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staidec Work Plan to Destroy Existing Monitoring and Remedial Wells 76 Station No. 5886 - Apnl 18,2011 INTRODUCTION On behalf of ConocoPhillips Company, Stantec Consulting Corporation (Stantec) has prepared; this Work Plan to Destroy Existing Monitoring and Remedial Wells (work plan)for 76 Station No. 5886, located at 2701 West March Lane, Stockton, California (Figures 1 and 2). This work plan was prepared in response to a March 3, 2011 letter from the San Joaquin County Environmental Health Department(SJCEHD [Attachment 1]). The scope of work consists of destroying all existing monitoring and remedial wells at the site in: preparation for case closure, utilizing a combination of over-drilling and pressure-grouting:; Table 1 summarizes which wells will be over-drilled and which wells will be pressure-grouted. A brief discussion of site background and proposed scope of work are presented below. SITE DESCRIPTION I In 1967, Union Oil Company of California (Unocal) purchased the unoccupied property at 2701 West March Lane, Stockton, California. As described in a February 6, 1996 report, prepared by Delta Environmental Consultants (Delta), Unocal constructed a service station at the site in 1982, and installed three 12,000-gallon underground storage tanks (USTs) to hold gasoline and diesel fuels (replaced in 1995 with two 15,000-gallon and one 12,000-gallon' USTs). Currently the above-ground site features consist of a single-story convenience store building, six product dispensers on three islands, and a canopy covering the dispenser islands. PREVIOUS ASSESSMENT In 1995, Unocal removed three 12,000-gallon USTs, product piping, and dispensers, and replaced the fuel USTs with two 15,000-gallon and one 12,000-gallon USTs. The UST replacement activities included disposal of approximately 1,200 cubic yards of soil at Forward' Landfill in Manteca, California and disposal of approximately 15,000 gallons of groundwater at Unocal's Rodeo, California facility. In September 1995, Unocal advanced 21 soil borings and collected soil and groundwater: samples from the borings utilizing a Geoprobe®sampling system. In September 1996, Unocal drilled four additional soil borings, and constructed groundwater monitoring wells(MW-1 through MW-3) in three of these borings. In April 1997, Tosco performed a soil and groundwater investigation at the site in accordance' with a work plan and addendum prepared by Environmental Resolutions, Inc. (ERI). The investigation included hand-augering three soil borings adjacent to catch basins, drilling four soil, borings, and constructing dual-completion groundwater monitoring wells (MW-4S/D through MW-7S/D)at discrete depths in the borings. From July 1997 to December 1998, Tosco performed the following tasks: 1) conducted an underground utility search; 2) performed quarterly groundwater monitoring and sampling; 3) obtained access agreements to drill soil borings .at selected properties located south and southwest of the site; and 4) requested Sequoia Analytical Laboratories to -review: chromatograms of groundwater samples collected during the September 1995 Geoprobe'; 1 I