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WORK PLAN FOR ADDITIONAL SITE ASSESSMENT Gillies Trucking 3931 Newton Road Stockton, CA 1.0 INTRODUCTION Upgradient Environmental Consultants (Upgradient) has been retained by Gillies Trucking Co. to perform further assessment of soil and groundwater contamination at the Gillies Truckirg yard on Newton Road in Stockton, California(Figure 1). The scope of work is to drill two soil borings near the former location of two underground storage tanks and related fuel dispensers and to install a monitor well in the shallower boring. The proposal to prepare the work plan was accepted and pre- approved by the State UST Cleanup Fund on February 19, 1999. The project is under the supervision of San Joaquin County Environmental Health Divison(EHD). 2.011ACKGROUND 2.1 Tank Removal and Overexcavation According to the assessment report prepared by Richesin & Associates in 1990, two underground storage tanks (UST's) were removed from 3931 Newton Road in January 1989 by SEMCO, Inc. Soil samples collected during the UST removal in 1989 contained petroleum hydrocarbons in the gasoline and diesel ranges,prompting assessment of the extent of soil contamination. Contaminated soil was excavated to a depth of 35 feet by SEMCO in July, 1990, but residual hydrocarbons were detected in all eight soil samples collected from the floor and sidewalls of the excavation. Clean soil was excavated from the south end of the property and placed into the open excavaion. The contaminated soil was then treated with bacteria and stored at the south end of the property. It was later sampled and approved for disposal by San Joaquin County Public Health Service, Environmental Health Division(PHS/EHD). The soil was then spread out and the area was graded. 2.2 Initial Drilling and Sampling Richesin& Associates supervised the drilling of two borings in November, 1990, and collected soil and groundwater samples for analysis. One boring (TB-2, Figure 2) was terminated z.t 70 feet. Diesel concentrations were detected between 25 and 40 mg/kg (ppm)between a depth of 60 and 70 feet. The other boring was terminated at 100 feet and completed as monitoring well MW-1 (shown as GT-3 in Figure 2). Gasoline and diesel were detected at low to moderate concentrations to a depth of 60 feet; below this, diesel was reported in three of four samples at a concentration of 3.4 mg/kg. 2.3 Further Assessment and Monitoring Upgradient was retained in August, 1994 and supervised the installation of two additional monitoring wells in September. Both wells were drilled to 100 feet. Five soil samples f-om GT-1