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<br /> 7-Eleven Store#19976
<br /> Quarterly Groundwater Monitoring and Remediation Progress Report (lQ2004)
<br /> . June 3, 2004
<br /> Page 2
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<br /> Current Quarter Monitoring Data (See Figure 2 and Table 1)
<br /> Wells Monitored and Sampled Ten wells- MW-1 through MW-7, MW-1A,
<br /> MW-4A, MW-5A
<br /> Dissolved Oxygen Readings Collected From Ten wells- MW-1 through MW-7, MW-1A,
<br /> MW-4A, MW-5A
<br /> Approximate Depth to Groundwater(DTW) 20 26 to 22 73 ft bgs
<br /> Average Change to Groundwater Elevation Since Last
<br /> Event 0 83 foot increase
<br /> Groundwater Flow Direction and Gradient Northwest @ 0 001 feet per foot(Figure 2
<br /> Current Quarter Analytical Data (See Figure 3 and Table 1)
<br /> Maximum TPHg Concentrations MW-1, 180 pg/L
<br /> Maximum Benzene Concentrations MW-2, 65 pg/L
<br /> Maximum MtBE Concentrations MW-2, 5,900 pg/L
<br /> Maximum TBA Concentrations MW-2, 1,600 /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 (MtBE) 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 the excavation, contained TPHg, benzene, toluene,
<br /> ethylbenzene, and total xylenes (BTEX), and MtBE at low concentrations Prior to backfilling the
<br /> excavation, two horizontal vent lines were installed in the northern part of the tank pit floor at a depth
<br /> of 11 to 11 5 feet bgs (Figure 2) The vent lines were constructed of 4-inch diameter Schedule 40
<br /> poly-vinyl-chloride (PVC) piping, each line with 20 feet of screened interval The lines were stubbed r
<br /> up inside a fiberglass box located in a planter east of the tank pit The new USTs were installed
<br /> close to (partly overlapping)the location of the original USTs (Figure 2)
<br /> Geoprobe soil borings GP-1 through GP-3 were advanced on April 29, 1998, near the hydrocarbon � �
<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 ;4 n
<br /> mg/kg TPHg, and 27 mg/kg MtBE The highest concentrations of hydrocarbons in soil were detected
<br /> in boring GP-3 at a depth of 35 feet bgs Grab groundwater samples collected from the base of each
<br /> boring contained up to 5,800 micrograms per liter (Ng/L) benzene and 290,000 pg/L TPHg a
<br /> Dissolved MtBE levels ranged up to 3,700 pg/L
<br /> Groundwater monitoring wells MW-1/1A, 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 50 feet bgs Soil samples
<br /> collected during well installation contained up to 0 07 mg/kg benzene and 0 08 mglkg MtBE The
<br /> highest concentrations were at 20 and 25 feet bgs (capillary fringe) in wells MW-1 and MW-2
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<br /> 19976 Q4-1Q Report doc SECOR International incorporated
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