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_ Ms.Lori Duncan,Senior REHS September 2,2002 <br /> r : Mr.Nuel C.Henderson Jr.,R� <br /> Page: 2 <br /> Contaminant Masses Released - As stated on page 1, section 1.1, Site Description and <br /> History. The release was discovered by the San Joaquin County Environmental Health <br /> Department who observed hydrocarbon odors in soils at the location of the former <br /> dispenser island and tank pit. There is no record of the volume of fuel released. <br /> Potential Contaminant Migration Pathways - Potential pathways are described on page 4, <br /> section 3.3 -Receptors and Potential Pathways as "onsite and offsite ingestion and dermal <br /> exposure to contaminated surface soils; onsite and offsite exposure to contaminate vapors <br /> sourced from surface contaminated soil, subsurface contaminated soil, and contaminated <br /> groundwater, and onsite and offsite ingestion of contaminated groundwater". <br /> Residual Contamination - Residual contamination was estimated using the analytical data <br /> generated in the site investigation. The extent of soil contamination is presented in <br /> contaminant cross-sections in Figures 5 and 6 of the Risk Assessment Report. <br /> Data and Method used for Calculating Volumes and Average Concentrations of Impacted <br /> Media and Resultant Calculated Contaminant Mass - The volume of impacted material <br /> for the release located at tank 2 (1130 cu ft) and at the dispenser island (4415 Cu ft) was <br /> estimated using the contaminant maps and cross-sections. The soil volume is multiplied <br /> by an average contaminant concentration, by compound, and the resultant volume <br /> converted to mass based on the specific gravity ratio of the contaminant to water. The <br /> resultant volumes and estimated mass are presented in the chart "Estimated Soil <br /> Contaminant Mass"on page 4. Risk Assessment Report. <br /> Sources and Ranges of Default Values used in the RBCA Modeling — Default values for <br /> the RBCA Modeling are presented In Appendix B, Output Table 1, Physical Property <br /> Data, Toxicity Data, Miscellaneous Chemical Data, and Exposure Limits in Groundwater <br /> and Air. Site Specific Data for the Model is presented in Appendix A. "Hydrogeologic <br /> Model" and Representative COC concentrations in Source Area. References for the Risk <br /> Based Corrective Action Model are attached. <br /> Conclusion - The model assumes a commercial environment with an 8 hour per day/ 40 <br /> hours per week exposure (Permissible Exposure Limit) which is conservative for the <br /> towing and repair activities at McDowell and Davis Towing. Based on the current usage <br /> of the facility the potential for indoor vapor exposures in excess of existing conditions is <br /> unlikely. Based on the above discussion, additional corrective action should not be <br /> undertaken and a "No Further Action Letter issued for the site using the Risk assessment <br /> Report and this discussion in place of a Tri-Regional Guidelines, Appendix B review. <br /> The above represents our best professional opinion as to environmental conditions at this <br /> site. These opinions were derived using data at hand, and standards of care common to <br /> the environmental consulting community in Northern California. There is the possibility <br /> that even with proper application of the above methodologies, there may exist on the <br /> subject property conditions that could not be identified within the scope of this sampling <br />