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Mr Brian Tay]or•---- F---tted:ainht <br /> anuM31,2008 _ Deleted:Mr.Brian Taylorl <br /> Deleted:rvwvy 31,20081 <br /> Hydrogeology <br /> The San Joaquin River acts as a source of recharge to groundwater beneath the site and shows an average <br /> change in tidal flux of 3 feet(Port of Stockton). A tidal study conducted at the site in 1991 reported that <br /> tidal fluctuations of approximately 4.46 feet had a very slight effect(0.11 feet) on groundwater levels .. <br /> onsite(Harding Lawson Associates, 1991). Saltwater infiltration from the Stockton Deep Water Channel <br /> has been observed in the shallow groundwater beneath the site(URS,2003). <br /> Based on previous investigations by others, the aquifer beneath the site appears to be unconfined with <br /> both fine and coarse-grained layers separating the shallow and deep monitoring wells. The horizontal <br /> groundwater gradient is relatively flat ranging from 0.002 to 0.03. The groundwater flow direction is <br /> generally southwesterly. Upward and downward vertical gradients have been observed between shallow <br /> well MW-26 and deep well DW-2. The depth to groundwater generally ranges from 10 to 15 fbg. <br /> Aquifer testing at well MW-20 indicates that hydraulic conductivities range from 1 to 3 feet per day <br /> within shallow groundwater,but this location contains more sandy strata than most other portions of the <br /> facility. <br /> DISTRIBUTION OF PETROLEUM HYDROCARBONS <br /> Soil <br /> Soil sample locations and historical soil analytical data are presented in Attachment B. In general,soil <br /> analytical data collected from the unsaturated soils beneath each of the source areas are limited and the <br /> extent of impact is unknown. However,as previously presented in the investigation reports submitted by <br /> Groundwater Technology, 1986; Law Engineering, 1988; Harding Lawson Associates, 1993; Pacific <br /> Environmental Group, 1994 and 1995; and TOXICHEM, 2000 and 2001; the horizontal extent of <br /> petroleum constituents is adequately defined beneath the site. The vertical extent of impacted soil most <br /> likely extends to the groundwater surface beneath each of the source areas, which is approximately 10 <br /> fbg. There are numerous monitoring well and soil borings that provided both horizontal and vertical soil. <br /> definition around the site's perimeter. The primary constituents of concern are total petroleum <br /> hydrocarbons as gasoline (TPHg), total petroleum hydrocarbons as diesel (TPHd), benzene, methyl <br /> tertiary butyl other(MTBE)and tertiary butyl alcohol(TBA). <br /> 241729 3 <br />