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1 •` A September 2002 ' GE-NC Project No. 95-0100 Page 2 of 9 ' 2.3• REGIONAL HYDROGEOLOGIC SETTING The site is located within the 13,500 square-mile San Joaquin Valley Ground Water Basin[California ' Department of Water Resources(CDWR)Basin No. 5-22]. The principal subsurface units that supply ground water are the Modesto, Riverbank and Turlock Lake Formations and overlying modern alluvium. This basin is drained primarily by the San Joaquin River. AGE has installed and monitored six ground water monitoring wells on the subject property. Based on these wells, the average depth to ground water is approximately 54-feet bsg. The ground water flow direction beneath the site is to the northwest, however, the flow direction can vary locally due to pumping of production wells in the area or through seasonal recharge. A total of three domestic ground water wells were located on the site. One well (DW-1, now ' abandoned)was located approximately 150 feet west ofthe UST investigation area.A secondary well (DW-2) is located approximately 250 feet north of the former UST investigation area. The primary production water well (DW-3) is located approximately 350 feet northwest of the former UST investigation area. Based on the Well Completion Report for the domestic well drilled on the site in ' 1992, the well was constructed with a sanitary sea] to a depth of 100 feet, and is screened over the depth interval of 160 to 200 feet bsg. Ground water was measured at a depth of 54 feet below surface grade (bsg). ' Jefferson School is located on the southeast corner ofLinne Road and Chrisman Road a r xi 600 feet.southeast of the site. The ground water well utilized by the Jefferson School Districteis ' located approximately 700 feet southeast of the former UST area. 3.0. PREVIOUS INVESTIGATIONS 3.1. UNDERGROUND STORAGE TANK REMOVAL Two 1,500-gallon USTs,piping and dispensers were removed from the property in September 1993. Analysis of soil samples collected from beneath the dispenser at a depth of six feet below surface grade(bsg) indicated that total petroleum hydrocarbons quantified as gasoline(TPH-g) and volatile ' aromatics(benzene,toluene,ethylbenzene andlenes:BTE xy X)were present at concentrations as high as 48,000 parts per million(ppm,reported as nvlligrams per kilogram). Total petroleumhydrocarbons quantified as diesel were not detected in the soil sample analyzed. Total lead was detected in the sample collected from the dispenser area at 270 ppm. Ethylene-dibromide was also detected in the same sample at 70 parts per billion(ppb,reported as micrograms per kilogram). The analytical results of the soil samples from the UST removal are summarized on Table 1. -` - --3.2:`- INJTIAL:SITE'ASSESSMENT On 12 August 1994, two borings (boring B 1 and boring B2) were drilled under the supervision of Geological Audit Services, Inc. (GAS) in the area of the former fuel dispenser island. TPH-g was detected at concentrations as high as 13,000 milligrams per kilogram(mg/kg)and hydrocarbons were Advanced GeoEnvironmiental,Inc. t -