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7-Eleven Store #19976 Quarterly Groundwater Monitoring and Remediation Progress Report (2Q2006) August 8, 2006 Page 2 Current Quarter Monitorma Data (See Figure 2 and Table 1) _ Wells Monitored and Sampled Monitored Ten wells - MW-1 through MW-7, MW-1A, MW-4A, and MW-5A Sampled Nine wells - MW-1 through MW-3, MW-5 through MW-7, MW-1A, MW-4A, and MW-5A Dissolved Oxygen Concentrations Measured In Ten wells— MW-1 through MW-7, MW-1A, MW-4A, and MW-5A Depth to Groundwater (DTW) (Measured Below Top of Casing) 16 79 to 19 32 ft Average Change in Groundwater Elevation Since Last Event 2 16 foot increase Groundwater Flow Direction and Gradient North-Northwest @ 0 001 foot per foot (Figure 2) Current Quarter Analytical Data (See Figure 3 and Table 1) Maximum TPHg Concentrations MW-1, 1,000 pglL • Maximum Benzene Concentrations MW-1, 21 pg/L Maximum MtBE Concentrations MW-1, 730 pglL - Maximum TBA Concentrations MW-1, 150 pglL BACKGROUND Underground storage tank (UST) replacement and associated sod sampling activities were conducted in September 1996 Five soil samples (T1-N, T1-S, T2-S, T3-N, T3-S) (Figure 2) 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 (mglkg), 220 mg/kg, and 0 35 mg/kg, respectively After over excavation, analytical results of soil samples E1-W, E1-C, E1-E, E2-W, E2-C, E2-E (Figure 2), collected at the edge of the excavation, contained TPHg, benzene, toluene, ethylbenzene, and total xylenes (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 were constructed of 4-inch diameter Schedule 40 polyvinyl- chloride (PVC) piping, each line with 20 feet of screened interval The lines were 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 mg/kg benzene, 2,600 mg/kg TPHg, and 27 mg/kg MtBE The highest concentrations • of hydrocarbons in soil were detected in boring GP-3 at a depth of 35 feet bgs Grab 19976 06-2Q Report doc SECOR Intematfonal Incorporated