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S E C ® R 7-Eleven Store#19976 Quarterly Groundwater Monitoring and Remediation Progress Report(2Q2003) September 11, 2003 Page 2 4 Approximate Change in DTW Since Last Event 0 76 foot decline Groundwater Flow Direction and Gradient Northwest @ 0 005 feet per foot(Figure 2) Current Quarter Analytical Data (See Figure 3 and Table 1) Maximum TPHg Concentrations MW-2, 55,000 uglL Maximum Benzene Concentrations MW-2, 3,100 uglL _ - I Maximum MtBE Concentrations MW-2, 2,300 uglL Maximum TBA Concentrations MW-2, 1,200 uglL BACKGROUND Underground storage tank (UST) replacement and associated sod sampling activities were conducted in September 1996 Five soil samples were collected at depths of 15 feet below ground surface (bgs) from beneath the USTs Benzene, total petroleum hydrocarbons as gasoline (TPHg), and methyl-tert-butyl ether (MtBE) were detected at concentrations of 0 93 milligrams per kilogram (mg/kg), 220 mg/kg, and 0 35 mg/kg, respectively After over excavation, analytical results of samples collected at the edge of excavation contained TPHg, BTEX, and MtBE at low concentrations Prior to backfilling the excavation, two horizontal vent lines were installed in the northern part of the tank pit floor at a depth of 11 to 11 5 feet bgs (Figure 2) The vent lines are A' constructed of 4-inch Schedule 40 PVC piping, each line with 20 feet of screened interval The lines , are stubbed up inside a fiberglass 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) Geoprobe soil borings GP-1 through GP-3 were 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 mglkg benzene, 2,600 mg/kg TPHg, and 27 mg/kg MtBE The highest concentrations of hydrocarbons adsorbed to sod 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 (pglL) benzene and 290,000 pglL 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-111A is a dual-nested well 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 Highest concentrations were at 20 and 25 feet bgs (capillary fringe soils) in wells MWA and MW-2 x Soil boring SB-1 and nested wells MW-4/4A and MW-515A were installed in May 1999, as requested by the SJCPHS, to provide additional lateral and vertical plume delineation Sod 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 a ; 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) 1 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 sod samples collected The 19976 03-2Q Report doc SECOR International Incorporated