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