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CAP Addendum:Former FuXg Station, 7500 West Eleventh Street, Tracy, Page 19 It was also observed when a sample of groundwater was first recovered from Monitoring Well MW-13 on April 30, 2004 that it was also free of LNAPL and the natural attenuation evaluation made at the site showed that groundwater conditions in that area are aerobic in character. Monitoring Well MW-13 is approximately 100 ft. down-gradient from Tank Pit No. 1. It is some 80 ft. up-gradient from Monitoring Well MW-7 where LNAPL has been persistently present on the groundwater. The mean groundwater gradient in the vicinity of all of those cited wells is 0.003 ft/ft (the San Joaquin Company 2004). Those observations demonstrate that the extraction of LNAPL from the surface of the groundwater in Tank Pit No. 1 cleared floating product from the water table over a wide area, potions of which were down-gradient from, as well as co-gradient to, the pumping location. When considering the success of the removal of LNAPL from Tank Pit No. 1 in clearing floating product from distant locations, it is useful to note some of the hydraulic effects of the pumping. The hydrostratigraphic setting of Tank Pit No. 1 is generally similar to that of Monitoring Well MW-7. At both locations, a surficial clayey aquitard is present from near the ground surface to a general depth of some 10 ft. BGS. Beneath that stratum is a sandy silt aquifer having a similar 10 ft. thickness, which overlies a second aquitard that serves, essentially, to prevent hydraulic and geochemical communication with deeper aquifers. As the vacuum truck was used to extract a total of 2,000 gallons of LNAPL and highly- contaminated groundwater from Tank Pit No. 1, the water table fell steadily but slowly so '- that it did not cause turbulence and emulsification of the floating product. At maximum drawdown, the water table had fallen some 3 ft. and inflow to the pit was from the , western wall where silty and sandy soils were present. With the knowledge that floating (too')2 Io Qroduct at a distances up to 100 ft. from the pumping site was being drawn into_thepit, a ; , s--------------- rti reasonable estimate of the mean gradient of th—c-flow'into tfie pit some 0.03 ft/ft., which is ten times the gradient of the groundwater table at the site when it is in its undisturbed = condition. Under those circumstances, the natural gradient of flow in the down-gradient direction is essentially of no consequence compared to the high gradient of the flow that is induced in the opposite direction (i.e., up-gradient with respect to the natural flow regime). The observations considered above indicate that, based on the practically identical model available from extraction of LNAPL from Tank Pit No. 1, it is reasonable to expect that pumping of LNAPL from the proposed extraction test will successfully remove the floating product from the remaining area of the subsurface that it affects. If that occurs, for the reasons developed in earlier sections of this addendum, SJC believes that the remediation program will successfully initiate aerobic processes of natural groundwater contaminant attenuation in the area of the site currently affected by LNAPL. Although the above assessment is based on observed conditions when only 2,000 gallons of LNAPL and highly-contaminated groundwater were extracted from Tank Pit No. 1, for the proposed remediation of groundwater in LNAPL-affected areas to the north of the