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Remedial Action Plan Canepa's Car Wash, 6230 Pacific Avenue, Stockton, California March 3, 2003 Page 4 The site groundwater gradient is to the northeast and the groundwater elevation contours from the January 2003 sampling event are shown in Figure 2. The average depth to groundwater measured was approximately 36 feet bgs, 20 feet higher than the groundwater depth in early 1993 , as shown in the site I hydrograph, Figure 3 . Figures 4 through 6 provide general geologic cross sections. Contaminant groundwater isoconcentation contours are provided in Figure 7. ? � In the Problem Assessment Report and Corrective Action Plan for Canepa's Car Wash (PAR/CAP) dated October 18, 2002, the mass of benzene calculated in the groundwater was approximately 6.7 gallons. i, 3.0 TREATMENT SYSTEM 3.1 STATEMENT OF QUALIFICATIONS ` i The PAR/CAP evaluated the potential remedial design alternatives and concluded that pump and treat with soil vapor extraction was the technically-feasible, cost-effective remedial strategy. Pump and treat has an extensive history as a remediation strategy. The traditional concern with a pump and treat strategy is the difficulty in reaching site closure concentrations in a reasonable time frame and with reasonable total project cost. However, the significant rise in groundwater elevation, resulting in the saturation of previously contaminated vadose zone soil, indicates that some groundwater drawdown in conjunction with soil vapor extraction could significantly reduce site contaminants in addition to mitigating further rc down-gradient plume migration. -� 3.2 TREATMENT SYSTEM DESIGN The proposed treatment system is comprised of two extraction wells with submersible pumps, an air stripping unit, and a thermal oxidizer, in combination with the existing vapor extraction system. The 1 extraction wells are the two 44nch diameter wells on site: MW- 1 , screened from 50 to 70 feet below l ground surface (bgs), and MW- 11 , screened from 25 to 45 feet bgs. Underground piping will also be installed to MW-2, a 2- inch well screened from 50 to 70 feet bgs, as an optional extraction point. The treatment system location and conceptual design are shown in Figures 8 and 9, Appendix A. The design is intended to remove the air-borne contaminants through air stripping and treating both the air stripper exhaust and the vapor extraction exhaust in the thermal oxidizer. ; I The area in which the system will be located is restricted by width, so the treatment train will be assembled side by side. Although we are still soliciting equipment bids, the current design uses Sofleco equipment mounted on 4-foot by 6-foot skids. r The vapor extraction unit is a 300 ACFM Liquid Ring Blower. This is a very quiet blower minimizing the noise impact to the surrounding businesses and residences. The unit runs on 240 volts and has a maximum capability of 300 ACFM and up to 29 inches of mercury. The thermal oxidizer is also rated at 300 ACFM and will be fired by natural gas plumbed to the site, and includes a built-in differential pressure transmitter with pitot tube for flow measurement and a chart recorder. The air stripper includes a j 210 ACFM blower and six trays designed to handle flows from I to 40 gpm. Total flow is designed to be t less than 10 gpm. 3.3 PERMITTING AND MONITORING The system, as designed, will require a revision of the existing air discharge permit and a sanitary sewer permit. The system is expected to operate 24 hours a day, seven days a week. Discharge to the sanitary sewer of more than 10 gpm results in characterization as a major discharger with added requirements, so the discharge will be kept to below 10 gpm. As noted in the PAR/CAP, a pumping rate of 5 gpm is � � CONDOR