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ATTACHMENT B SUMMARY OF PREVIOUS INVESTIGATIONS Chevron 9-6258, 2409 East Main Street at F Street, Stockton, CA PREVIOUS INVESTIGATION In February 1995, The Mark Group performed a Phase I Environmental Site Assessment (ESA) at the site and concluded that further investigation be performed. In April 1995, The Mark Group performed a preliminary subsurface investigation involving the drilling of six soil borings. The Mark Group reported residual concentrations of hydrocarbons in the former UST excavation backfill. Groundwater was not encountered and was reported to be in excess of 50 feet below the ground surface (bgs). In June 1996, NorCal Geophysical Consultants, Inc. (NorCal) performed a geophysical survey at the site. NorCal identified five anomalies associated with the waste oil tank, sump drain line, two corners of the former building where utilities entered the building, one location adjacent to the former pump islands; and one location where the former product lines and utility lines crossed. Subsequent excavation in these areas revealed debris within approximately 2 feet of the surface, including asphalt, both clay and metal piping, and wires. In September and October 1996, Pacific Environmental Group, Inc. (PACIFIC) observed excavation of the waste oil tank and former product lines, excavation of debris from the former gasoline UST pit, and limited over-excavation of the former gasoline UST pit. Gasoline hydrocarbon concentrations of up to 4,000 parts per million (ppm) were reported in the southeast corner of the former UST pit. After limited over-excavation of approximately 380 cubic yards of impacted soil and 200 cubic yards of concrete debris from the former gasoline UST pit, perimeter soil samples collected from the excavation were reported nondetectable to 2,300 ppm TPH-g. No petroleum hydrocarbons were detected in the soil samples colleted from beneath the former waste oil tank and former product lines. As approved by San Joaquin County Environmental Health Division (SJCEHD), material excavated from the waste oil tank pit was reused as backfill in the waste oil tank excavation. Excavated debris and soil from the former gasoline UST pit was disposed of offsite. In May 1997, one boring (B-7) was drilled within the southeast portion of the footprint of the former gasoline UST pit. Groundwater was encountered in Boring B-7 at a depth of approximately 58 feet bgs. A groundwater"grab" sample collected from B-7 was reported to contain 32,000 parts per billion (ppb) TPH-g and nondetectable concentrations of benzene. On April 1999, three monitoring wells were installed (MW-1 through MW-3) at the site by SECOR. Soil samples collected from the installation were found to contain nondetectable concentrations of petroleum hydrocarbons (Table 1). Quarterly monitoring and sampling of site monitoring wells commenced in April 1999. Groundwater samples are analyzed for TPHg, BTEX and fuel oxygenates.