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. 16 August 2001 AGE-NC Project No 97-0312 ` I Page 2 of 20 r - i_ y i Unauthorized releases of hydrocarbons were reported from fuel piping at the site on three occasions ' in 1988 and 1989 The PHS-EHD subsequently required Investigation of the extent of impacted soil and ground water i ' On 18 September91 98,lslx product USTs were removed from the site A:totalof nine UST soli samples and thirteen-piping/dispenser soil samples were collected and analyzed Total petroleum ' hydrocarbons quantified as gasoline and diesel (TPH-g and TPH-d) were detected in the samples collected beneath the USTs at concentrations as high as 2,500 milligrams per kilogram(mg/kg)and 32,000 mg/kg, respectively TPH-g and TPH-d were detected in piping dispenser samples at concentrations as high as 1,200 mg/kg and 16,000 mg/kg,respectively Excavation of approximately ' 400 cubic yards of hydrocarbon-impacted soil was performed after removal of the USTs Excavated soil was stockpiled on site On 16 March 1998,approximately 180 cubic yards of impacted soil was ' disposed,the remaining clean soil was used as backfill Sample locations are illustrated on Figure 3 23 REGIONAL GEOLOGIC/HYDROGEOLOGIC SETTING The Geologic Map of California,published in 1966 by the California Department of Conservation Division ofMines and Geology,shows the site areawithin the Great Valley Syncline(GVS),a large, elongate, northwest trending structural trough The GVS is subdivided into two mayor divisions designated the Sacramento and Sari Joaquin Valleys,which have been filled to the present elevation ' with thick sequences of sediment ranging in age from Jurassic to present day, creating a nearly flat-lying alluvial plain extending from the Tehachapi Mountains in the south to the Klamath Mountains in the north The western and eastern boundaries ofihis province are comprised of the ' California Coast Range and the Sierra Nevada, respectively Based on General Soil Map from the San Joaquin County Soil Survey,published by the United States ' Department of Agriculture Soil Conservation Service in 1992, the site area is within the Guard- Devnes-Rioblancho (GDR) association The GDR soils are,located within'basin rims along the ' eastern edge of the Sacramento-San Joaquin Delta area,and generally consist of poorly drained,fine to coarse textured soils The GDR soils generally formed inlalluvium from mixed rock sources The Modesto,Riverbank,Turlock Lake Formations and overlying recent alluvium are the principal ' source of domestic ground water in the 13,500-square mile San Joaquin Valley Ground Water Basin (Basin 5-22) This basin is drained primarily by the San Joaquin River The nearest surface water ' feature in the vicinity of the property is Stockton Diverting Canal,located approximately 2,500 feet southwest of the site ' Ground water in the area of the Cherokee Truck Stop site is considered to be of beneficial use Advanced GeoEnvironmentai,Inc 1