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S EtC ® fR 7-Eleven Store#f19976 . Quarterly Groundwater Monitoring and Remediation Progress Report(4Q2003) March 4, 2004 Page 3 Soil boring SB-1 and nested wells MW-4/4A and MW-5/5A were installed in May 1999, as requested by the SJCPHS, to provide additional lateral and vertical plume delineation Sod boring SB-1 was advanced to 65 feet bgs near the UST area, to evaluate the vertical extent of the plume The nested wells were installed to access the saturated zone at depths of 10-30 feet bgs (MW-4 and MW-5), and 45-50 feet bgs (MW-4A and MW-5A) Soil sample data from boring SB-1 indicated that the base of the benzene, TPHg, and MtBE/oxygenate plume lies at between 40 and 55 feet bgs, as evidenced by non-detectable concentrations of all constituents Samples from well MW-5 contained low concentrations of gasoline constituents from 30-35 feet bgs, with no detectable concentrations at deeper depths (Shaw, 1999) Well MW-6 was installed in December 1999, to a depth of 30 feet bgs (Figure 2) Benzene and MtBE were not detected above laboratory reporting limits in soil samples The maximum TPHg concentration in sod was reported at 20 feet bgs (15 4 mg/kg) Delineation of hydrocarbons in soil is considered complete at this time (Shaw, 2000) On September 18, 2000, as requested by the SJCEHD, eleven sod-gas survey points were advanced to 3 feet bgs Significantly elevated MtBE concentrations (2,200 pg/L) were detected in only one of the points (SG-9), located along the western edge of the UST pit about 20 feet south of MW-2 On November 1, 2000, monitoring well MW-7 was installed to a depth of 30 feet bgs and soil boring SB-2 was drilled to a depth of 50 feet bgs (Figure 2) Sod samples analyzed from both well MW-7 and sod boring SB-2 did not contain benzene, toluene, ethylbenzene, xylene (BTEX), TPHg, and MtBE above laboratory reporting limits A grab groundwater sample was collected from boring SB-2 s^^i at a depth of 40 feet bgs The water sample contained total xylenes at a concentration of 1 5 pg/L Soil vapor extraction (SVE) testing conducted in 2001 using the two horizontal vent wells installed in the former UST tank pit (IT Corporation, September 2001) indicated extracted vapor concentrations of over 10,000 parts per million by volume (ppmv) at applied vacuums ranging between 9 and 32 inches of water Induced vacuum responses of more than 0 2 inches of water were observed in wells MWA, MW-2, and MW-3 during extraction from the horizontal wells Vacuum response in these wells is likely facilitated by relatively higher-permeability conditions in the tank cavity backfill as X�' opposed to native sods Testing was also conducted on the groundwater monitoring wells, lower permeability and limited screen above the water table likely account for low extraction rates/low induced vacuum responses observed during extraction from the wells Extracted vapor y concentrations ranged up to 0 91 ppmv benzene, 220 ppmv MtBE, and 880 ppmv TPHg The dataf indicated that vapor extraction is a viable remediation method at the site, particularly when the horizontal wells are used Double-nested sparge well SPA was installed on October 29-30, 2001 to a depth of 50 feet bgs and screened from 30-35 and 45-50 feet On February 6, 2002, an air sparge test was conducted using well MW-2 for vapor extraction, and double-nested sparge well SPA for air injection Pre-sparging extracted vapor levels ranged up 1,000 ppmv, with dissolved oxygen (DO) levels of up to 0 72 mg/L v Air was then injected into the deeper section of well SPA using an oil-less air compressor Vapor levels in well MW-2 increased to 1,600 ppmv, and DO levels to 8 mg/L, after 30 minutes of sparging The data indicated the area of sparging influence in SPA includes the region of MW-2, and that treatment using well SPA should be successful at reducing dissolved hydrocarbons/oxygenates in well MW-2 and the surrounding area Air sparge testing was repeated on May 23, 2002, using well MW-1A as the injection point and the horizontal wells as the SVE point Results indicated no breakthrough of sparged air into the vadose zone accessible by the horizontal wells 19976 03-4Q Report SECOR International Incorporated