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<br /> 7-Eleven Store #19976
<br /> Quarterly Groundwater Monitoring and Remediation Progress Report (2Q2004)
<br /> August 20, 2004
<br /> • Page 2
<br /> Current Quarter Monitormtg Data (See Figure 2 and Table 1) I
<br /> Wells Monitored and Sampled Ten wells- MW-1 through MW-7, MW-
<br /> 1A, MW-4A, MW-5A
<br /> Dissolved Oxygen Concentrations Measured In Ten wells- MWA through MW-7, MW-
<br /> 1A, MW-4A, MW-5A
<br /> Depth to Groundwater (DTW) 22 37 to 24 79 ft bgs
<br /> Average Change to Groundwater Elevation Since
<br /> Last Event 2 10 foot decrease
<br /> Groundwater Flow Direction and Gradient Northwest @ 0 003 feet per foot
<br /> (Figure 2)
<br /> Current Quarter Analytical Data (See Figure 3 and Table 1)
<br /> Maximum TPHg Concentrations MW-2, 15,000 pg/L
<br /> Maximum Benzene Concentrations MW-2, 480 /L
<br /> Maximum MtBE Concentrations MW-2, 2,800 /L
<br /> Maximum TBA Concentrations MW-2, 1,500 pg/L
<br /> BACKGROUND
<br /> Underground storage tank (UST) replacement and associated soil sampling activities were
<br /> conducted in September 1996 Five sod samples (T1-N, T1-S, T2-S, T3-N, T3-S) (Figure 2)
<br /> were collected at depths of 15 feet below ground surface (bgs) from beneath the USTs
<br /> Benzene, total petroleum hydrocarbons as gasoline (TPHg), and methyl-tert-butyl ether
<br /> (MtBE) were detected at concentrations of 0 93 milligrams per kilogram (mg/kg), 220
<br /> mg/kg, and 0 35 mg/kg, respectively After over excavation, analytical results of soil
<br /> samples E1-W, E1-C, E1-E, E2-W, E2-C, E2-E (Figure 2), collected at the edge of the
<br /> excavation, contained TPHg, benzene, toluene, ethylbenzene, and total xylenes (BTEX),
<br /> and MtBE at low concentrations Prior to backfilling the excavation, two horizontal vent
<br /> lines were installed in the northern part of the tank pit floor at a depth of 11 to 11 5 feet bgs
<br /> (Figure 2) The vent lines were constructed of 4-inch diameter Schedule 40 poly-vinyl-
<br /> chloride (PVC) piping, each line with 20 feet of screened interval The lines were stubbedA
<br /> up inside a fiberglass box located in a planter east of the tank pit The new USTs were
<br /> installed close to (partly overlapping) the location of the original USTs (Figure 2) M ,
<br /> w
<br /> Geoprobe soil borings GP-1 through GP-3 were advanced on April 29, 1998, near the
<br /> hydrocarbon source area (the UST pad and pump island) (Figure 2) Nineteen soil samples
<br /> were collected from depths ranging between 5 and 45 feet bgs Concentrations ranged up
<br /> to 15 mg/kg benzene, 2,600 mg/kg TPHg, and 27 mg/kg MtBE The highest concentrations
<br /> of hydrocarbons in soil were detected in boring GP-3 at a depth of 35 feet bgs Grab
<br /> groundwater samples collected from the base of each boring contained up to 5,800
<br /> micrograms per liter (pg/L) benzene and 290,000 pg/L TPHg Dissolved MtBE levels
<br /> ranged up to 3,700 pg/L
<br /> Groundwater monitoring wells MW-1/1A, MW-2, and MW-3 were installed in September
<br /> • 1998 (Figure 2) Well MW-1/1A is a dual-nested well completed to depths of 30 and 50 feet
<br /> 19976 04-20 Report doc SECOR Intemabonal Incorporated
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