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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 ' carcinogenic slope factors for inhalation and oral exposure to MTBE (Office of Environmental Health Hazard Assessment [OEHHA] 2000) were also incorporated into the risk calculations ' 5.5 TIER I RBCA RESULTS Based on the data presented in Table 1 of Appendix H, 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 in Appendix H Table 2 of Appendix H 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 of Appendix H, 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 10 ' Table 3 of Appendix H summarizes a comparison of highly conservative Tier I RBSLs with COPC source concentrations for exposure scenanos associated with offsite residential receptors ' As indicated in Table 3 of Appendix H, 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 10 5.6 REPRESENTATION OF SITE CONDITIONS In the absence of site-specific data, the ASTM RBCA process promotes the use of conservative ' default values for various parameters representing site conditions This RBCA analysis consistently incorporated conservative assumptions for selection of input parameters, while ' attempting to maintain a reasonable, site-specific evaluation Examples of conservative assumptions used to formulate input parameters include ' • The maximum chemical concentrations in groundwater over the past two years are representative of current and future source concentrations, • The entire soil column is impacted at levels detected in confirmation soil samples, ' • The onsite building exists directly over the locations of maximum groundwater concentrations detected over the past two years, • Cracks exist in the foundation of the onsite building, and ' • The paved surface has a negligible impact on vapor emission to outdoor air Use of these conservative assumptions result in an over-estimation of health risks and lower ' RBSLs F Propec1s1701371MastcAWp5CIasare I IMO 137C,text doc 1 7