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• 31 August 2005 AGE-NC Project No 96-0232 Page 24 of 30 meters), and sand as the general soil type (Appendix 0-1) The mean MTBE concentration in soil was calculated at the 95%UCL to be 0 179 mg/kg,the maximum concentration detected in soil was 0 57 mg/kg (Table 11) The `Saturated Soil To Ground Water' model predicts that the MTBE in sandy soil will leach into groundwater at a relatively fast rate of approximately 550 grams/day(Appendix 0-1) MTBE source concentration after 20 years was predicted to be non-detectable at<0 005 mg/kg (Figure 21) The liquid phase concentration over a 100-year period would ranged from an initial 893 µg/1 to non- detectable levels (<0 5 µg11) in 40 years RISC model calculations utilizing the `Saturated Soil To Ground Water' input parameters predicts that MTBE will not be detected in ground water(at<10"13µg/1 range)at the down-gradient CSA well (Appendix 0-1), 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 12) In comparison, the RISC model predicts that MTBE may be detected at concentrations of 3 0 µg/1 in sentinel well MW-10, at the shallowest-screened interval A(15-20 feet bsg), but not detected at . the second screened interval B (40 -50 feet bsg), however, sentinel well MW-10 is not impacted if the soil type in interval A,which is screened in the Primary Aquifer Zone,is finer-grained(e g, silty sand) Sensitivity analysis of well MW-10 predicts that as the input parameters increase, screened intervals A and B may be impacted by MTBE (up to 52 µg/l 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 12) The `Saturated Soil To Ground Water' model predicts that the rate of vertical migration of MTBE as soil gas into a building will be 0 024µg/m2/day(Appendix O-1),the annual average concentration inside the building is predicted to be 0 57 micrograms per cubic meter (µg/m3) 7 1 2 Dissolved Source To Ground Water Model Since ground water in the vicinity may be impacted by MTBE releases from three separate areas (Flag City Chevron,Three B's Truck Plaza,and New West Petroleum)that are possibly co-mingled, fate and transport analysis was conducted for three `Dissolved Source To Ground Water' models (Figure 22),an on-site plume model only(Area I),an on-site to Banner Street plume model(Area II), and a regional plume model (Area 111), to determine the effects of an increasingly larger MTBE- impacted ground water plume on down-gradient wells The mean MTBE concentrations at the 95% UCL for each ground water model,determined from the four latest quarterly monitoring events(four seasons), were utilized to determine the impact to the down-gradient CSA well and sentinel well . MW-10 from the lateral migration of an increasingly larger contaminated ground water plume Advanced GeoEnveronmental,Inc