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Risk-Based Corrective Action Assessment& Over-Excavation Request Moore Petroleum,Inc.,5491 'T'Street,Banta,CA Page 4 with current U.S. EPA guidelines. In order to apply the GSI System to RBCA evaluation in California, several chemical-specific parameters in the system were adjusted. The benzene cancer slope factor was adjusted from 0.029 (mg/kg/day)-' to 0.1 (mg/kg/day)-'. The diesel cancer slop factor was also added into the GSI System. The most recent California drinking water Maximum Contaminant Levels for BTEX and MTBE were also updated in the model. Under complete exposure pathways, the Tier I GSI modeling compares affected media concentrations of constituents of concern (COCs) to conservative, generic Tier 1 Look-Up Table values (RBSLs) based on possible risk to human health and the environment. A constituent reduction factor (CRF) can be assigned to a specific COC. The CRF value can be calculated as the ratio of the source concentration for that COC divided by the RBSL for that COC. Constituents with a calculated CRF value less than one require no reduction to meet excess risk limits. For CDCs with CRF values exceeding one, the relative magnitude of the CRF is indicative of the degree of source remediation required. A RBCA Tier 2 evaluation under the GSI System takes into account site-specific data to assess whether the site is a risk to human health and the environment. In a Tier 2 evaluation, a concentration reduction factor (CRF) for a specific COC can be calculated as the ratio of the source concentration for that COC divided by the SSTL for that COC. Constituents with calculated CRF values less than one require no reduction to meet excess risk limits. For COCs with CRF values exceeding one, the relative magnitude of the CRF is indicative of the degree of source remediation required. Tier 2 GSI System Modeling Results This RBCA assessment used Tier 2 GSI model because several site-specific subsurface parameters are available. They are the average depth to groundwater, the approximate extents of the soil and groundwater impact, the average soil hydraulic conductivity and the average groundwater velocity. The soil TPHd and BTEX concentrations for soil sample MW-1 collected on April 28, 1989, and the groundwater TPHd concentration for groundwater sample MW-1 collected on January 16, 1998, were used in the modeling. The complete exposure pathways included in the Tier 2 GSI System modeling are: groundwater ingestion, soil leaching to groundwater ingestion, volatilization of soil to outdoor air, and volatilization of groundwater to outdoor or indoor air. The groundwater ingestion pathways are modeled in a residential setting, all other pathways were evaluated under a commercial setting. The Tier 2 GSI System modeling results indicate that due to the existence of the groundwater ingestion pathways, the site soil and groundwater impact levels had exceeded the Tier 2 RBCA cleanup goals by a magnitude of 3 to 28,000. The Tier 2 GSI System modeling results are included in Appendix B. 1Y:D:1 PRO.TECTT-'STRA02 004-Moore Petroleum.Banta\Qmr0I98CRaBCAAssessment.doc