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September 28, 2000 Project 6447.003 Mr. Robert Mihalovich Chevron Environmental Management Company P.O. Box 6012 San Ramon, California 94583 Subject: Revised Soil and Grab Groundwater Sampling and Work Plan for Additional Sampling Pombo Property, Tracy, California Dear Mr. Mihalovich: Geomatrix Consultants, Inc. (Geomatrix)has prepared this revised work plan for work to be conducted at the Pombo Property(the site; Figure 1) on behalf of Chevron Environmental Management Company(CEMC). Soil and groundwater investigations were conducted at the site by Earth Systems Consultants in July 1999 and by Geomatrix in September 1999. An initial investigation of the off-site lateral extent of petroleum hydrocarbon-affected soil and groundwater quality was performed by Geomatrix in May 2000. The proposed scope of work involves advancing three boring to further investigate the lateral extent of petroleum hydrocarbon-affected soil to the north of the site and collection of a grab groundwater sample in the zone of petroleum hydrocarbon affected soil on site. The proposed work is described below. In May 2000, Geomatrix advanced three direct push borings (GMX-25, 26, and 27)to collect soil and groundwater samples to assess the lateral extent of petroleum hydrocarbon-affected soil and groundwater quality downgradient of the site (Figure 2). Total petroleum hydrocarbons quantified as crude (TPHc)was detected in one of the three soil samples analyzed(GMX-26) at a concentration of 120 milligrams per kilogram (mg/kg). TPHc was not detected in soil in borings GMX-25 or GMX-27,which define the lateral extent of petroleum hydrocarbon-affected soil in the areas shown on Figure 2. This work plan outlines further investigation activities to define lateral extent of affected soil in the area of GMX-26 and to collect groundwater samples from within the area of affected soil. A report documenting results of the May 2000 investigation and this investigation will be submitted to you following completion of this investigation. Geomatrix will contract with Precision Sampling, Inc., of Richmond, California, to advance three off-site soil boring in the West Side Irrigation District(GMX-30, 31, and 32; Figure 2). One boring will be located to the northeast of GMX-26 near the sound wall along the southwest boundary of the residential development. Two borings will be advanced to the northwest and northeast of GMX-26, respectively. A fourth boring(GMX-29)will be advanced within the center of the area of petroleum hydrocarbon affected soil defined by previous investigations to collect a grab groundwater sample. During previous investigations at the site, groundwater was encountered at 11.0 to 12.0 feet below ground surface(bgs). Prior to drilling, Underground Service Alert(USA)will be notified and the soil boring locations will be cleared by a private underground utility locator. A soil boring permit will be obtained from the San Joaquin County Public Health Services, Environmental Health Division, and an encroachment permit will be obtained from the City of Tracy. The boring will be advanced using a hydraulically driven,