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WORK PLAN FOR 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 Trucking yard on Newton Road in Stockton, California(Figure 1). The scope of work, described in Upgradient proposal no. 96008, is to drill five soil borings in a circular pattern around the former location of two underground storage tanks and related fuel dispensers in order to collect samples for laboratory analysis of petroleum hydrocarbons. The proposal was submitted on February 23, 1996. The project is under the supervision of San Joaquin County Environmental Health Divison(EHD). 2.011ACKGROUND 2.1 Tank Removal and Overeacavation 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 excavation. 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/EBD). 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 at 70 feet and encountered diesel-contaminated soil from the surface to total depth. The other boring was terminated at 100 feet and completed as monitoring well MW-1 (shown as GT-3 in Figure 2), and encountered diesel-contaminated soil to 85 feet and gasoline-contaminated soil from 45 to 60 feet. Low concentrations of gasoline, diesel, and aromatics were also detected in the water sample from MW-1. A domestic water well (G-1) located near the shop building was also sampled, but no hydrocarbons were detected.