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' ARCADIS Ms. Kathryn Dominic <br /> November 5, 2010 <br /> ' derived toxicity values, such as cancer slope factors, for the purpose of assessing <br /> carcinogenic risk to TPH-d (USEPA 2010; OEHHA 2010). <br /> ' In California, carcinogenic risk associated with TPH mixtures is typically evaluated by <br /> assessment of the carcinogenic constituents of TPH mixtures, namely benzene, <br /> ' ethylbenzene, and carcinogenic polycyclic aromatic hydrocarbons (PAHs). Of the 55 <br /> soil samples collected from a depth of 0 to 10 feet below ground surface (bgs) and <br /> analyzed for benzene and ethylbenzene, benzene was detected in only one sample, <br /> ' or less than 2 percent (%) of the samples. Ethylbenzene was detected in only 2 of 55 <br /> samples, or in less than 4% of the samples (IC 1994; Terranext 1997a and b). <br /> Benzene and ethylbenzene would not be selected as Compounds of Potential <br /> ' Concern (COPCs) at the Site because their frequency of detection is less than 5% <br /> following USEPA If 989) and California Department of Toxic Substances Control <br /> (DTSC 1992) guidance. <br /> ' Fluorene, naphthalene, 2-methylnaphthalene, and phenanthrene were the only PAHs <br /> detected in soil, from the surface to 10 feet bgs, based on historical site <br /> ' assessments. Of these, only naphthalene is classified as a carcinogen (USEPA <br /> 2010, OEHHA 2010). Naphthalene was detected in only 1 of 22 samples, or less <br /> than 5% of all samples. Therefore, naphthalene was not selected as a COPC for the <br /> site. <br /> 1 In the absence of carcinogenic COPCs, potential carcinogenic risk associated with <br /> ' TPH-d to human receptors appears minimal. In addition, for ecological receptors, <br /> cancer is not evaluated as an endpoint consistent with USEPA and DTSC ecological <br /> risk assessment guidance (USEPA 1997; DTSC 1996). Therefore, soils remaining in <br /> ' the biotreatment beds do not pose a risk to human health or sensitive ecological <br /> receptors. <br /> ' RWQCB Comment: Soil TPHd concentrations must be shown to be incapable of <br /> leaching to groundwater at levels that exceed beneficial use protective <br /> concentrations. <br /> ' Response:As a comparative reference, TPH-d cleanup goals protective of <br /> groundwater were negotiated with the RWQCB for the UPRR Former Tracy Yard <br /> VCA, located west of MacArthur Drive adjacent to the Site during 2008. The cleanup <br /> ' goals were approved by the RWQCB in a letter, "Approval of Final Removal Action <br /> Work Plan for OU-3 and OU-4, Former Tracy Rail Yard, Tracy, San Joaquin County, <br /> California,"to UPRR dated May 19, 2008 (RWQCB 2008). <br /> Page: <br /> PWCA0I5Dd MttAd'rOUnbnPMftS BOMWC709TmryMMFatillyip&'ru"m QD10118o4e nlBMRgwseWRWQCB1Mao LBWSRTC_W(IIL4- )d= 2/7 <br />