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Civic Center Square 2444 Main Street, Suite 21 E. Fresno, California 93721-2734 (559) 264-2535 • FAX(559) 264-7431 GEOMATRIX July 8, 2003 Project 6447.001 Mr. M. Scott Mansholt Chevron Environmental Management Company Post Office Box 6012 San Ramon, California 94583 Subject: Amendment to the Off-site Soil and Groundwater Investigation Work Plan U.S. Can/Welty Road 35275 Welty Road, Vernalis, California Dear Mr. Mansholt: Geomatrix Consultants, Inc. (Geomatrix),has prepared this amendment to the February 11, 2003, Off-Site Soil and Groundwater Investigation Work Plan (the Work Plan) based on Geomatrix' recent discussions with Mr. Dean Hubbard of the California Regional Water Quality Control Board—Central Valley Region (RWQCB). The Work Plan was prepared for the subject site by Geomatrix on behalf of Chevron Environmental Management Company (ChevronTexaco) and was approved by the RWQCB in a February 28, 2003, letter(approval letter) to ChevronTexaco. In the Work Plan, two soil borings (GMX-10 and GMX-11) were proposed to be advanced using a sonic rig to about 90 feet below ground surface (bgs) for collecting soil and grab groundwater samples. In the approval letter, the RWQCB requested that an additional soil boring be placed between GMX-10 and MW-2 for similar sampling. The three proposed soil borings are located within the shoulders of Highway 33. Geomatrix obtained an encroachment permit on behalf of ChevronTexaco from the California Department of Transportation (Caltrans) for the soil borings. During a recent site visit to mark the proposed soil boring locations, Geomatrix staff observed overhead power lines above the three boring locations. The drilling contractor would not be able to set up the mast of the sonic rig beneath the power lines for advancing the proposed borings. Geomatrix discussed the possibility of relocating the proposed soil borings to within Highway 33 with Caltrans. Mr. Marcel De La Cruz of Caltrans (Stockton office) indicated that drilling would be permitted within a highway if Caltrans had plans to repave the highway within 6 months; currently, Caltrans has no plan to repave Highway 33 within the next 4 years. Based on these site constraints, Geomatrix proposes to use a rhino rig to advance the three proposed borings. A rhino rig has a shorter mast than a sonic rig and therefore will be able to drill beneath the overhead power lines. However, due to soil conditions at the site, a rhino rig will not be able to advance the borings to about 90 feet bgs as proposed in the Work Plan. We Geomatrix Consultants, Ince Engineers, Geologists, and Environmental Scientists