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S E C ® R 7-Eleven Store#21756 Quarterly Groundwater Monitoring Report(1 Q2003) May 16, 2003 "A Page 2 9 a' BACKGROUND s On December 9, 1998, three 10,000-gallon steel single-wall underground storage tanks (USTs) were , n removed from the site and not replaced (IT Corporation, January 1999) Six soil samples were collected from the base of the UST pit, and two soil samples were collected from beneath the pump islands and associated fuel dispensing piping Tank pit soil samples were collected from about 14 feet below ground surface (bgs), pump-island-area samples were obtained from depths of about 4 feet bgs Total petroleum hydrocarbons as gasoline (TPHg), TPH as diesel (TPHd), and benzene, toluene, ethylbenzene, and total xylenes (BTEX) were not detected above laboratory reporting limits Methyl-tert-butyl ether (MtBE) was reported at a concentration of 018 milligrams per kilogram (mg/kg) in the northwest corner of the UST pit at a depth of about 14 feet bgs Initial site assessment was conducted in June 1999 (IT Corporation, 1999) with the installation of five push-probe borings to total depths of up to 25 feet bgs Soil samples collected during drilling contained up to 0 167 mg/kg MtBE, benzene or TPHg were not detected above laboratory reporting limits Grab groundwater samples collected at the base of the borings contained up to 607 micrograms per liter (ug/L) benzene, 32,200 ug/L TPHg, and 46,300 ug/L MtBE The highest + concentrations were observed in boring GP-3, located near the western end of the former dispenser island location On December 20, 1999, three groundwater monitoring wells (MW-1 to MW-3) were installed to depths of 25 feet bgs, and screened from 10 to 25 feet bgs (IT Corporation, February 2000) Benzene and TPHg were not detected above laboratory reporting limits MtBE was found in one location (GP-4) at a concentration of 0 167 mg/kg Groundwater analytical data indicated dissolved concentrations of benzene (up to 0 32 ug/L), TPHg (up to 55 ug/L), MtBE (up to 230 ug/L), and tert- butyl alcohol (TBA) (3,710 ug/L) The reported TBA detection appears to be anomalous, as TBA has not been detected in subsequent quarterly samples The MtBE plume was defined downgradient of the hydrocarbon source area (the former UST and pump island locations) (Shaw 2000) On April 1-3, 2002, monitoring well MW-4 was installed to 30 feet bgs and screened from 15 to 30 feet bgs (Shaw, August 2002) Dual-nested sparge/vent wells (SPV-1 and SPV-2) were advanced to 29 feet bgs, and screened from 12 to 22 feet bgs as a soil vapor extraction (SVE) well, and 24 to 29 feet bgs as a sparge well A Geoprobe soil boring (GP-6) was advanced to a depth of 40 feet bgs Soil samples collected from GP-6 and MW-4, and a grab groundwater sample from GP-6, did not contain BTEX, TPHg, and MtBE/oxygenates above laboratory reporting limits An SVE test was conducted on April 24, 2002, by application of vacuum to well SPV-1 Magnehelic gauge measurements of wells MW-1, MW-2, MW-3, and SPV-2 indicated an estimated radius of vapor extraction influence of 25 to 30 feet around the extraction point A second SVE test was conducted October 17, 2002 Vapor extraction was performed on SPV-2 and MW-1, which resulted in an estimated radius of influence of 25 to 30 feet MONITORING AND SAMPLING PROCEDURES t The depth to water was measured in monitoring wells MW-1 through MW-4 from top of casing (TOC) f using a water level indicator to within 0 01 feet bgs Dissolved oxygen readings were also collected from each well using a YSI Model 550A dissolved oxygen meter equipped with a down hole sensor Well purging and sampling equipment is thoroughly cleaned prior to purging and sampling each well • The sampling procedure for each well includes measuring the water level and checking for the presence of liquid-phase hydrocarbons (LPH), using either an electronic indicator and a clear i� 21756 03 10 Report SECOR Internatrona!Incorporated rk�j7 4 41 k