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S ECOR �- 7-Eleven Store#19976 Quarterly Groundwater Monitoring and Remediation Progress Report (1Q2006) May 16, 2006 Page 2 Current Quarter Monitoring 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 Monitored Ten wells - MW-1 through MW-7, MWAA, 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 rnwa- 1rvG Top of Casing) 17 86 to 20 35 ft Average Change in Groundwater Elevation Since Last Event 1 25 foot increase Groundwater Flow Direction and Gradient Northwest @ 0 001 foot per foot (Figure 2) Current Quarter Analytical Data See Figure 3 and Table 1 Maximum TPHg Concentrations Not Detected, X50 pg/L Maximum Benzene Concentrations MW-1, 0 79 pg/L Maximum MtBE Concentrations MW-1, 56 NgIL Maximum TBA Concentrations MW-5, 62 NgIL E70,C- Gv-r o.G� JL V),h BACKGROUND 10 GJ7-2-10t Underground storage tank (UST) replacement and associated soil sampling activities were conducted in September 1996 Five sod 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 (mg/kg), 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), �nl -and 4At6Z 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 115 feet bgs 1 (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 sod 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 groundwater samples collected from the base of each boring contained up to 5,800 19976 06-1Q Report doc SECOR lntematlonal Incorporated