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Site Closure Request Pombo Property June 14,2005 r... All detected constituents except for TPH measurements were quantitatively evaluated;the maximum detected concentration for each constituent was conservatively used as the representative concentration. TPH results were not directly evaluated as prescribed in the PEA. The toxicity of the petroleum mixtures is best described by the aggregated toxicity of key individual constituents in the mixture,such as benzene and PAHs(Cal-EPA, 1999). Appendix B includes the equations,models,and input parameters used to calculate the theoretical lifetime cancer risk and non-cancer hazard index from exposure to petroleum constituents detected in soil and groundwater at this site;all input assumptions were based on conservative default parameters provided in the PEA. The estimated non-cancer hazard indexes and theoretical excess cancer risks for all scenarios are below or at the lower end of the range generally considered acceptable by regulatory agencies(i.e., a hazard index of 1 for noncarcinogens and a cancer risk of 1 x 10,6 to 1 x 10-4 for carcinogens). Geomatrix noted that the presence of petroleum constituents currently measured in soil and shallow groundwater at the site should not pose an unacceptable noncarcinogenic or carcinogenic risk to future on-site residential receptors. 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 workers at the site. The Human and Ecological Risk Division(HERD)of the DTSC,provided comments on the Geomatrix risk assessment(2001)in a January 2,2002 memorandum concerning risk levels, limitations on data, lack of vertical extent delineation,and lack of data to address the source of BTEX,creating an uncertainty in the estimated risks. Geomatrix responded to comments in a May 9,2002 letter(Appendix C);the Geomatrix response included a cross-section showing delineation of vertical extent. In an August 5,2002 letter(Appendix A),DTSC states that while the total risk for the focused screening level risk assessment is in excess of the benchmark target of 1 x 10-6, it falls within an acceptable risk range of 1 x 10-6 to 1 x 10"4. In addition,the DTSC states the current land use is acceptable within this range provided that a land use covenant(i.e.,deed restriction)is applied to the site. The DTSC agrees that the current land use `-' designation for the Pombo Property is acceptable. 4. CONFORMANCE TO APPENDIX A OF TRI-REGIONAL BOARD STAFF RECOMMENDATIONS: REQUEST FOR CLOSURE This section presents the documents that support the No Further Action request needed by the RWQCB in "Appendix A Reports Tri-Regional Board Staff Recommendations for Preliminary Investigation and Evaluation of Underground Tank Sites." 4.1 CHECKLIST OF REQUIRED DATA FOR NO FURTHER ACTION REQUESTS AT UNDERGROUND TANK SITES A completed"Checklist of Required Data for No Further action Requests at Underground Tank Sites"is included in Table 3. 4.2 REQUIRED DOCUMENTATION FOR NO FURTHER ACTION REQUIRED REQUEST A discussion of the required minimal documentation supporting the request for closure of the subject site is provided in this section. 1. Site history and current site conditions. The OVP was located along the northern boundary of the Pombo Property, adjacent to the UPRR ROW. The former OVP was used to transport crude oil and Bunker C fuel oil from Bakersfield to the Richmond Refinery,California, from the early 1900s to through the mid-1930s,at which time the pipeline system was shut down due to the depressed economy. Pump station equipment and the pipelines were removed south of Tracy and used for other pipeline construction projects;the lines from Tracy to Richmond were 4