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1 ' Second Quarter 2002 Monitoring, Sampling,and Interim Remediation Report 3 7-Eleven Inc Store#19976, 1399 North Main Street, Manteca,CA July 30 2002 • ' Upon receipt of these data, the SJCPHS requested further lateral delineation Well MW-6 was Installed in December 1999, to a depth of 30 feet bg (Figure 2) Soil samples collected during well Installation contained no detectable benzene or MTBE, and up to 15 4 mg/kg TPH-G, with the highest levels detected ' at 20 feet bg (capillary fringe soils) Soil hydrocarbon/oxygenate delineation Is considered complete at this time ' On September 18, 2000, eleven soil-gas survey points were pushed to 3 feet bg 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 Sod vapor extraction (SVE) testing conducted 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 (ppm[v])at applied vacuums ranging between 9 and 32 Inches of water(IW) Induced vacuum responses of more than 0 2 IW were observed in wells MW-1, MW-2, and MW-3 during textraction from the horizontal wells Vacuum response in these wells Is likely facilitated by relatively higher-permeability conditions in the tank cavity backfill as opposed to native soils ' 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 concentrations ranged up to 0 91 ppm(v) benzene, 220 ppm(v) MTBE, and 880 ppm(v)TPH-G The data indicated that vapor extraction is a viable remediation method at the site, particularly when the horizontal wells are used On February 6, 2002, an air sparge test was conducted using well MW-2 for vapor extraction, and double- nested sparge well SPA (total depth of 50 feet, perforated at 30-35 feet and 45-50 feet bg) for air infection Pre-sparging extracted vapor levels ranged up 1,000 ppm[v], with dissolved oxygen (DO) levels of up to 0 72 mg/L Air was then Infected into the deeper section of well SPA using an oil-less air compressor ' Vapor levels in well MW-2 increased to 1,600 ppm(v), 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 SP-1 should be successful at reducing dissolved hydrocarbons/oxygenates in well ' MW-2 and the surrounding area Air sparge evaluation was repeated on May 23, 2002, using well MWAA as the infection 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 Specific recommendations for remediation are discussed in further ' detail later in this report ' 19976a22 ooc