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ERI 205505 R02 Tosco Service Station 5886,Stockton, California September 25, 2001 20 SITE DESCRIPTION 221 Background It is ERI's understandingthat Union Oil Company of California Unocal purchased the land in 167 P Y (Unocal) 9 , at which time the property was vacant In 1982, Unocal constructed a service station at the site and installed three 12,000-gallon USTs to store gasoline and diesel fuels (Delta Environmental Consultants, Inc [Delta], February 6, 1996) During 1997, Tosco Marketing Company assumed ownership of the property During September 2001, Phillips merged with Tosco Marketing Company and assumed ownership of the property Environmental activities previously preformed at the site include the following 1995 — Unocal removed three 12,000-gallon fuel USTs, product piping, and dispensers Unocal replaced the fuel USTs with one 12,000-gallon and two 15,000-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 (Delta, February 6, 1996) • September 1995 - Unocal advanced 21 soil borings and collected soil 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 soil borings, and constructed groundwater monitoring wells in three of these borings (Delta, December 5, 1996) ' First Quarter 1997 - Tosco assumed ownership of the property • 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 soil and groundwater investigation at the site The investigation included hand-auguring three soil borings adjacent to catch basins, drilling four soil borings, and constructing dual-completion groundwater monitoring wells (MW4S/D through MW7S/D) at . discrete depths in the borings (ERI, June 3, 1997) 2