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2101 Webster SGreeG <br /> 12th Floor <br /> Oakland, GA 54612 <br /> (5101 663-4'100 - FAX (510) 663.4141 GEOMATRIX <br /> October 4, 2001 <br /> Project 7243.001 <br /> Mr.Robert D. Mihalovich <br /> Chevron Environmental Management Company <br /> P.O.Box 6012 <br /> J San Ramon, California 94583-0712 <br /> Subject: Results of the Updated Human Health Screening Risk Evaluation <br /> Pombo Property(San Joaquin County APN 232-170-07) <br /> 1755 West l ltn Street <br /> Tracy, California. <br /> Dear Mr. Mihalovich: <br /> Geomatrix Consultants,Inc. (Geomatrix)has prepared this letter summarizing the results of <br /> the updated human health screening risk evaluation of the additional soil and groundwater <br /> samples collected at the Pombo Property(the site)in Tracy, California. This letter supple- <br /> ments the Report ofAdditional Soil and Groundwater Investigation,dated December 21, 2000 <br /> (Geomatrix,2000a),which summarized the results of investigations conducted at the site in <br /> May and October 2000. <br /> r••-- Prior to the May and October 2000 investigations, Geomatrix prepared the Soil and Ground- <br /> water Investigation and Human Health Screening Evaluation (Geomatrix,2000b)report, <br /> dated January 17, 2000,documenting the results of the September 1999 field investigation <br /> and human health risk screening evaluation. The.results of the screening risk evaluation indi- <br /> cated that potential exposure to residual petroleum constituents at the site should not pose an <br /> unacceptable health risk to hypothetical future on-site residents under the conditions evalu- <br /> ated. The Department of Toxic Substance Control (DTSC)concurred with this conclusion in <br /> an April 27,2000 letter(Attachment 1), After completing this assessment, additional soil and , <br /> groundwater samples were collected in May and October 2000,to assess the lateral extent of <br /> petroleum-affected soil encountered during previous investigations, and to further assess <br /> groundwater conditions at and downgradient of the site. <br /> Analytical results from the May and October investigations were generally consistent with the <br /> results from previous investigations. With the exception of low:detections of benzene, tolu- <br /> ene,ethylbenzene and xylene(BTEX) in one shallow grab groundwater sample,the concen- <br /> trations and the chemicals detected in soil and grab groundwater are similar to the samples <br /> previously collected. BTEX compounds have not been detected in the previous grab ground- <br /> water sample. Benzene was detected in the new sample at a concentration of 0,6 micrograms <br /> per liter(gg/L),which is slightly above the laboratory reporting limit of 0.5 µg/L. Toluene, <br /> ethylbenzene, and xylenes were detected at a concentration of less than 4 µg/L. Results of <br /> soil and grab groundwater samples were combined with previous analytical data from the site <br /> and evaluated in this screening risk evaluation. <br /> Geometrix Consultants, Inc. <br /> Engineers, Geologists, and Environmental Scie=ntists <br />