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31 August 2005 •' AGE-NC Protect No 96-0232 Page 25 of 30 (Table 13) Input parameters and results for the three models are presented in Appendices 0-2a through 0-2c Utilizing the most recent 1-year analytical data,the `Dissolved Source To Ground Water' model for all three plume areas predicts that MTBE concentrations will be non-detectable at the CSA well (Appendices 0-2a through 0-2c), sensitivity analysis suggest that the non-detection was stable for a range of increasing hydraulic gradients, MTBE concentrations, aquifer soil types, and depths, up to the maximum values encountered at the Flag City Chevron site (Table 14) In comparison, the `Dissolved Source To Ground Water' model detected MTBE in low concentrations (-10 4g11 and less) within the upper 50 ft of ground water at sentinel well MW-10 for plume Areas I and 11 (Appendices 0-2a through 0-2c) Sensitivity analysis predicts that as the input parameters increase, screened intervals A and B may be impacted by MTBE (up to 38 4g11 at a maximum hydraulic gradient of 0 01 ft/ft),but screened interval C(70-75 feet bsg)and deeper will not be impacted (Table 15) 72 RISK ASSESSMENT AND CLEANUP LEVELS Assessment of risk and clean-up levels were determined based on the results of the soil and ground water fate and transport studies, which determined that the CSA well was unlikely to be impacted by MTBE from a soil or ground water source,but levels A and B of sentinel well MW-10 potentially may be impacted by low concentrations of MTBE (-3 0 µg/1 and less under typical conditions) 7 2 1 Assessment of Risk The risk assessment analyzed the risks to a residential adult and child (receptors) inside a building on the site Exposure for the receptors is to possibly MTBE-contaminated ground water and MTBE vapor in indoor air, default reasonable maximum exposure (RME) intake values for an adult and child were selected from the RISC program menu(Appendix O) The cancer risk is defined as the incremental probability of an individual developing cancer over a lifetime as a result of exposure to the potential carcinogen The hazard quotient is defined as the ratio of exposure to toxicity for an individual pathway and chemical, the hazard index is the sum of the hazard quotients Based on the results of the fate and transport studies, there was no risk (less than 1 case per 10 million inhabitants)associated with exposure to deep ground water(I 10 to 160 feet bsg)and indoor air in both the CSA well and sentinel well MW-10(Appendix O) However,there is a very low risk associated with exposure to shallow ground water (upper 50 ft) at sentinel well MW-10 The carcinogen risk calculated for MTBE from the`Saturated Soil To Ground Water'model ranged from Advanced GeoEnvironmentel,Inc