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ERI 205503.W01 Unocal Service Station 5886, Stockton, California March 28, 1997 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 ground 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 grade. Concentration of total petroleum hydrocarbons as gasoline (TPHg) and benzene, toluene, ethylbenzene, and total xylenes (BTEX) have not been 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, January 21, 1997). In response to concerns raised by the San Joaquin Public Health Department related to the detection of MTBE in groundwater during successive quarters, 76 Products Company requested an inspection of the site for fuel leaks. On March 20, 1997, Banks and Company of Fresno, California performed an inspection. The inspection did not note any leaks in the existing piping or in underground storage tanks interstitial spaces. The inspection report is attached (Attachment A). PROPOSED WORK This investigation is specifically designed to investigate the integrity of the existing product storage and delivery system, the source of potential discharges of petroleum hydrocarbons to the subsurface, and evaluate the lateral extent of residual MTBE in soil and dissolved MTBE detected in groundwater. The specific tasks are summarized below. ERI will perform field work in accordance with standard protocol (Attachment B), and ERI's site-specific Health and Safety plan. 76 Products Company will obtain data related to the integrity of the existing system. The following tasks will be completed: Task 1: Evaluation of System Integritv • Contact the laboratory to review chromatograms of groundwater samples collected from the 21 geoprobe locations prior to evaluate groundwater conditions with respect to MTBE prior to placement of the existing tanks and lines. • Obtain product reconciliation records. Task 2: Evaluation of Soil and Groundwater • Prepare and obtain appropriate well permits. • Observe the drilling of four on-site soil borings (135 through 138) utilizing a hollow-stem auger drilling rig and constructing groundwater monitoring wells MW4 through MW7 in these borings. The borings will be advanced to a minimum depth of 10 feet below first encountered groundwater. ERI expects groundwater to be encountered approximately 7 to 8 feet bgs. The locations of the proposed wells are shown on Plate 2. 2