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ERI 205503 R01 Unocal Service Station 5886, Stockton, Califomia June 3 1997 . 2.0 BACKGROUND The site is immediately west of Interstate 1-5 on the northwestern corner of March Lane and the I-5 offramp in Stockton, California, as shown on the Site Vicinity Map (Plate 1) The locations of existing underground storage tanks (USTs), dispenser islands, and other selected site features are shown on the Generalized Site Plan (Plate 2) Properties in the vicinity of the site are generally occupied by commercial developments Unocal Corporation purchased the land in 1967, at which time the property was 4acant In 1982, Unocal constructed a service station at the site and installed three 12,000-gallon UST's used to store gasoline and diesel fuels (Delta Environmental, February 6, 1996) Between July 1995 and September 1996, Unocal replaced the original UST's and associated product distribution lines, enlarged the existing tank pit to accommodate three new 15,000-gallon UST's, advanced 21 soil borings and collected soil and groundwater samples from the borings utilizing a geoprobe sampling system, and drilled four additional soil borings and constructed groundwater monitoring wells in three of these borings The location of existing wells are shown on Plate 2 Unocal initiated quarterly groundwater monitoring during December 1996 Data collected to date by others indicate the groundwater flow direction ranges from northwest to west and the groundwater surface is approximately 7 to 8 feet below ground surface (bgs) Concentrations of total petroleum hydrocarbons as gasoline (TPHg) and benzene, toluene, ethylbenzene, and total xylenes (BTEX) were not detected in groundwater except for 6 parts per billion (ppb) of xylenes detected in well MW2 during December 1996 Concentrations of MTBE have been detected in each on-site well (MPDS Services, Inc [MPDS], January 21, 1997) 2