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Work Plan for Refined Plume Definition and Management of floating Product-7500 W I1 th St., Tracy, CA. Page 16 estimated that the retardation factor for benzene being transported by groundwater from beneath the site is 5.5 ft/yr. That means that benzene is estimated to migrate down-gradient from its point of release on the Navarra property at a rate that is 5.5 times slower than the groundwater flow rate (i.e., at approximately 14 ft/yr) (The San Joaquin Company Inc. 2002c). r. As noted in Section 5.0 above, groundwater carrying concentrations of diesel and gasoline in excess of the solubility of those products in water had arrived at Monitoring Well M-7 by September 11, 2001, so that, although LNAPL could not be readily observed floating on the �. water in that well at that time, that date can be used to conservatively estimate the distance down-gradient from Monitoring Well MW-7 that floating product has advanced by the present time (November 2003), For the purpose of making that estimate, SJC has elected to 6-- use the estimated migration rate of benzene that, as noted above, is 14 ft/yr. It is recognized that the retardation factor for free fuel hydrocarbon products, such as diesel or gasoline, in 4 groundwater is likely to be considerably higher than the retardation factor for benzene. �- However, for the purpose of making a general estimate for the likely down-gradient distance to which floating product may have migrated, it is prudent to use very conservative assumptions so that a remediation system design would not be improperly located when { { -- subsurface conditions are revealed at the time of its installation. , Based on the above parameters, it is estimated that between September 2001 and November 2003, the advancing front of floating product flowing down-gradient from the Navarra property has reached a point some 30 ft to the north of Monitoring Well MW-7. "`" As was discussed previously, the floating product released on the Navarra property leaked from the piping beneath the pump islands, which were located some 200 ft. up the groundwater gradient from Monitoring Well MW-7. That would produce an estimate for the down-gradient length of the plume of floating product from the source on the Navarra property to its estimated present advanced front beneath the property at 7501 West Eleventh Street of 230 ft. However, as has been described previously, when the storage tanks were removed from the Navarra site in December 1998, some 2,000 gal. of floating product were removed from the subsurface at that time and.shipped off-site in vacuum trucks. The floating product extraction procedure used at that time included significant draw-down of the groundwater in the vicinity of the tank pit and pump islands from which the product was extracted. We estimate that the process evacuated all of the floating product present not only beneath the 7500 West Eleventh Street property, but also from an area beneath the southern. �. carriage-way of West Eleventh Street itself. To avoid making an unreasonably high estimate Grp of the floating product remaining in the subsurface, we will assume that floating product was, at that time, removed from an area beneath that thoroughfare that extended some 75 ft. north .� of the location of the pump island piping from which it was released. From the above estimates, the area over which floating product is present on the water table is estimated to extend some 150 ft. from north to south. It4s` S 1�4 With respect to the east to west extent of the area where LNAPL is present in the subsurface, an estimate can be made based on observations made at the time that the storage tanks were removed from the Navarra Site and the degree of lateral dispersion of the plume of affected sic