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P Extended Site Characterization Report; 7500 West Eleventh Street, Tracy, CA. Page 42 i of diesel, gasoline or any of the BTEX compounds, but did contain MTBE at a concentration of 110 pg/L. MTBE at a concentration of 120 µg/L was also detected on that date in the groundwater in exploratory boring PP-16. This indicates that those two Jlocations are on the eastern fringe of the primary plume where MTBE, due to its relatively high mobility in the subsurface when compared with other components of gasoline, has migrated by lateral dispersion a greater distance from the axis of the plume than have other components of fuel hydrocarbons. When a sample of groundwater was recovered from Exploratory Boring PP-15, which was drilled, as is shown on Figure 2, on the western side of Chrisman Road in front of Aubin Industries at 23833 Chrisman Road, it was found to contain the relatively high concentration of 1,100 µg/L of diesel-range hydrocarbons, with 63 µg/L of gasoline and 90 µglL of MTBE. Further down gradient at Monitoring Well MW-11, the groundwater was found to be free of any components of hydrocarbon fuels except MTBE at. a concentration of 5.1 µ.g/L. These observations are again indicative of a fringe of MTBE that has migrated in advance of the other components of fuel hydrocarbons in the plume of affected groundwater. That interpretation is shown on Figure 13, where the primary plume is shown to have a plume of MTBE surrounding the area where groundwater is affected by diesel and gasoline. >_1 7.3.2.3 Hydraulic Conductivity and Rate of Primary Plume Migration Historical information and the results of groundwater sampling and analyses can be used to estimate the effective hydraulic conductivity of the strata through which the primary plume has migrated and the rate of that migration. Although the concentration of MTBE detected in groundwater recovered from Monitoring Well MW-11 on April 11, 2002 was very low (5.1 µg/L), the location of that well is clearly located some distance up gradient from the precise down-gradient limit of the plume of MTBE in groundwater. However, that limit can be estimated by M /r extrapolation of the reduction in the concentration of MTBE from 90 µg/L to 5.1 µg/L G detected in samples of groundwater recovered from exploratory boring PP-15 and Monitoring Well MW-11, respectively, the locations of which are approximately 75 ft. apart. The extrapolation estimates the down-gradient limit of the plume of MTBE to be s f located some 5 ft to the north-northeast of Well MW-11, which point is some 735 ft. down the groundwater gradient from the former dispenser pump area on the 7500 West Eleventh Street property, from which the majority of the fuel hydrocarbons affecting the �.t subsurface leaked. I ` If it is assumed that the porosity of the sands beneath the site, which are the primary conduits for the down-gradient migration of the MTBE, is 0.3 and the MTBE was immediately released into the subsurface from piping that was already leaking at the 7500 -- West Eleventh Street site when gasoline containing that oxygenate was likely to have been first supplied to that site in October 1992 (California Environmental Protection Agency 1997), then it is possible to use the down gradient distance that MTBE has '3 sic