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`wo «� WELLS FARGO BANK PROPERTY 49 SOUTH WILSON WAY STOCKTON, CALIFORNIA DECEMBER 28, 1998 Page 5 Based on the general layout of the former gasoline service station, the location of the pump dispensers and the current concentrations of TPH-g and benzene in the soil, it is Building Analytics' opinion that the underground storage tanks (USTs) for the former service station were located near the southeast corner of the property. More specifically, the USTs would have been located east of the former pump island close to the corner of East Market Street and South Wilson Way, which is beneath the current bank building. Based on previous .� borings completed inside the bank building, it is Building Analytics' opinion that the USTs no longer exist on-site. Area Impacted by TPH-g Between February 1996 and October 1998, Building Analytics installed six soil borings and four groundwater monitoring wells in order to assess the vertical and lateral extent of TPH contamination beneath the subject site. The locations of the soil borings and monitoring wells are shown in Figure 2. The general area impacted by TPH-g is located in the southeast corner of the property in the area of Monitoring Wells MW-2 and MW-4, and Boring B-5 (Figure 3 and Figure 4). The vertical extent of TPH-g contamination is limited to between 35 and 60 feet bgs (Figure 5). In one boring (Boring B-5), the 78 feet deep sample was reported to have contain insignificant concentrations of TPH-g (2.8 mg/kg) -- and no benzene. It is Building Analytics opinion that this concentration is insignificant and that the vertical and lateral extent of the TPH-g plume has been defined. TPH-g Concentration in the Soil TPH-g has been detected in soil samples collected from three of the borings. The highest y concentration have been reported in soil samples from the boring for Monitoring Well MW- 2. The concentration in samples from this boring range from 65 to 1,300 ppm of TPH-g, and are located between 35 and 60 feet bgs. Samples collected from Boring B-5 have been reported to contain TPH-g in concentrations ranging from 1.3 to 590 ppm and are located between 35 and 78 feet bgs. Samples collected from the boring for MW-4 have been reported to contain 1.3 and 75 ppm of TPH-g and are located at 35 and 50 feet bgs. Samples collected from other depths within these borings as well as from all other borings installed at the subject site have been reported not to contain detectable concentrations of TPH-g. Mass Balance Analysis for TPH-g Remaining in Soil A Mass Balance Analysis was conducted to determine the approximate amount of TPH-g " remaining within the soil beneath the subject site. In order to calculate the approximate amount of TPH-g the total area of impacted soil was divided into three areas based on TPH-g concentrations. Additionally, the total porosity was assumed to be equal to the �y effective porosity. TPH-g concentrations within the soil in each area were averaged. The total amounts of TPH-g remaining in the soil beneath the site are as follows: Porosity = 38.76% Average unit weight of soil = 102 pounds/cubic foot Estimated area and thickness of soil in Area A= 364 square feet, 26 feet thick