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ion Report: Groundwater-quality Monitoring—October 27,2003: 7500 West Eleventh Street, Tracy, CA. Page 4 Clay, which acts as a confining bed within the regional groundwater basin. At the subject site, the top of the Corcoran Clay is estimated to be at a depth of approximately 230 ft. beneath the ground surface and to be some 100 ft. thick. The total thickness of the underlying Lower Tulare Formation is not well documented; however, estimates suggest that it ranges in thickness from 300 ft. to greater than 1,400 ft. The depth to groundwater beneath the site varies seasonally between 7 and 11 ft. Regionally, the general direction of groundwater flow is to the north toward the Old River anastomosis branch of the San Joaquin River, the closest tributary of which, the Tom Paine Slough, is one and one-quarter miles north of the 7500 West Eleventh Street 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 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 hydraulic conductivity of approximately 1.5 to 10.2 cm/sec (The San Joaquin Company Inc. 1994, 2002c). 1.5 Distribution of Hydrocarbons in the Subsurface 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. The site is affected by a primary plume of diesel and gasoline that has affected both soil and groundwater. As is shown on Figure 2, it emanates from the area where the pump islands were formerly located on the 7500 West Eleventh Street property and extends north-northeast some 750 ft. down the groundwater gradient. The main body of the plume includes groundwater affected by both gasoline and diesel, but moving ahead of that mass is a fringe of groundwater affected solely by MTBE. A secondary plume of diesel and gasoline, which is also shown on Figure 2, emanates from an area to the rear and slightly to the east of the Casa Mendoza restaurant on the 7500 West Eleventh Street property. Although no physical evidence of an underground or above-ground storage tank has been found there, it appears that, at some time in the past, there was a source of fuel hydrocarbons located in the vicinity of Monitoring Wells MW- 12 and MW-12A (see Figure 2 for locations). When the 6,000 linear feet of underground piping was removed from the site in December 1998, a disconnected pipeline was found that ran from the general area of those monitoring wells toward the West Eleventh Street -� SJC