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OFOMATRIX 0.2 and 0.3 micrograms per liter(µg/1),respectively). Historical grab groundwater analytical results are presented along with results from the current investigation in Table 2. Geomatrix performed a human health screening evaluation(Geomanis,2000)based on data collected by Earth Systems and Geomatrix in 1999; the evaluation was submitted to Department of Toxic Substances Control(DTSC). The potential health risk posed by the residual petroleum constituents detected in on-site soil and groundwater was evaluated based on the methodology presented in the Human Health Screening Evaluation section of the Preliminary Endangerment Assessment Guidance manual(PEA;Cal-EPA, 1994). h3 the evaluation,the maximum detected concent limon for each constituent detected in soil and groundwater was conservatively used as the representative concentration. This evaluation indicates that exposure to residual petroleum constituents present in soil and groundwater should not pose an unacceptable health risk to hypothetical future on-site residents under the conditions evaluated. The theoretical excess lifetime cancer risk for an adult and child resident is 1x106,which is at the lower end ofthe acceptable risk range of 1x10 to We used by Cal-EPA and United States Environmental Protection Agency (U.S.EPA)at sites where remediation is considered(CCR Division 4.5,Title 22,Section 67500.15 and U.S.EPA, 1990). The non-cancer hazard index for an adult and child resident is 0.37;this value is less than 1,indicating that the predicted exposure to petroleum at the site should not result in an adverse non-carcinogenic health effect(U.S.EPA, 1989). In addition,based on the conservative nature of the PEA evaluation,the presence of residual petroleum constituents in soil and groundwater should also not pose an unacceptable human health risk to future commercial and construction workers at the site. On March 14,2000 DISC issued a memorandum with comments on Geomatrix's assessment Geomatrix responded to DTSC's comments in a March 29,2000 letter to CEMC. On April27,2000 DTSC issued a letter stating that the analytical data at the site were adequate in both quantity and quality and the residual levels of petroleum-related hydrocarbons in soils are below levels that would constitute an unacceptable risk to human health and the environment. The DISC letters and Geom mix's response to DTSC comments are presented in Appendix A. 3.0 ADDITIONAL INVESTIGATIVE OBJECTIVES Geomatnx conducted two off-site investigations to assess the lateral extent of petroleum-affected soil encountered in two areas during previous investigations,and to further assess groundwater conditions at and downgradient of the site. On May 16,2000,borings GMX-25 and GMX-26 were advanced to assess conditions north of the site and GMX-27 was advanced to assess groundwater conditions \\SF3\DEr ATA\DOC SAFE\ 05\baa7.003TOMBOSOMWa OV00 .DOC 3