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westward through Lodi and turns northwest until its confluence with the Cosumnes River near Thornton <br /> (approximately 7 miles west of Lodi)(Figure 1) At the town of Walnut Grove,the Mokelumne River Joins <br /> with the series of sloughs and other rivers that form the Delta <br /> Regional Groundwater Flow Groundwater flow in the Central Valley is controlled primarily by the <br /> northwest-trending, asymmetric structural trough Groundwater generally moves from the flanks of the <br /> valley toward its center and toward the Delta area(Page, 1986) In the Lodi area this is generally west but <br /> because of the location of the site to the Mokelumne River the flow direction is south In the Lodi area the <br /> 1'water bearing zone is generally around 45 to 50 feet bgs <br /> Previous investigators described the Sacramento Valley groundwater system as comprising one overall <br /> unconfined aquifer and the San Joaquin Valley groundwater system as having separate aquifers separated by <br /> the Corcoran clay member(Bloyd, 1978,Poland and Lofgren, 1984) However,more recent investigations <br /> suggest that the entire Central Valley acts as a single aquifer system with varying degrees of "leakiness" and <br /> confinement,depending primarily upon depth(Williamson and others, 1989) Lithologic studies suggest that <br /> the system consists of numerous fine-grained lenses that constitute more than 50 percent of the aquifer system <br /> and have an aggregate thickness of as much as several thousand feet(Page, 1986) <br /> Near Lodi, the unconfined water table begins at approximately 45 to 50 feet below ground surface (San <br /> Joaquin County, 2000) Fresh water reportedly is present to depths as great as 600 to 700 feet The water <br /> table varies seasonally,rising in the spring with river recharge and declining through the summer in response <br /> to irrigation demand <br /> 4.2 Soil Conditions <br /> The sediments penetrated in the soil borings consisted of brown to yellow brown colored,moderately dense <br /> fine to medium grained sand with silty layers from the ground surface to a depth of 61 5 feet bgs Where <br /> petroleum hydrocarbon contamination was present the color of the sand has been altered to a green/gray color <br /> Soil boring logs are presented in Appendix IV <br /> Several of the soil borings(MW-4, 5,and 6)encountered flowing sand conditions at depths below the water <br /> table and below 50 feet bgs where fine to medium grained sand entered the hollow stem auger ofthe B-53 drill <br /> system and plugged the bit This required removal of the drill string from the soil boring and placement of <br /> Mam1D V2nwao=cnW%Parn Tcxaw\R tU%SAR1203 vipd Page 20 <br />