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' GERAGHTY & MILLER, INC 3 <br /> during the environmental investigations are included in Attachment 2 A cross section prepared <br /> by Groundwater Technology Inc (GTI) is included in Attachment 3 <br /> ' VERTICAL DELINEATION <br /> ' Geraghty & Miller conducted an investigation during July 1997 to determine the vertical <br /> extent of contamination at the service station A vertical delineation soil boring was installed at <br /> ' the downgradient periphery of the former tank pit This location was chosen to assess the <br /> possibility of vertical migration of petroleum hydrocarbons in soil and groundwater near the <br /> area of the suspected source Results from this investigation demonstrated that hydrocarbons <br /> ' present in the vadose zone and groundwater near the area of the former tank pit are not <br /> 'T;. 7 migrating downward. A summary of the installation activities, a boring log, and analytical <br /> results are presented in Attachment 4 <br /> ' DISSOLVED PETROLEUM HYDROCARBONS IN GROUNDWATER <br /> The Regional Water Quality Control Board -- Central Valley (CVRWQCB) Basin Plan <br /> identifies groundwater in the area around the service station as a potential drinking water and <br /> agricultural water source In accordance with CVRWQCB Basin Plan requirements, the <br /> ultimate project goal is to reduce concentrations of petroleum hydrocarbons to below the MCLs <br /> established for drinking water <br /> A municipal drinking water well, with screened intervals below 180 feet bgs, is located <br /> across Plymouth Avenue from the site (Figure 2) No hydrocarbons have been detected in the <br /> imunicipal well <br /> Analytical results of the May 1997 sampling event are shown in Figure 3. Maximum <br /> concentrations of petroleum hydrocarbons in groundwater detected during the May 1997 <br /> ' sampling event were 38,000 micrograms per liter(µg/L) of TPH as gasoline (MW-5) and 590 <br /> gg/L of benzene (MW-2) An increase in the concentration of TPH as gasoline observed in <br /> Well MW-5 during the May 1997 sampling event is attributable to the onset of air sparging, <br /> ' which can mobilize hydrocarbons sorbed to soil <br /> ' The extent of TPH-as-gasoline contamination is defined to the west (upgradient) by MW-7 <br /> (historically nondetect for all analytes) and to the north by Monitoring Wells MW-3 and MW-4 <br /> Occasional detections near the method-detection limit for TPH as gasoline have been reported <br /> in Wells MW-3 and MW-4 The southern boundary of the plume commingles with the northern <br /> Project No RC0309 010 <br />