the on-gomg monitoring program It is anticipated that when hydrocarbon concentrations in the process gas begin to approach 500 ppmv, an alternate off-gas treatment technology (such as carbon filtration) can be considered Changes in system operation will be formally addressed with the San Joaquin Valley APCD prior to i modification 7.1 2 Well Installation Spacing Alton's extraction feasibility testing of the extraction well VW-3 (screened interval of 21 to 36 feet ' bgs) indicated that there was a vacuum response in the upper vadose zone's screened intervals of adjacent monitoring wells (near or at 0 1 inches of water column) approximately 33 feet from the extraction well Vapor extraction well radii of influence are indicated on Figure 16 A.11528J1 ASA 18