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f j Fourth Quarter 2002 Monitoring,Sampling and Remediation Progress Report 2 7-Eleven, Inc, Store#19976, 1399 North Main Street Manteca,CA December 24,2002 1 Underground storage tank(UST) replacement and associated soil sampling activities were conducted In September 1996 Five samples were collected at depths of 15 feet below grade (bg)from beneath the USTs Sample analysis Indicated up to 0 93 milligrams per kilogram(mg/kg) benzene, 220 mg/kg TPH-G, and 0 35 mg/kg MTBE After overexcavation, analytical results of samples collected at the edge of excavation were generally non-detectable Prior to backfilling the excavation, two horizontal soil vapor extraction (SVE) Imes were installed in the northern part of the tank pit floor at a depth of 11 to 11 5 feet ' bg (Figure 2) The SVE lines are constructed of 4-inch Schedule 40 PVC piping, each line having 20 feet of screened interval The lines are stubbed up inside a flush-mounted equipment box located in a planter east of the tank pit The new USTs were installed close to (partly overlapping) the location of the original USTs(Figure 2) ' A workplan for initial site assessment was requested in February of 1998 by the SJCPHS Geoprobe soil borings GP-1 through GP-3 were subsequently Installed on April 29, 1998, near the hydrocarbon source area (the UST pad and pump island) (Figure 2) Nineteen soil samples were collected from depths ' ranging between 5 and 45 feet bg for analysis for BTEX, MTBE, and TPH-G (EPA Methods 80201modlfied 8015) One sample from each soil boring was also analyzed for MTBE and other oxygenates using EPA Method 8260 Concentrations ranged up to 15 mg/kg benzene, 2,600 mg/kg TPH-G, and 27 mg/kg MTBE (EPA Method 801518020) Highest concentrations were detected in boring GP-3 at a depth of 35 feet bg Grab groundwater samples collected from the base of each boring were also analyzed for the same I) constituents and contained up to 5,800 micrograms per liter(pg/L) benzene and 290,000 pg/L TPH-G I Dissolved MTBE levels ranged up to 3,700 pg/L (EPA Method 8260) (Table 1A) ' Additional assessment was requested by the SJCPHS in June 1998 Groundwater monitoring wells MW-1/1A, MW-2, and MW-3 were installed in September 1998 (Figure 2) Well MW-1/1A Is a dual-nested well completed to depths of 30 and 66 feet bg Soil samples collected during well installation contained up 1 to 0 07 mg/kg benzene and 0 08 mg/kg MTBE Highest concentrations were at 20 and 25 feet bg (capillary fringe soils) in wells MW-1 and MW-2 Initial dissolved gasoline levels ranged up to 245 pg/L benzene, 2,430 pg/L TPH-G, and 449 pg/L MTBE (Table 1A) 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 (Figure 2) Soil boring SB-1 was installed to about 65 feet bg 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 20-30 feet bg (MW-4 and M-5), and 45-50 ' feet bg (MW-4A and MW-5A) Soil sample data from boring SB-1 Indicated that the base of the benzene, TPH-G, and MTBE/oxygenate plume lies at between 40 and 55 feet bg, as evidenced by non-detectable ' concentrations of all constituents at depths of more than 40 feet bg Samples from well MW-5 contained low levels of gasoline constituents from 30-35 feet bg, with no detectable concentrations at deeper depths 199761,14 1,0C