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0 19 June 2000 AGE-NC Project No 96-0232 Page 17 of 35 sedimentary units thin and pinch out In the immediate area ofP16,MW-11 and RDW-1,the plume Of impacted ground water extends vertically from approximately 10 feet bsg to as much as 150 feet bsg,as evidenced by detectable concentrations of MTBE in MW-7 and MW-11D Vertical migration to this depth,despite the presence of a fine-grained sedimentary unit between 74 feet and 86 feet bsg, may have been facilitated by the gravel pack around the casing of RDW-I reported to be between 50 feet and 150 feet bsg Further westward from RDW-1, the plume of water impacted by oxygenates and ethanol, appears to extend to P-17 at the west end of the Rocky's Restaurant parking lot,with a vertical extent from possibly 10 feet bsg to possibly 105 feet bsg(MTBE), TBA was detected from 111 to 114 feet bsg the low ethanol concentrations detected in these samples maybe valid data as the coated bentonite pellets had not been utilized prior to or during installation of P-17 MTBE was detected in the samples between 37 and 83 feet bsg The plume may or may not extent to MW-13 at the west side of the Three B's site Contaminants in ground water at MW-13 include TPH-d,benzene,MTBE and TAME at shallow depth (10 to 20 feet bsg), TPH-d and MTBE at intermediate depth (50 to 60 feet bsg), ethanol at deeper depth(90 to 100 feet bsg), and less than detectable concentrations near total depth (130 to 150 feet bsg) Ethanol detected at high concentrations in the upper three samples is believed to be the result of localized contamination introduced during well construction The TPH-d, benzene and higher concentration of MTBE in the shallow sample suggest that most of the contaminants are from a source other than the Flag City plume as TPH-d and benzene were not detected at shallow depth in the RDW-1 area and the MTBE concentrations are higherin the shallow MW-13 area than in the shallow P-17 of RDW-1 areas It is not known if the two suspected plumes are co-mingled in the area between MW-13 and P-17, AGE has modeled them as separate plumes (Figures 21, 22 and 23) Relative to the inferred source area near MW-1, the northern extent of the plume appears to have reached MW-4 at depths between 55 and 65 feet bsg, the eastern extent of the plume appears to have migrated slightly past MW-2 at depths between 45 and 55 feet bsg, and the southern extent of the plume appears to have migrated slightly past MW-6 at depths between 10 and 50 feet bsg Based on the models depicted in Figures 21, 22 and 23, the average impacted ground water on-site is assumed to be 45 feet thick near the Flag City Chevron site and is assumed to be 55 feet thick off-site near MW-11,based on porosity analysis of three soil samples(see Section 4 0 ), porosity is assumed to be 37 percent Utilizing Figure 37, the distribution of dissolved petroleum hydrocarbons can be approximated by utilizing an equation for an ellipsoid,the dimensions of the ellipsoid was estimated at a short radius of 105 feet, a long radius of 215 feet and an average thickness of 50 feet The calculated volume of hydrocarbon-impacted groundwater is 9,400,679 gallons Volume calculations are included in Appendix I Advanced GeoEnrironmental,Inc