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S E C O R Conceptual Site Model Chevron Service Station#9-6171 March 25, 2005 IPage 22 450 ppm (B6), ethylbenzene ranged from 0 2 ppm (B4) to 130 ppm (B6), and total xylenes Iranged from 0 3 ppm (B4) to 770 ppm (B6) In March 1989, seven additional groundwater monitoring wells (MW4 through MW10) were Installed The concentrations of TPHg ranged from below the method detection limits (MW4) to 3 ppm (MW8) There was no TPHd detected in these monitoring wells Concentrations of BTEX were reported from below the method detection limits at groundwater monitonng well MW4 to 1,300 ppm (MW8), 63 ppm (MW8), 46 ppm (MW8), and 377 ppm (MW8), respectively There is no available data for MW10 until 1990 due to the presence of floating product in the well The concentrations of lead were reported as either not detected or below the method detection limits The extent of the petroleum hydrocarbon groundwater plume extends from 25 feet bgs to the top of groundwater at 40- 45 feet bgs The lateral extent of the plume is roughly 350 feet from east to west In September 1992, groundwater monitoring wells MW11 and MW12 were Installed downgradlent from the service station MW11 was Installed onsite and MW12 was Installed downgradlent offsite, Into the leading edge of the gasoline plume Both wells were turned Into air sparge wells The vapor extraction system is discussed In greater detail in the next section In June 1997, groundwater monitoring well MW13 was Installed and Joined to the existing vapor extraction system as an air sparge well In addition, soil borings SB4 through SB7 were completed to assess soil contaminant concentrations Analyses of the soil samples reported TPHg concentrations from below the method detection limits at SB4 to 87 mg/kg (SB6) Concentrations of benzene and ethylbenzene in soil were reported at below the method detection limits The concentrations of toluene and total xylenes in soil ranged from below the method detection limits (SB4) to 180 mg/kg (SB5) and 7 7 mg/kg (SB7), I respectively The concentrations of MtBE in soil ranged from below the method detection limits (SB4) to 3 mg/kg (SB7) The concentrations of TPHg in groundwater ranged from 28 µglL (SB6) to 46 n/L (SB7) The concentrations of BTEX in groundwater ranged from ` below the method detection limits (SB7) to 15 µglL (SB5), 590 ug/L (SB7), 540 µg/L (SB5), I and 15,000 µg& (SB7), respectively MtBE was reported at below the method detection limits in all of the groundwater samples I643 Remediation History In 1991, Western Geo Engineers (WEGE) Installed a thermal oxidizing groundwater treatment system onsite It consisted of a "Convault" pump and treat remediation system attached to a "Hirtz" incinerator Contaminated groundwater was pumped from the aquifer, stripped of hydrocarbons, and then stored in a tank to wait testing prior to disposal The hydrocarbons were extracted from the vapor and delivered to the incinerator for destruction Since soil contamination extends from the UST excavation pit to groundwater, In-situ vapor N IFdes\Projects\Chevron\Retaiil#9-61711Site Conceptual ModelTINAL_CSM doe