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CLOSURE REQUEST 5-31-94 range hydrocarbons benzene, toluene and ethylbenzene from the ground water. EPA models "VLEACH" and "BIOPLUME" were used to predict the long- term fate of the remaining gasoline range hydrocarbons that remain in the soil as of February 2 , 1994, see UPDATE STATUS REPORT #27, dated April 22, 1994 . These models predicted that in 100 years, only the area near SB14 would impact the ground water, but due to natural oxidation and slow migration velocities, that the impact would be insignificant and would never leave or cross the site boundaries. VI. REMEDIATION ACTIVITIES a) Rationale for selected remedial option. OVER-EXCAVATION Initial sampling and assessment of the product line and underground storage tank excavations indicated that the majority of the contaminant would be found above the twenty foot depth. The soil within this interval consisted of silty clay to clayey silts and would perform poorly to in-situ methods that are conducive to much sandier soils . Over-excavation was accomplished to the 13 foot depth of the central pump island area, the 20 foot depth for the eastern and western pump island areas and the 26 foot depth for the underground storage tank area This soil was subjected to uncontrolled aeration on site, until clean, and then used as compacted backfill in the top of the excavations An estimate of total pounds of gasoline range hydrocarbons remaining in the soil was calcualated and a pilot study for vapor extraction using activated carbon vapor scrubs as Best Available Control Technology (BACT) was performed. The pilot study indicated that approximately 60 pounds a day of hydrocarbons would be removed. And vapor extraction would take approximately 195 days to remove the estimated 2865 pounds of hydrocarbons remaining in the soil after the pilot test . � b) Soil -remediation method and effectiveness Over Excavating and Soil Aeration On Site Approximately 800 cu yds of soil were excavated and piled on site during removal of the underground storage tanks and product dispensing lines . The hottest of these piles (SP1) was used to classify the soil as non hazardous, see PRELIMINARY ASSESSMENT REPORT, DECEMBER 15, 1989 (Updated January 16, 1990) - Appendix E -"HAZARDOUS CLASSIFICATION REPORT" . This soil was aerated in 54 cubic yard increments as approved by the San Joaquin County Air Pollution Control District on November 1, 1989 . Aeration was page2l