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GEOMATRIX Mr.Robert D.Mihalovich Chevron Environmental Management Company October 4,2001 Page 2 This screening evaluation is based on the human health risk methodologies presented in the Preliminary Endangerment Assessment Guidance Manual(PEA; Cal-EPA, 1999). The PEA evaluation consists of a compilation of methods,models, and assumptions commonly used by California and United States Environmental Protection Agencies(Cal-EPA and U.S.EPA)to quantify human health risk and hazard at a site. The PEA evaluation is intended to provide conservative estimates of the theoretical lifetime excess cancer risk and noncancer hazard index based on reasonable maximum exposure(RME)in a residential setting. The site is cur- rently used for a commercial office. It is anticipated that the future use of the site will remain commercial. Potential likely receptors at the site include future on-site commercial workers and construction workers involved in site redevelopment. Nonetheless,to avoid restricting land use,the PEA methodology was used to evaluate a hypothetical future residential receptor as a conservative measure: According to the PEA, the maximum concentration for each of the constituents detected in each medium at a site should be used to represent the exposure point concentrations under a residential setting. In cases where thereis adequate characterization,the 95.percent upper confidence limit(95%UCL)of the arithmetic mean may be used to represent the exposure point concentration(Cal-EPA, 1999). U.S.risk assessment guidance(U.S. EPA, 1992) also stipulates that the exposure point concentration estimates should be based on the 95%UCL of the arithmetic mean to estimate a reasonable maximum exposure(RME)scenario. In the event that the calculated 95%UCLs exceed the maximum detected value,the maximum value should be used as the exposure point concentration. The 95%UCL concentration is defined I as a value that,when calculated repeatedly for randomly drawn subsets of site data, equals or exceeds the true mean 95 percent of the time(U.S. EPA, 1992). The 95%UCL concentration is a more accurate predictor of actual chronic exposure because it is based on the probability of long-term random contact with impacted areas. The purpose of using the 95%UCL is to account for natural variability and uncertainty in the estimate of the mean. Analytical results are available for twenty(20)soil and six(6)grab groundwater samples at and immediately north of the site. Soil samples were collected for analysis between 8 and 14.5 feet below ground surface (bgs)where themost visibly affected soil was encountered. For the purpose of this screening evaluation,the health risks were estimated using the maxi- mum detected concentrations and the calculated 95%UCLs of all data as the representative exposure point concentrations. Sampling results of soil and shallow grab groundwater sam- ples are presented in Tables 1.and 2. The maximum concentrations and the calculated 95% UCLs of the arithmetic means are also presented in these tables. For chemicals that were detected in at least one sample and reported as"not detected"in another sample, it is assumed that the chemical was present in that sample at a concentration of one-half the detection limit. This method of establishing representative concentrations follows DTSC guidance(Cal-EPA, 1992) and is consistent with estimating exposure for the RME (U.S. EPA, 1989).