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I S E C ® R Ir i 7-Eleven Store Number 19976 Fuel System Upgrade Report August 7, 2003 Page 2 east of the tank pit The new USTs were Installed close to (partly overlapping) the location of the original USTs (Figure 2) Geoprobe soil borings GP-1 through GP-3 were subsequently advanced 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 bgs Concentrations ranged up to 15 mg/kg benzene, 2,600 mg/kg TPHg, and 27 mg/kg MtBE in the soil samples The highest concentrations of hydrocarbons adsorbed to soil were detected in boring GP-3 at a depth of 35 feet bgs Grab groundwater samples collected from the base of each boring contained up to 5,800 micrograms per liter(Ng/L) benzene and 290,000 pg/L TPHg Dissolved MtBE levels ranged up to 3,700 pg/L 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, with well casings completed to depths of 30 and 66 feet bgs Soil samples collected during well installation contained up to 0 07 mg/kg benzene and 0 08 mg/kg MtBE The highest concentrations of hydrocarbons in soil were at 20 and 25 feet bgs in wells MW-1 and MW-2 Soil boring SB-1 and nested wells MW4/4A and MW-5/5A were installed in May 1999, as requested Q by the SJCEHD, to provide additional lateral and vertical plume delineation Soil boring SB-1 was installed to about 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 20-30 feet bgs (MW-4 and MW-5), and 45-50 feet bgs (MW4A 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 levels 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 the soil samples collected The maximum TPHg concentration was reported at 20 feet bgs (15 4 mglkg) (Shaw, 2000) On September 18, 2000, as requested by the SJCEHD, eleven soil-gas survey points were pushed 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) Soil samples analyzed from both well MW-7 and soil boring SB-2 did not contain benzene, TPHg, and MtBE above laboratory reporting limits A grab groundwater sample was collected from boring SB-2 at a depth of 40 feet bgs Total xylenes were detected at a concentration of 1 5 pg/L 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 MW-1, 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 opposed to native Bolls Testing was also conducted on the groundwater monitoring wells, lower permeability and 19976 Fuel Line&USTs Upgrade Reportl doc SECOR International Incorporated