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S E C ® R 7-Eleven Store 419976 Quarterly Groundwater Monitoring and Remediation Progress Report (1Q2003) May 13, 2003 Page 2 Current Quarter Monitoring Data (See Figure 2 and Table 1) Wells Monitored and Sampled Ten wells- MW-1 through MW-7, MW-1A, MW-4A, MW-5A Dissolved Oxygen Readings Collected From Ten wells- MW-1 through MW-7, MW-1A, MW-4A, MW-5A Approximate Depth to Groundwater(DTW) 19 01 to 21 47 ft bgs Approximate Change in DTW Since Last Event 1 56 foot rise Groundwater Flow Direction and Gradient East-southeast @ 0 003 feet per foot (northwest vicinity of site, shallow zone) Northwest @ 0 011 feet per foot (in vicinity of MW-4 offsite, shallow zone) North @ 0 009 feet per foot (deep zone) (Figure 2) Current Quarter Analytical Data (See Figure 3 and Table 1) Maximum TPHg Concentrations MW-2, 54,000 ug/L l Maximum Benzene Concentrations MW-2, 3,600 ug/L Maximum MtBE Concentrations MW-2, 2,900 ug/L Maximum TBA Concentrations MW-2, 1,700 ug/L BACKGROUND Underground storage tank (UST) replacement and associated soil sampling activities were a conducted in September 1996 Five 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 overexcavation, 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 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 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 bgs Concentrations ranged up to 15 mg/kg benzene, 2,600 mg/kg TPHg, and 27 mg/kg MtBE 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 (erg/L) benzene and 290,000,ug/L TPHg Dissolved MtBE levels ranged up to 3,700 Ng/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 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 MW-1 and MW-2 ' 19976 03 1 Q Report SECOR International Incorporated 1 1"