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31 August 2005 AGE-NC Project No 96-0232 Page 26 of 30 about 2 to 6 cases in 10 million inhabitants for typical and maximum input parameters (Table 12), the hazard index is less than 0 01 The typical carcinogen risk calculated from the`Dissolved Source To Ground Water' model is 2 to 5 cases in 100 million inhabitants for the three plume areas,but can increase to 1 case in 1 million inhabitants with increasingly higher values for hydraulic conductivity, concentrations, and more permeable soil types, the hazard index is 0 01 or less(Tables 14 and 15) 7 2 2 Cleanup Levels RISC Workbench back-calculated cleanup levels using the models and assumptions selected for the risk analyses (Tables 12, 14 and 15, Appendices N, O) In the `forward' risk assessments,risks are calculated from input or estimated receptor point concentrations and receptor input parameters In the back-calculations,the user specifies target risks or concentrations(e g ,MCLs)and the software calculates `allowable' source concentrations (clean-up levels)using equations presented in EPA's Risk Assessment Guidance for Superfund(EPA, 1989),all the pathways associated with each source are added together when calculating the cleanup level for that source The RISC Workbench back- calculations determined that for the typical site models,MTBE concentrations in site soil and ground water were below the site-specific target cleanup levels(SSTL),therefore no clean-up was necessary However, MTBE concentrations were above SSTL for well MW-10 soil and ground water models where input values for hydraulic gradients or soil types were set at the maximum values encountered at the site It should be noted that results of the risk analysis are conservative estimates,based on the use of the Johnson-Ettinger calculations for soil-vapor modeling,the model does not account for degradation of MTBE over time In addition, exposure to MTBE vapor at the Flag City Chevron site would be further reduced due to the presence of a "cap" of concrete and asphalt directly over the impacted area 8.0. DISCUSSION AND CONCLUSIONS Based on data collected during site assessment activities, AGE concludes • MTBE has been established as the contaminant of concern at the site • Between May 1991 and August 2000 (pre-ground water remediaiton), ground water flow direction was typically inferred to be towards the north-northeast-northwest at gradients between 0 001 and 0 003,between December 2000 and August 2004 (during ground water remediation), ground water flow direction at the site was typically inferred to be towards a ground water depression at the Flag City Chevron site at gradients between 0 003 and 0 01, . between September 2004 and March 2005 (post-ground water remediation), ground water Advanced GeoEnvironmental,Inc