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NfINICK-�-IfRNIR Mr. Charles Leubner 20 January 1997 Page 5 Because the 200 mg/Kg concentration was above the 100 mg/Kg threshold that Mr. Yoshioka indicated would most likely be acceptable as a maximum concentration of diesel in soil that could be left in place or be returned to the excavation, it was determined to overexcavate this area to investigate whether the staining was limited to a small area. However, the staining and odors increased with depth. The overexcavation continued to the maximum depth that the backhoe could reach - 18 feet below the ground surface. Diesel hydrocarbons were detected at a concentration of 6,150 mg/Kg in the sample collected from the soil at the base of the overexcavation. As indicated previously, this soil was stockpiled near the Bus Shelter (S 13). Pipeline Trench Excavation Samples Petroleum hydrocarbons were detected only in one of the 17 samples collected from the pipeline trench (Sample P-17) and one of the trench stockpiles (SP -3). Although Sample P-17 contained diesel hydrocarbons at a concentration of 1,550 mg/Kg, this area was not overexcavated prior to backfilling the trench. Because the 2.2 mg/Kg TPH-D concentration detected in SP -3 was below the 100 mg/Kg threshold for acceptable use as backfill material, the soil removed the pipeline trench was used to backfill the trench. Stockpile Samples Three of the four stockpiles of soil removed from the diesel tank excavation were sampled to determine if the soil could be used to backfill the tank excavation. Only the material stored in the stockpile adjacent to the excavation (Slthrough S8) was deemed suitable as backfill material. Diesel hydrocarbons were below the method detection limit of 1.0 mg/Kg in the composite sample analyzed from this stockpile, and the only BTEX constituent that was detected was xylenes at a concentration of 8 ug/Kg. The other stockpiles will be disposed of at Forward Landfill of Stockton, California. Clean pea gravel and Class A/B subbase was imported to complete backfilling of the diesel tank excavation and pipeline trench. REMOVAL OF GASOLINE TANKS Excavation of Tanks The asphalt over the two gasoline tanks was removed and soil was excavated from the sides of the tanks prior to the removal of the tanks on 3 January 1997. This soil was stored adjacent to the excavation (see Figure 4).