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QCAP Addendum:Former Fue2'P1rg Station, 7500 West Eleventh Street, Tracy, G Page 2 (1984) calls for a "formation factor" to be subtracted from the measured thickness of floating product in a well to derive the estimated thickness of floating product in the formation. Formation factor values range from 5.0 cm. for coarse sand to 12.5 cm. for clay. Clearly, if the measured thickness of floating product in the well is 0.05 ft., the application of the Hall, et al methodology would generate an irrational result. In the present case, to avoid such difficulty, SJC has very conservatively assumed that the thickness of floating product measured in Monitoring Well MW-7 is the same as the thickness of LNAPL actually present in the adjacent formation. With that assumption, the thickness of LNAPL assumed in making an estimate of the volume of LNAPL in the subsurface will be 0.05 ft. 2.1.3 Area of Subsurface Affected by Floating Pr duct SJC previously estimated the area of the subsurface beneath the 7500 West Eleventh Street Site affected by floating product to be 15,000 ft2 (The San Joaquin Inc. 2003). That estimate was based on the assumed rate at which LNAPL has migrated down gradient from the point at which it was released beneath the pump island of the former fueling station, the distribution of floating product in the subsurface that could be implied from observations of the flow of LNAPL into the tank pits when the tanks were exhumed and the presence and absence of floating product in the groundwater-quality monitoring wells that were extant at the Site at that time. Since that estimate was made, additional groundwater-quality monitoring wells have been installed at the sight. Those include wells MWFP-1, MWFP-2, MWFP-3, MWFP-4, MWFP-5, and MW-13(see Figure 1 for locations). Monitoring Well MW-13 is free of LNAPL. This finding confirms SJC's earlier assumption that pumping of floating product from the tank pits that was performed in December 1998 was successful in extracting floating product from beneath the southern carriage way of 7500 West Eleventh Street. No LNAPL has been detected in any of the floating product monitoring wells (designated MWFP). This indicates that floating product has not advanced down-gradient as far as monitoring well MWFP-3. Given those observations, it would appear that the down-gradient length of the area of the subsurface affected by the floating product does not exceed a maximum of 115 ft. With respect to the east to west extent of the area where LNAPL is present in the subsurface, SJC estimated that prior to its removal from that area, floating product extended along the southern side of West Eleventh Street over a distance of some 80�ft. and on the northern side of the street in the vicinity of Monitoring Well MW-7, it has an east to west extent of some 100 feet. Those estimates were based on the presence and absence of floating product in the monitoring wells extant on the Site at that time and the observed flow of LNAPL into the tank pits at the time that the underground storage tanks were removed from the property in 1998. The subsequent installation of Monitoring Wells MWFP-2, MWFP-3 and MWFP-4, which consistently have been found to be free of floating product, has permitted a revised estimate to be made for the possible east to west extent of the plume of floating product along the northern boundary of West