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<br /> SE (C0R
<br /> 7-Eleven Store#19976
<br /> Quarterly Groundwater Monitoring and Remediation Progress Report (3Q2003)
<br /> November 26, 2003
<br /> Page 2 I
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<br /> Current Quarter Monitoring Data See Figure 2 and Table 1
<br /> Wells Monitored and Sampled Ten wells- MWA through MW-7, MW-1A,
<br /> MW-4A, MW-5A
<br /> Dissolved Oxygen Readings Collected From Ten wells- MW-1 through MW-7, MWAA,
<br /> MW-4A, MW-5A
<br /> Approximate Depth to Groundwater(DTVV) 20 06 to 22 42 ft bgs
<br /> Average Change to Groundwater Elevation Since Last
<br /> Event 0 28 foot decrease
<br /> Groundwater Flow Direction and Gradient Northwest @ 0 005 feet per foot(Figure 2)
<br /> Current Quarter Analytical Data (See Figure 3 and Table 1)
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<br /> Maximum TPHg Concentrations MW-2, 40,000 u /L
<br /> Maximum Benzene Concentrations MW-2, 1,100 ug/L
<br /> Maximum MtBE Concentrations MW-2, 810 u /L
<br /> Maximum TBA Concentrations MW-2, 540 ug/L
<br /> BACKGROUND
<br /> Underground storage tank (UST) replacement and associated soil sampling activities were
<br /> conducted in September 1996 Five soil samples (T1-N, T1-S, T2-S, T3-N, T3-S) (Figure 2) were
<br /> collected at depths of 15 feet below ground surface (bgs) from beneath the USTs Benzene, total
<br /> petroleum hydrocarbons as gasoline (TPHg), and methyl-tert-butyl ether (MtSE) were detected at
<br /> concentrations of 0 93 milligrams per kilogram (mg/kg), 220 mg/kg, and 0 35 mg/kg, respectively
<br /> After over excavation, analytical results of soil samples E1-W, E1-C, E1-E, E2-W, E2-C, E2-E
<br /> (Figure 2), collected at the edge of excavation, contained TPHg, benzene, toluene, ethylbenzene,
<br /> and total xylenes (BTEX), and MtBE at low concentrations Prior to backfilling the excavation, two "
<br /> horizontal vent lines were installed in the northern part of the tank pit floor at a depth of 11 to 11 5 F
<br /> feet bgs (Figure 2) The vent lines are constructed of 4-inch Schedule 40 PVC piping, each line with fi
<br /> 20 feet of screened interval The lines are stubbed up inside a fiberglass box located in a planter
<br /> east of the tank pit The new USTs were installed close to (partly overlapping) the location of the 3 4
<br /> original USTs (Figure 2)
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<br /> 'yb
<br /> Geoprobe soil borings GP-1 through GP-3 were advanced on April 29, 1998, near the hydrocarbon y`s
<br /> source area (the UST pad and pump island) (Figure 2) Nineteen soil samples were collected from
<br /> depths ranging between 5 and 45 feet bgs Concentrations ranged up to 15 mg/kg benzene, 2,600
<br /> mglkg TPHg, and 27 mg/kg MtBE The highest concentrations of hydrocarbons adsorbed to soil
<br /> were detected in boring GP-3 at a depth of 35 feet bgs Grab groundwater samples collected from Af<
<br /> the base of each boring contained up to 5,800 micrograms per liter (Ng/L) benzene and 290,000 pg/L
<br /> TPHg Dissolved MtBE levels ranged up to 3,700 pg/L
<br /> Groundwater monitoring wells MW-111 A, MW-2, and MW-3 were installed in September 1998 (Figure
<br /> 2) Well MW-1/1A is a dual-nested well completed to depths of 30 and 66 feet bgs Soil samples
<br /> collected during well installation contained up to 0 07 mg/kg benzene and 0 08 mg/kg MtBE Highest
<br /> concentrations were at 20 and 25 feet bgs (capillary fringe soils) in wells MW-1 and MW-2
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<br /> 19976 03-30 Report doc SECOR International incorporated
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