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S E C 0 R • QUARTERLY SUMMARY REPORT FOURTH QUARTER 2003 76 Service Station#5886 2701 West March Lane, Stockton, CA City/County ID# Stockton County San Joan BACKGROUND Union Oil Company of California (Unocal) purchased the land in 1967, at which time the property was vacant In 1982, Unocal constructed a service station at the site and installed three 12,000- gallon underground storage tanks (USTs) to store gasoline and diesel fuels (Delta Environmental Consultants, Inc [Delta], February 6, 1996) As presented in previous reports, a summary of site activities including remedial efforts at the site is as follows 1995—Unocal removed three 12,000-gallon fuel USTs, product piping, and dispensers Unocal replaced the fuel USTs with two 15,000-gallon and one 12,000-gallon UST The UST replacement activities included disposal of approximately 1,200 cubic yards of sod at Forward Landfill in Manteca, California and disposal of approximately 15,000 gallons of groundwater at Unocal's Rodeo, California facility(Delta, February 6, 1996) September 1995 - Unocal advanced 21 soil borings and collected sod and groundwater samples from the borings utilizing a Geoprobe®sampling system (Delta, February 6, 1996) June 1996- Pacific Environmental Group, Inc (PEG)performed a water-supply well survey within a one-quarter-mile radius of the site The survey identified 10 wells located within the survey area These wells are located northeast to southeast of the site and are apparently utilized as cathodic protection or irrigation wells (PEG, June 24, 1996) September 1996 - Unocal drilled four additional sod borings, and constructed groundwater monitoring wells in three of these borings (Delta, December 5, 1996) March 1997—Tosco submitted a Work Plan for Supplemental Environmental Investigation (ERI, March 28, 1997)to the County April 1997—Tosco submitted an Addendum to Work Plan(ERI, April 3, 1997)to the County April 1997—Tosco performed a sod and groundwater investigation at the site The investigation included hand-auguring three sod borings adjacent to catch basins, drilling four soil borings, and constructing dual-completion groundwater monitoring wells (MW4SID through MW7SID)at discrete depths in the borings (ERI, June 3, 1997) July 1997—Tosco submitted a Work Plan for Supplemental Environmental Investigation (ERI, July 14, 1997) to the County August 1997—Tosco submitted the Addendum to Work Plan (ERI, August 8, 1997) to the County • July 1997 through December 1998—Tosca performed an underground utility search (ERI, October 6, 1997) (Plate 2), continued to perform quarterly groundwater monitoring and sampling, obtained access agreements to drill soil borings at select properties located south