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ARCADiS Site Investigation Work Plan 401 North San Jose Street,Stockton,California According to reports prepared by others, groundwater was encountered at approximately 12 feet bgs in temporary borings during a 2001 investigation(Geological Technics,2002).No other historical measurements of the depth to groundwater have been obtained at the Site. The Stockton area has historically experienced substantial fluctuations in groundwater elevation due to groundwater pumping In the San Joaquin Valley,with declines in the elevation of the groundwater table east of Stockton of up to 80 feet(Eastern San Joaquin Groundwater Basin Groundwater Management Plan, 2004).The Site's proximity to the Stockton Deep Water Channel makes it unlikely that groundwater table fluctuations of this magnitude would have occurred at the Site. However,groundwater pumping, as well as seasonal and tidal cycles,may have resulted In a certain amount of historical groundwater table fluctuation at the Site. The predominant direction of shallow groundwater flow in the area north of the Stockton Deep Water Channel Is reportedly to the northeast,as documented by the majority of groundwater investigations in the area. (Apex, 1995;Black and Veatch,2005;Advanced GeoEnviron mental, Inc.,2006)However,variations in the direction of shallow groundwater flow have been observed and are likely due to the influence of water 18veis in the Stockton Deep Water Channel.The permeability of Solis at the Site and in the surrounding area Is reportedly tow and groundwater flow velocity is also likely to be low. 2.3 Previous Site Investigations An environmental assessment was reportedly performed at the Site in 2001 (Geological Technics,2002). Ten borings were reportedly advanced to a depth of approximately 16 feet bgs on a 50-by-80-foot grid.The regular distribution of sample locations suggests that the locations were not selected to coinclde wlth specific historical site features. Soil and groundwater samples were collected and analyzed for total petroleum hydrocarbons(TPN)as gasoline(TPHg);diesel-range organics(approximately equivalent to TPH as diesel jTPHdj);TPH as motor oil(TPHmo); and benzene, toluene,ethylbenzene,and total xylenes(BTEX).Diesel-range organics and TPHmo were analyzed using U.S. Environmental Protection Agency(EPA)Method SW8100,which is listed in the EPA document SW 846 as a polynuclear aromatic hydrocarbons method. Several states list the Modified SW8100 method as an acceptable analytical method for TPH extractables.However, Modlfted EPA Method 8015 Is recommended forTPHd and TPHmo analysis in California, and it is the method most commonly used for this purpose.The two methbds use similar preparation procedures and Instrumentation. Therefore,the results of the two methods can be interpreted as equivalent. Soil and groundwater analytical data from this investigation are presented in Tables 1 and 2. 2