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S EC ® R
<br /> 7-Eleven Store#21756
<br /> Quarterly Groundwater Monitoring Report(4Q2003)
<br /> • January 20, 2004
<br /> Page 2
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<br /> Current Quarter Analytical Data (See Figure 3 and Table 1)
<br /> Maximum TPHg Concentrations Not Detected, <50 pq/L
<br /> Maximum Benzene Concentrations Not Detected, <0 50 pg/L
<br /> Maximum MtBE Concentrations Not Detected, <0 50 /L
<br /> Maximum TBA Concentrations Not Detected, <5 0 pg/L
<br /> BACKGROUND
<br /> On December 9, 1998, three 10,000-gallon steel single-walled USTs were removed from the site and
<br /> not replaced (IT Corporation, January 1999) Six sod samples were collected from the base of the
<br /> UST pit approximately 14 feet below ground surface (bgs), and two soil samples were collected from
<br /> beneath the pump islands and associated fuel dispensing piping at approximately 4 feet bgs Total
<br /> petroleum hydrocarbons as gasoline (TPHg), TPH as diesel (TPHd), and benzene, toluene,
<br /> ethylbenzene, and total xylenes (BTEX)were not detected above laboratory reporting limits Methyl-
<br /> tert-butyl ether (MtBE)was reported at a concentration of 0 18 milligrams per kilogram (mg/kg) in the
<br /> northwest corner of the UST pit
<br /> Initial site assessment was conducted in June 1999 (IT Corporation, 1999) by advancing five push-
<br /> probe borings to total depths up to 25 feet bgs Soil samples collected during drilling contained up to
<br /> • 0 167 mg/kg MtBE, benzene and TPHg were not detected above laboratory reporting limits Grab
<br /> groundwater samples collected at the base of the borings contained up to 607 micrograms per liter
<br /> (pg/L) benzene, 32,200 pg/L TPHg, and 46,300 pg/L MtBE The highest concentrations were '
<br /> observed in boring GP-3, located near the western end of the former dispenser island location
<br /> i
<br /> On December 20, 1999, three groundwater monitoring wells (MW-1 to MW-3) were installed to
<br /> depths of 25 feet bgs, and screened from 10 to 25 feet bgs (IT Corporation, February 2000)
<br /> Benzene and TPHg in soil were not detected above laboratory reporting limits MtBE in soil was
<br /> detected in one location (GP-4) at a concentration of 0 167 mg/kg Groundwater analytical data
<br /> indicated dissolved concentrations of benzene (up to 0 32 pg/L), TPHg (up to 55 pg/L), MtBE (up to
<br /> 230 pg/L), and tert-butyl alcohol (TBA) (3,710 pg/L) The reported TBA detection appears to be
<br /> anomalous, as TBA has not been detected in subsequent quarterly samples The MtBE plume was
<br /> defined downgradient of the hydrocarbon source area (the former UST and pump island locations)
<br /> (Shaw 2000)
<br /> On April 1-3, 2002, monitoring well MW-4 was installed to 30 feet bgs and screened from 15 to 30
<br /> feet bgs (Shaw, August 2002) Dual-nested sparge/vent wells (SPV-1 and SPV-2)were drilled to 29
<br /> feet bgs, and screened from 12 to 22 feet bgs as a soil vapor extraction (SVE) well, and also
<br /> screened from 24 to 29 feet bgs as a sparge well A Geoprobe sod boring (GP-6)was advanced to a
<br /> depth of 40 feet bgs Soil samples collected from GP-6 and MW-4, and a grab groundwater sample
<br /> from GP-6, did not contain BTEX, TPHg, and fuel/oxygenates above laboratory reporting limits
<br /> An SVE test was conducted on April 24, 2002, by application of vacuum to well SPV-1 Magnehelic
<br /> gauge measurements of wells MW-1, MW-2, MW-3, and SPV-2 indicated an estimated radius of
<br /> vapor extraction influence of 25 to 30 feet around the extraction point A second SVE test was
<br /> conducted October 17, 2002 Vapor extraction was performed on SPV-2 and MW-1, which resulted it
<br /> in an estimated radius of influence of 25 to 30 feet
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