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S EC 0 IR 7-Eleven Store#21756 Quarterly Groundwater Monitoring Report (3Q2004) . October 28, 2004 Page 2 ; N Depth to Groundwater (DTW) 22 21 to 23 95 ft bgs Average Change in Groundwater Elevation Since Last Event 1 34 foot decrease Groundwater Flow Direction and Gradient Northeast @ 0 017 foot per foot (Figure 2) Current Quarter Analytical Data (See Figure 3 and Table 1) k Maximum TPHg Concentrations Not Detected, X50 pg/L Maximum Benzene Concentrations Not Detected, <0 50 pg/L < Maximum MtBE Concentrations Not Detected, <0 50 pg/L ' X Maximum TBA Concentrations Not Detected, <5 0 p9IL BACKGROUND On December 9, 1998, three 10,000-gallon steel single-walled USTs were removed from the site and not replaced (IT Corporation, January 1999) Six soil samples were collected from the base of the UST pit approximately 14 feet below ground surface (bgs), and two soil samples were collected from beneath the pump islands and associated fuel dispensing d piping at approximately 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 Y concentration of 0 18 milligrams per kilogram (mg/kg) in the northwest corner of the UST pit Initial site assessment was conducted in June 1999 (IT Corporation, 1999) by advancing five push-probe borings to total depths up to 25 feet bgs Soil samples collected during drilling contained up to 0 167 mg/kg MtBE, benzene and TPHg were not detected above laboratory reporting limits Grab groundwater samples collected at the base of the borings t contained up to 607 micrograms per liter (pg/L) benzene, 32,200 pg/L TPHg, and 46,300 x pg/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 in soil were not detected above laboratory reporting limits MtBE in soil was detected 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 pg/L), TPHg (up to 55 pg/L), MtBE (up to 230 pg/L), and tert-butyl alcohol (TBA) (3,710 pg/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) a , 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 drilled to 29 feet bgs, and screened from 12 to 22 feet bgs as sod vapor extraction , 21756 04-3Q Report doc SECOR International Incorporated 4r,