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PIA Report:Groundwater-quality Monitoring—July 29,2002: 7500 West Eleventh Street, Tracy, C4, Page 4 Tom Paine Slough, is one and one-quarter miles north of the 7500 West Eleventh g � q v 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. � f Based on pump tests that SJC has 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, r 2002a). i � 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, h drostrati ra hic and geo-chemical data. That synthesis has resulted m 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 1 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 close'to the southeastern corner of the truck service bay that it located at the rear 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 is 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 that general area of those monitoring wells toward the West Eleventh Street frontage of the property where the underground storage tanks that were removed at that time were a located. I As shown on Figure 2, underground fuel tanks were formerly located on the property at 24195 Chrisman Road, which adjoins the 7500 West Eleventh Street site to the south. They had contained diesel but were removed in December 1998. Although no additional investigation of the 24195 Chrisman Road site has been performed, SJC believes that because samples recovered from the bottom of the tank pit at the time of removal SJC