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CLEARWATER ' G R O U P Envirvnmrnte! 5rrwrs 1 currently evaluating Interim Risk-Based Screening Levels (RBSLs) for application as soil and ' groundwater cleanup levels in a variety of site and hydrogeologic settings The RSBLs are In the form of look-up tables based on Tier 1 RBCA models for 4 different combinations of soil, groundwater, and land-use characteristics The RBSLs were calculated using MCLs, cancer risk estimates, contact/inhalation hazards, aquatic life threat, and other risk factors Athough the ' RBSLs have not been adopted as formal cleanup levels, they are currently under evaluation as such by the SFRWQCB and a number of other regulatory agencies in California, and offer a reasonable approximation of a site-specific Tier 1 RBCA The RBSLs, as applicable to the ' subject site, are summarized as follows (from SFRWQCB Application of Rask-Based Screening Levels and Decision Making to Site with Impacted Soil and Groundwater, Interim Final Volume ' 1 Summary Tier 1 Lookup Tables, December, 2001) ' RSBLs for Subsurface Soil(>3m bgs)(mg/kg) Groundwater RBSL(µg/L) Groundwater is Groundwater Is Not Drinking Drinking Drinking Drinking Water Water Resource Water Water Not Resource Threatened Theatened ' Contaminant Residential, Residential Industrial/ Industrial/ Land Use Commercial Commercial Permitted Land Use ' Land Use Only Total Petroleum 100/100 400/500 400/500 100/100 500/640 ' Hydrocarbons (gas/diesel) Benzene 0 045 018 039 1 0 46 ' Toluene 26 84 84 40 130 Ethvlbenzene 25 24 24 30 290 ' Xvlenes 1 0 1 0 1 0 13 13 MTBE 0 028 10 1 0 5 0 1,800 ' Clearwater believes that the listed RBSLs provide adequate guidance in determining final soil ' and groundwater cleanup levels The most conservative soil RBSLs listed are 100 mg/Kg TPHg • and TPHd for soil and 100 pg/L TPHg and TPHd for groundwater, assuming drinking water is ' ZB 178C CAP 10 November 20,2002