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Ms.Lori Duncan CAM B R I A September 9,2005 Well MW-3 contained SPH from April 1991 to April 1993. Historically, TPHg and benzene concentrations in wells MW-2, MW-3, MW-4, and MW-6 were significantly higher than they are currently. As discussed above, groundwater depths have risen approximately 25 feet since groundwater monitoring began at the site. Wells MW-1, MW-2, and MW-3 are screened from 45 to 65 fbg, and the well screens are currently submerged. Wells MW-4 through MW-7 are screened from 35 to 55 fbg. These wells are not currently submerged,but have been in the past. RBCA ANALYSIS © To determine site specific clean-up levels, Cambria conducted a Tier II RBCA analysis following the guidelines outlined by the American Society for Testing and Materials (ASTM) Designation E-1739-95'. The ASTM-RBCA approach is consistent with general U.S. Environmental Protection Agency (USEPA) and California Environmental Protection Agency (Cal-EPA) risk assessment guidance. Cambria presents an abbreviated conceptual site model (CSM) and the site specific target levels (SSTLs) calculated by RBCA analysis below. CSM A CSM describes the relationship between the impacted media and receptors that may be exposed to chemical constituents originating from the site. Cambria developed this abbreviated CSM for the site based on a review of available geological and analytical data and on evaluation of potential transport and exposure pathways. The following information is included in the CSM: (a) chemical sources and impacted media, (b) potentially exposed receptors and exposure pathways,and(c) the protective target risk levels. Chemical Sources and Impacted Media: Analytical data collected since 1990 indicate that soil and groundwater beneath the Shell site are impacted by petroleum hydrocarbons. The chemicals of concern (COCs) addressed in this RBCA analysis are the BTEX compounds. The highest COC concentrations were detected in and near the former UST excavation. TPHg has also been detected beneath the site, but because it is made up of many (and varying) constituents, its potential human health risks are not evaluated in the RBCA analysis. Instead, the individual BTEX components are evaluated, as discussed above. Groundwater data suggest that the former UST area is minimally impacted by MTBE(1.4 ppb during the second quarter 2005 event in well VEW-2). Because the MTBE detected is below the California primary maximum contaminant level (MCL)of 5 ppb,MTBE is not evaluated as a COC in the RBCA analysis. ' Standard Guide for Risk-Based Corrective Action Applied at Petroleum Release Sites,E 1739-95 (Revised December 1996):American Society of Testing and Materials,100 Barr Harbor Drive,West Conshohocken,PA 19428. 6