ARCHIVED REPORTS_XR0012962 - Laserfiche WebLink

Home Browse Search

• 31 March 2005 AGE-NC Project No 03-1080 Page 2 of 7 ' 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) According to previous site investigations,ground water is approximately 30 feet to 35 feet below surface grade (bsg), and flowing under a northeast-trending hydraulic gradient, however, ground water elevation and gradient may vary both locally and seasonally ' '22 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 collected during the well installation In November 1989,three 10,000-gallon USTs 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 y 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 Monitonng well MW-9 remained free of contamination throughout the monitoring period ' 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 of methyl tertiary-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 Envirotech, Inc (Apex)residual mass balance calculations are provided in Appendix A ' During the last ground water monitoring event, conducted in November 1997, 16 micrograms per liter(p.g/l) 1,2-DCA and 5 5 µg/l MTBE were detected in monitoring well MW-1,5 7µg111,2-DCA and 16 µg11 MTBE were detected in ground water samples collected from monitoring well MW-2, 610 �Lg/l TPH-d, 0 72 µg/l toluene, 2 3 µg/l 1,2-DCA, and 8 4 µg/1 MTBE were detected in well ' Advanced CeoEvironmental,Inc 1