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Report: Groundwater-quality Monitoring--March 29-31,2006, 7500 West Eleventh Street, Tracy, CA Page 6 Old River anastomosic branch of the San Joaquin River, the closest tributary of which, the Tom Paine Slough, is one and one-quarter miles north of the Navarra Site. However, locally, the shallow groundwater gradient tends to follow the topography, which, at the subject property, slopes gently to the north-northeast. The local direction of groundwater flow is also affected by the local sedimentary geology, particularly where continuous or semi-continuous sand or gravel strata provide channels for subsurface flow through less permeable facies. Based on pump tests that SJC conducted in similar strata at another location in Tracy, and from the observed rate of migration of methyl tertiary butyl ether (MTBE) through the subsurface, it is estimated that the sands beneath the site have a mean horizontal hydraulic conductivity of approximately 1.5 to 10-2 cm/sec (The San Joaquin Company Inc. 2002c). The vertical permeability of the clay strata that separate the several aquifers present at depths in the range from ground surface to 50 ft. BGS was assessed by a constant-head permeability test that was conducted on a sample of clayey soil recovered in April 2004 from a depth of 7.5 ft BGS. The vertical permeability of that sample was found to be 10.8 x 10-' cm/sec. Distribution of Hydrocarbons in the Subsurface 1.5 y , The several stages of tank removal, excavation and removal of contaminated soil and groundwater, site characterization and groundwater-quality monitoring that have been �-. conducted at the 7500 West Eleventh Street site since December 1998 have permitted synthesis of historic, geologic, hydrostratigraphic and geo-chemical data. That synthesis has resulted in the following interpretation of the distribution of hydrocarbons in the subsurface. 1.5.1 Primary Plume The site is affected by a primary plume of diesel and gasoline that has affected both soil and groundwater. As is shown on Figures 3 through 9, it emanates from the area where the pump islands were formerly located on the 7500 West Eleventh Street property and extends north-northeast down the groundwater gradient. When it was at its maximum extent, the long axis of the plume measured some 750 ft down the groundwater gradient. The main body of the plume includes groundwater affected by both gasoline and diesel, but ahead of that mass is a fringe of groundwater affected solely by MTBE. Historically, in the interior of the primary plume, LNAPL in the form of free floating product was resent beneath the um islands of the former fueling station at the Navarra p p pump g Site where it had leaked from piping, and by April 2002, it had migrated down-gradient as far north as groundwater-quality Monitoring Well MW-7 (see Table 2). The locations of the pump islands at the former fueling station and of Monitoring Well MW-7 are both shown on Figure 2. Pumping of floating product and heavily contaminated groundwater from the pits from which the underground tanks were exhumed in December 1998 successfully eliminated E sic