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Work Plan for Refined Plume Definition and Management of Floating Product-7500 W 11th St., Tracy, CA, Page 15 necessary to evaluate the amount of the floating product that is actually recoverable from the �.,. subsurface. Due to adhesion and absorption of liquid-phase hydrocarbons onto soil particles, a significant portion of those materials present in a zone just above and below the water table do not flow freely out of the soil when the pore space evacuated when the water table falls, nor when clean water flows into the pores from up-gradient locations. Thus, the volume of LNAPL that can actually be extracted from the in situ soil by any given remediation ,.. technology (other than removal by excavation) is considerably less than the total volume of non-aqueous phase hydrocarbons present in the soil mass. In fact, a good rule of thumb that is used by experienced engineers states that 70% of the contamination present in a typical .� formation, particularly if it is formed from fine-grained deposits such as clay, silt or silty sands, is bound tightly onto the soil and will remain there for very long periods because cyclical sorption and desorption transfers the contaminants into the dissolved phase in +.- groundwater at extremely low rates. By corollary, only 30% of the product moves readily when the soil is flushed by natural groundwater flow or by remedial pumping. *- The relationships between the apparent thickness of floating product measured in a well, the actual thickness in the adjacent formations, and the amount of product that is recoverable are complex and are affected by soil-specific and LNAPL-specific parameters and the dynamics •-- of the hydrogeologic regime prevailing at any given moment. Methods have been developed to approximate the volume of recoverable LNAPL in an aquifer based on the thickness of product floating in a monitoring well by, among others, Farr, Houghtalen and McWhorter (1990). The mathematics of the relationship does not lend itself to closed-form solutions, but plots of numerically-computed solution curves are available. For the thickness of 0.58 fl. (18 cm) of floating product in Monitoring Well MW-7 measured on November 8, 2003, the available solution curves that have been developed for a sandy, porous formation, which is the closest available type to the subsurface beneath the 7500 West Eleventh Street site, indicate that the volume of recoverable free product in the subsurface would be approximately 0.2 cm31cm2 (i.e., 0.2 cm3 of product is present in each cm2 of the plan area over which floating product is present on the water table) (Farr, Houghtalen and McWhorter 1990). That quantity is equivalent to 0.05 gal. of LNAPL per ft of the area affected by floating product. Alternative methods of estimating the volume of recoverable free product in a formation include those proposed by Testa and Paczkowski (1989). Like Hall, et al, who were concerned with estimating the thickness of floating product in a formation, Testa and Paczkowski presented a simple procedure that used a single parameter that is dependent upon soil type to estimate the volume of recoverable product from a unit volume of affected soil. They estimated the oil retention capacity of soil to range from 5 Lim' (0.34 galft3) for gravel to 40 U m3 (2.96 gallft) for silty sand. 5.2.3 Area of Subsurface Affected by Floating Product Based on estimates of the permeability of the formations beneath the 7500 West Eleventh Street site and observations made during the various stages of site characterization and groundwater-quality monitoring, it was previously estimated that the down-gradient rate of migration of groundwater beneath the site is approximately 77 ft/yr. It has also been SJC