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w <br /> Geomatrix <br /> February 17, 2006 <br /> Project 7065.001 TAIOC ITlracyPump Station <br /> age�I��lJ�M"DJeport <br /> Mr. Dean Hubbard FEB 2 1 '1006 <br /> California Regional Water Quality Control Board ENVIRONMENT HEALTH <br /> Central Valley Region PERMIT/SERVICES <br /> 1685 E Street <br /> Fresno, California 93706 <br /> Subject: Additional Information for the Human Health Risk Assessment <br /> TAOC Tracy Pump Station - ggo p-� - <br /> Tracy, California <br /> Dear Mr. Hubbard: <br /> On behalf of Chevron Environmental Management Company (CEMC) and Union Pacific <br /> Railroad (UPRR), Geomatrix Consultants, Inc. (Geomatrix), submitted a report presenting the <br /> findings of the human health risk assessment (HHRA) for the TAOC Tracy Pump Station <br /> property (the site) in Tracy, California to the California Regional Water Quality Control Board— <br /> Central Valley Region (RWQCB)`. We understand that RWQCB will forward the report to the <br /> Office of Environmental Health Hazard Assessment (OEHHA) for review. Prior to submitting <br /> the HHRA to OEHHA, you reviewed the report and provided verbal comments to Geomatrix in <br /> telephone conversations on August 29, 2005 and January 19, 2006. This letter was prepared in <br /> response to your verbal comments and provides additional information supporting the HRRA for <br /> OEHHA's review. <br /> Below is additional information pertaining to five comments received from the RWQCB: (1) <br /> need for additional information to support why it is not necessary to evaluate exposure to <br /> groundwater, (2) presentation of historical chemical data, (3) filtering of groundwater samples <br /> for lead analyses, (4) clarification of text describing soil data used in the HHRA, and (5) an <br /> electronic version of the risk calculations presented in the HHRA. <br /> 1) The HHRA did not evaluate exposure to groundwater because no recent on-site <br /> groundwater samples had been collected for chemical analysis, and therefore, no current <br /> chemicals of potential concern (COPC) had been identified. Additionally, as shown on the <br /> Site Conceptual Model (SCM) on Figure 6 of the HHRA, there is no complete exposure <br /> pathway to groundwater. To support this conclusion, we note that depth to groundwater has <br /> historically been greater than 12 feet below ground surface, and therefore, direct contact with <br /> groundwater is extremely unlikely. Additionally, it is also extremely unlikely that a domestic <br /> t Geomatrix,2005,Human Health Risk Assessment,TAOC Tracy Pump Station,Tracy,California,June 30. <br /> 2101 Webster Street, 12th Floor Tel 510.663.4100 www.geomatrix.com <br /> Oakland,California 94612-3066 Fax 510.663.4141 <br />