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_ _AW"&ff_AR <br /> 1992 Third Quarter Groundwater Monitoring Report Arm <br /> f; Western Stone Products, Tracy, California <br /> t + August 28, 1992 <br /> Page 3 <br /> during well and soil boring installation (Table 3). Figure 5 shows the vertical distribution of <br /> TPHD and benzene encountered in SBI. <br /> DISCUSSION <br /> Although unexpected levels of petroleum hydrocarbon analytes were detected in boring SBI <br /> soils, the transfer of the contaminants to the groundwater has been negligible, as borne out by <br /> contaminant levels found in groundwater from MW4. Well MW4 is located 6 feet downgradient <br /> of the former tank pit location. The contaminated soils have been in the tank pit since shortly <br /> after tank removal on August 23, 1988, and were never capped with asphalt. Apparently the <br /> adsorptive properties of the fine grained soils (clayey silts) have effectively retarded solution in <br /> groundwater at the site. <br /> Based on soil sampling from MW1, MW4, SBI, and SB2, petroleum hydrocarbon impacted soils <br /> appear to be essentially confined to the tank pit backfill material. The affected area is <br /> approximately 150 square feet, as depicted on Figures 3 and 4. The vertical profile of <br /> contamination (as observed in SBI) is depicted on Figure 5. Diesel concentrations fall off <br /> `== r= dramatically between 5.5 and 8 feet whereas benzene increases with depth in this interval. Table <br /> 4 summarizes the soil volumes and contaminant concentrations used to estimate residual soil <br /> impact. An estimated 12 gallons of diesel and .07 fluid ounces (1.81 grams) of benzene are <br /> contained in the impacted soils. This figure assumes a uniform concentration from the surface <br /> to the first sample at 5.5 feet. <br /> RESNA Industries recommends the removal of the impacted soils. <br /> WORKPLAN <br /> RESNA Industries, on behalf of Western Stone Products, Inc., proposes the following scope of <br /> work: <br /> EXCAVATION AND SAMPLING <br /> A backhoe will be used to excavate the former tank pit. If possible, the excavation will be <br /> opened to an approximate depth of 14 feet below grade (6 feet below groundwater) in order to <br /> remove contaminated soil found to exist above and below the water table. The areal dimensions <br /> of the excavation are expected to coincide with or be slightly larger than the former tank pit <br /> QR081292,➢RF <br />