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CAP Addendum:Former FueQg Station, 7500 West Eleventh Street, Tracy, C Page 4 LNAPL is 9,200 ftZ and the thickness of floating product present in the soil was estimated to be 0.05 ft. (see section 2.1.1). From an estimate of zero recoverable LNAPL that was made in Section 2.1.5 above using the Farr, Houghtalen and McWhorter method, the total estimated volume of LNAPL that can be recovered from the subsurface would also be zero, which, based on SJC's experience in recovering LNAPL from the south side of West Eleventh Street, would appear to be unreasonable. At the other extreme of estimation, if the estimated oil retention capacity is 2.96 gal/ft3, as would be assumed based on the Testa and Paczkowski method, then the estimated volume of recoverable LNAPL is 1,362 gallons. This estimate is also unreasonable, because, as is shown in section 2.1.4, the estimated total volume of LNAPL in the subsurface is only 1,050 gallons. This type of variation in estimates of recoverable volume is typical of the current state of the art. For practical purposes, then, we will limit our upper-bound estimate of the recoverable volume of LNAPL to 1,050 gallons, which is the estimated total volume of LNAPL in the subsurface. Based on SJC's previous experience with recovery of LNAPL from remediation pits and trenches, we will set the_yer_yfconservative lower-bound estimate as one-half of the estimated total volume, Le 525 gallons.,_These numbers are consistent with the thickness of LNAPL that has been observed in onitoring Well MW-7 over the last two years and, as is indicated by the absence of LNAPL in nearby wells, the limited areal extent of LNAPL in the vicinity of that well. In conclusion: ESTIMATED RECOVERABLE VOLUME OF LNAPL: 525-1,050 GALLONS 2.2 Mass of Contaminants and Portion to be Addressed by Extraction Trench It is difficult to compute the total mass of dissolved and sorbed contaminants that is distributed in the subsurface due to a release of fuels from an underground storage tank unless the volume of fuel that flowed from the tanks is known, which is rarely the case. The difficulty arises because the concentrations of sorbed contaminants in the soil and dissolved contaminants in the groundwater are usually distributed in a very complex pattern throughout the area of the contaminant plume, while the number of locations at which the concentrations are known (i.e., at wells and borings) are relatively few. This difficulty is particularly pronounced in the case of petroleum hydrocarbon contaminants in an alluvial fan such as is present at the Navarra Site. We also note that the proposed remediation methodology for the site, which involves pumping of LNAPL and contaminated groundwater into vacuum trucks, is not intended to remove contaminants from the whole of the plume. Its principal objective is to remove extractable LNAPL from the area around Monitoring Well MW-7. Accordingly, we will base our estimates of contaminant mass on that contained within the 9,200 ftZ area used in Section 2.1.3 to estimate the total quantity of LNAPL present in the subsurface.