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t to 31 October 2005 AGE-NC Project No 03-1080 Page 2 of 10 been completely defined, but are believed to be of metamorphic and igneous origins The Modesto, Riverbank, and Turlock Lake Formations and overlying Recent alluvium are the principal sources of domestic ground water in the 13,500-square mile San Joaquin Valley Ground ` ' Water Basin (Basin 5-22) The nearest surface water feature to the site is the San Joaquin River, located approximately 5 miles west of the site According to previous site investigations, ground water is located at approximately 30 feet to 35 feet below surface grade (bsg), and flows under a ' northeast-trending hydraulic gradient,however,ground water elevation and gradient may vary both locally and seasonally ry 212 SITE HISTORY ' According to records on file with the San Joaquin Country Environmental Health Department (EHD), the subject property was formerly part of San Joaquin County's Local Oversight Program (LOP)for the investigation of leaking underground storage tanks In late 1988 and early 1989,four ground water monitoring wells (MW-1 through MW-4) were installed to total depths ranging between 40 feet and 50 feet bsg Contamination was detected in soil and ground water samples 1 • collected during the well installation In November 1989,three 10,000-gallon gasoline USTs(regular,unleaded,and diesel)were removed, ' with soil excavated to 17 feet bsg The tanks were noted to,be in good condition, but hydrocarbon odor and discoloration were present in surrounding soil All soil samples collected at 17 feet bsg confirmed contamination In November 1989 and June 1992, five additional ground water monitoring wells (MW-5 through MW-9) were installed to approximately 40 feet bsg Quarterly ground water monitoring events ' occurred from April 1992 to November 1997 Ground water samples collected from monitoring wells MW-1 through MW-4 were consistently impacted by petroleum hydrocarbons Occasional detections of petroleum hydrocarbons occurred in monitoring wells MW-5 through MW-8 Monitoring well MW-9 remained free of contamination throughout the monitoring period Analytical ' results of ground water samples collected from April 1992 to November 1997 are included in Appendix A In October 1990,remediation at the site was initiated with the installation of soil venting components in the former UST pit From January 1995 to November 1996, soil vapor extraction and ground ' water extraction systems operated on the site,extracting approximately 2,391 8 pounds(lbs)of total petroleum hydrocarbons quantified as gasoline (TPH-g) and 8 17 lbs of benzene It was estimated that 0 068 lbs ofinethyltertiary-butyl ether(MTBE),0 032 lbs of 1,2-dichloroethane(1,2-DCA),and 0 56 lbs of total petroleum hydrocarbons quantified as diesel(TPH-d)remained in the ground water, while an estimated 0 01 lbs of 1,2-DCA remained in soil when remediation was halted Apex Advanced GeoEnvironmental,Inc