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PROBLEM ASSESSMENT REPOR Shc1l S--rncc Station 7910 Lower Sacramento Road,Stockton,California Shell WIC No.204-7524-OW Delta PrOjCCE No.40.88-147 Page 15 Soil excavation and treatment would require the excavation of approximately 600 cubic yards of both clean sod and soil containing petroleum constituents from the area of the former storage tanks. The excavated soil containing petroleum constituents would thm either In treated on site or hauled off site for treatment and disposal. Due to the limits of space on the site, on-site treatment is considered to be impractical. Thus,the material would have to be hauled off site for treatment and disposal. The excavation of the soil would cause a major disruption to the service station's operation. The estimated costs to perform excavation of the soil in the area of the former storage tanks, transportation and disposal costs to an off-site treaunent and/or disposal facility,backfilling the zxcavated hole,and replacement of ffie concrete surfaces ranges between$140,000 and 3210,000 depending upon the levels of petroleum constituents encountered in the soils excavated. The estimated costs do not include the loss of revenues due to station closure or any taxes that may have to be paid for the transportation and/or disposal of any soil classified as Itazardous waste." Soil vapor extraction would consist of the installation of one or two vapor extraction wells within y. the tank backfill area with treatment of the recovered hydrocarbon vapors prior to discharge to the k. atmosphere. It is estimated that this alternative would cost between 5135,000 and 5170,000 depending upon the Type of vapor destruction or adsorption treatment required. The no action alternative consists of leaving the soil containing petroleum hydrocarbons in place. The soil does not appear to represent a thmat to ground water quality because an impermeable surface overlies the former storage tank area,the soil beneath the site is predominantly silty clay and clayey silt,and the depth to ground water is greater than 55 feet. Therefore, based on the extensive costs for the removal of the limited amounts of petroleum constituents in the soil,and the minimal threat to ground water that these petroleum constituents represent,it is our opinion that the no action alternative is appropriate for this site. It is suggested that ground water monitoring be continued for a period of I year to fully evaluate the potential for- migration of the petroleum constituents from the soils in the area of the tank backfill to the ground water underlying the site. If monitoring of the ground water quality after the.1-year-period indicates that the ground water is not being impacted;site closure of the soils should be grante d i . If petroleum constituent levels in the ground water show an increasing trend,then itis suggested that a soil vapor extraction system be installed.