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Well Installation Report August 17,2001 ARCO Facility No. 5469 Stockton,California . Page 2 Site Background In February 1999, The IT Group (IT) observed the removal and replacement of product lines and associated dispensers. Soil samples were collected at depths ranging between 3.3 to 5.5 feet bgs in the vicinity of the product lines and dispenser locations following their removal. Samples collected in the vicinity of the dispenser islands indicated the presence of total petrolebm hydrocarbons as gasoline (TPHG) at concentrations up to 7.32 milligrams per kilogram (mg/kg), benzene at a concentration of 0.187 mg/kg, tert-butyl alcohol (TBA) at 44 mg/kg, and methyl-tert butyl ether (MTBE) up to 5.16 mg/kg (IT, 2000). Based on the analytical results, limited excavation of hydrocarbon impacted soil was performed in the vicinity of the north end of the two dispenser islands. Approximately 4 cubic yards of impacted soil was excavated from the north end of the western dispenser island. Due to subsurface utilities and native hardpan soils (encountered at approximately 3 feet bgs), only overlying pea-gravel was hand excavated from the north end of the eastern dispenser island. Throughout the project, approximately 60 cubic yards of soil and pea-gravel were excavated and removed from the site(IT, February 2000). • In a letter dated February 2, 2000, the San Joaquin County PHS/EHD requested further characterization of soil and groundwater at the site. The request was based on the findings of soil sampling performed during product line removal activities in February 1999. In response to the letter, IT proposed to install three onsite groundwater monitoring wells. In a letter dated April 4, 1999, PHS/EHD issued a Workplan Approval Letter that requested additional wells be installed. On May 1, 1999, Delta submitted a Workplan Addendum proposing a total of six onsite groundwater monitoring wells. Delta contracted Woodward Drilling Company of Rio Vista, CA to drill and install six groundwater monitoring wells between June 5 and June 8, 2000. The site was subsequently placed on a quarterly monitoring program. A Well Installation Report was submitted to San Joaquin County PHS/EHD on October 4, 2000. Sensitive Receptor Surve At the request of San Joaquin County PHS/EHD, Delta conducted a review of the California Department of Water Resources (DWR) confidential well files to locate water supply wells located within a 2,000-foot radius of the site. Delta also contacted California Water Service Company, the local water purveyor, to obtain additional information regarding current use'of water supply wells in the vicinity of the site. Appendix A contains two maps showing the location of the water wells, a table documenting well details, and e-mail correspondence between . Delta and California Water Service Company. DWR well drillers reports for wells located within 2,000 feet of the site are also included in Appendix A. C:lARC0154691REP0RTS15469We11 Installation 060t.doe