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6ETTLER - RYAN INC.-El - May 8, 2002 Mr Michael J. Infurna Jr San Joaquin County Public Health Services Environmental Health Division 304 East Weber Avenue, Third Floor Stockton, California 95202 Subject - Verification and Evaluation of TBA in groundwater at Tosco (76) Service Station No. 11192, 1403 Country Club Boulevard, Stockton, California. ` Mr Infurna At the request of Tosco Corporation (Tosco), a subsidiary of Phillips Petroleum Company, Gettler-Ryan Inc (GR) has prepared this letter report to document the most recent groundwater sampling at the subject site (Figure 1) All fieldwork was conducted in accordance with the GR's Standard Operating Procedure — Groundwater Sampling (enclosed) The purpose of this work is to provide verification of tertiary Butyl Alcohol (TBA) concentrations detected in wells MW-3 and MW-7 during the groundwater sampling event conducted by GR on February 18, 2002, and to provide an evaluation of the results This letter was also prepared to comply with a letter from the San Joaquin County Public Health Services — Environmental Health Division (PHS-EHD,) dated April 23, 2002, which required the verification and evaluation of the presence of TBA at the site. On February 18, 2002, during the quarterly monitoring and sampling event conducted by GR, TBA was detected in groundwater samples from wells MW-3 and MW-7 (Figure 2) at concentrations of 240,000 and 220,000 parts per billion (ppb), respectively, as analyzed by EPA Method 8260B As requested by the PHS-EHD in a letter dated April 23, 2002, GR resampled the two wells on April 30, 2002 These samples were also analyzed by EPA Method 8260B The laboratory reported TBA in wells MW-3 and MW-7 at concentrations of 230,000 and 210,000 ppb, respectively. The TBA concentrations for the two sampling events are essentially identical, and confirm the presence of TBA in wells MW-3 and MW-7 As part of an ongoing experiment by Tosco to determine if the laboratory detection of TBA is influenced by the hydrochloric acid preservative in the sampling containers, additional samples were collected on April 30, 2002 in unpreserved containers and in containers preserved with tn-sodium phosphate Preliminary evaluation of the results from this site and other Tosco sites appear to indicate that the presence or absence of various preservatives does not significantly affect the detected concentrations of TBA The analytical results are summarized in Table 1 The laboratory analytical report is attached to this letter 6747 Sierra Court , Suite J Dublin , California 94568 • ( 925 ) 55.1 - 7555