SITE INFORMATION AND CORRESPONDENCE

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QEOMATRIX Mr. Robert Mihalovich Chevron Environmental Management Company May 9, 2002 Page 4 hydrocarbon, specifically crude and/or fuel oil, from ChevronTexaco's former OVP located along the northern boundary of the site adjacent to the railroad right-of-way. 3. Four screening level risk assessments were included, using either maximum detected concentrations or 95th percentile upper confidence limit on the arithmetic mean (95% UCL) as the exposure point concentrations, and assuming groundwater either used as drinking water source or not used as a drinking water source. No response required. 4. With only 6 groundwater samples, it is not appropriate to calculate a 95% UCL on the mean for groundwater, due to the large uncertainty with such a small sample size. Therefore, the screening level risk assessment that employed 95% UCLs is not technically valid. Comment noted. The assessment using 95%UCL for groundwater was performed for comparison purposes. 5. In the absence of a designation by the Regional Water Quality Control Board that drinking water is not a beneficial use, DTSCIHERD recommends assuming all groundwater is a potential future drinking water source. Geomatrix acknowledges that the Regional Water Quality Control Board considers virtually all groundwater as a potential drinking water source and our screening level risk evaluation included a scenario where groundwater would potentially be used as future drinking water source. In reality, it is extremely unlikely that a domestic water well will be installed at or near the property, which is within the City of Tracy. The City of Tracy Municipal Code (Section 11-1.16)requires the use of water supplied by the City and declares water wells to constitute a public nuisance that must be abated. To consider this more realistic scenario, Geomatrix also estimated risk assuming that groundwater is not ingested. 6. Acceptable risk for Proposition 65 and the Air Toxics Hot Spots Programs are not applicable to the DTSC Site Mitigation Program. In accordance with the National Contingency Plan (NCP), use 1x10"6 as the point of departure when determining whether a human receptor is "at risk'; as per Risk-Assessment Guidance for Superfund (RAGS): Volume I-Human Health Evaluation Manual,Part B,Development of Risk- Based Preliminary Remediation Goals (PRGs) (U.S. EPA, 1991). As stated in 40 CRF 300.430: "For known or suspected carcinogens, acceptable exposure levels are generally concentration levels that represent an excess upper bound life-time cancer risk to an individual of between 104 and 10-6 using information on the relationship between dose and response. The 10-6 risk level shall be used as the point of departure for determining remediation goals for alternatives when ARARs are not available or are not sufficiently protective because of the presence of multiple