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3 <br /> • during the investigation have shown the extent of petroleum hydrocarbons in soil to be in the <br /> vicinity of the former USTs <br /> DISSOLVED PETROLEUM HYDROCARBONS IN GROUNDWATER <br /> Maximum concentrations of petroleum hydrocarbons in groundwater detected during <br /> the May 1997 sampling event were 38,000 4g/L TPH as gasoline (MW-5) d 590 µg/L <br /> benzene (MW-2) MTBE has been detected in all monitoring wells except o toring Well <br /> MW-7 The cause of an increase in the concentration of MTBE in Mo torn Well MW-4 <br /> during the May 1997 sampling event has not been determined on ed g ndwater <br /> monitoring will determine if additional lateral delineation will be required in the <br /> The extent of TPH-as-gasolme contamination is d in the west (upgradient) by <br /> MW-7 (historically nondetect for all analytes) and to the orth M tonng Wells MW-3 and <br /> MW-4, with occasional detections near the method-detectio limit r TPH as gasoline in both <br /> wells The southern boundary of the plume commingles wi the rthern boundary of the <br /> hydrocarbon plume surrounding the ARC;ation <br /> 'titian lova across Benjamin Holt Drive <br /> from the site The farthest downgradienhad a TPH-as-gasoline concentration of <br /> 90 pg/L in May 1997, no BTEX consere rep ed above the laboratory method- <br /> detection limit An increase in the conceas gasoline observed in Well MW-5 <br /> dunng the May 1997 sampling event isle to the onset of air sparging, which can <br /> mobilize hydrocarbons sor o soil. <br /> VERTICAL DELI O <br /> Geraghty & er co an investigation during July 1997 to determine the vertical <br /> extent of contamination a the rvice station. The soil boring was completed by Precision <br /> Samplin , (Precision). a Precision sampling rig utilizes a hydraulic hammer to drive <br /> the low cyhn cal EnviroCoreTm sampling rods into the ground to collect continuous soil <br /> c s Two es sampling rods were driven simultaneously the small-diameter inner <br /> s ling w used to obtain and retrieve the soil cores, and the larger-diameter outer <br /> rods ed temporary drive casings This equipment provides a technique for collecting <br /> soil sampl below the water table while minimizing the potential for cross-contamination from <br /> groundwater encountered in the boring <br /> Soil samples were collected from 5 5, 9 5, 18, 21, 26, 315, 35, 40 5, and 45 feet bgs <br /> to determine if a pathway for vertical migration existed for hydrocarbons present in shallow <br /> Project No RC0309 010 <br /> GERAGHTY&MILLER, INC, <br />