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' 1998) default values(see Appendix I), while plume dimensions correspond to highly conservative estimates based on the extent of hydrocarbon detection across the site Selected target risk/hazard levels represent the most conservative values outlined by USEPA (1989, 1991), ' ASTM(1995 and 1998), and DTSC (1999)risk assessment guidelines Use of a target risk level of I x 10-6 is particularly conservative with respect to standard risk assessment practice for evaluation of exposure under commercial/industrial land use ' In estimating risks associated with exposure to benzene, a carcinogenic slope factor of 0 1 (mg/kg/day)-', as outlined by the California EPA (DTSC, 1994), was used This value is more conservative than that ' outlined by the USEPA and ASTM Correspondingly, newly developed carcinogemc slope factors for inhalation and oral exposure to MTBE(Office of Environmental Health Hazard Assessment [OEHHA], 2000) were also incorporated into the risk calculations ' 3.2.1 Tier I Results. Based on the data presented in Table 1, a Tier I analysis consistent with ASTM (1995)guidelines was performed for the previously defined exposure scenarios associated with human receptors Tier I output data are included as Appendix III Table 2 summarizes a comparison of highly conservative Tier I RBSLs with COPC source concentrations ' for exposure scenarios associated with onsite receptors As indicated in Table 2, COPC source concentrations do not exceed the highly conservative Tier I RBSLs corresponding to a target risk level of 1 x 10-6 and a target hazard quotient of 1 0 Table 3 summarizes a comparison of highly conservative Tier I RBSLs with COPC source concentrations for exposure scenarios associated with offsite residential receptors As indicated in Table 3, COPC source ' concentrations do not exceed the highly conservative Tier I RBSLs corresponding to a target risk level of I x 10-6 and a target hazard quotient of 1 0 ' 3.3 Uncertainty Analysis The assumptions, procedures, and parameters used in estimation of risks and RBSLs are subject to various degrees of uncertainty To balance this uncertainty, use of data and protocols set forth by ASTM, USEPA, ' and DTSC employ significant conservatism in every component of the risk assessment process Uncertainties and corresponding conservatism implemented in this RBCA analysis include those associated ' with field sampling, use of fate and transport models, representation of site conditions, use of exposure factors, use of toxicity data, and interpretation of mathematical risks ' 3.3.1 Sample Collection and Analysis:Environmental sampling and analysis error can stem from improper sample collection and handling procedures, inadequate sample numbers, laboratory analysis errors, and the heterogeneity of the subsurface environment The use of standard techniques such as the ' collection of duplicates, and the use of trip and method blanks can be used to reduce the likelihood of errors Errors in data analyses can occur from the simplest tabulation and typographical errors to complex interpretational errors Matrix interference due to the presence of high concentrations often raise the detection limits of other chemicals in the analytical procedure and introduce uncertainty in the method of data analyses The sampling effort implemented at the site was specifically designed to identify areas that were suspected to have elevated chemical concentrations This sampling bias resulted in a data base that focused on the worst-case areas of the Site This focused approach resulted in assumptions related to representation of ' 10