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f <br /> N <br /> is vmn <br /> W <br /> 1 <br /> of the intersection of Hammer Lane and Pacific Avenue. Under- <br /> ground tanks were once used to store gasoline at this station. <br /> The station building has been removed from the site. EA has <br /> obtained the 22 October 1986 report of a subsurface investigation <br /> conducted at the Knowles station by Groundwater Technology, Inc. <br /> (GTI) . Water samples were obtained from three monitoring wells, <br /> all of which contained detectable 'levels of petroleum hydrocar- <br /> bons (350-•470 ug/L) and BTEX components (e.g., 36-85 ug/L ben- <br /> zene) (GTI 1986c) . <br /> 2.2 LOCAL HYDROGEOLOGY AND GROUNDWATER USE <br /> 2.2.1 Regional Hydrogeology <br /> The `soil section in the Stockton area consists of a thick <br /> sequence of continental alluvial fan, river channel, and flood , <br /> '. <br /> plain deposits. These deposits are composed of interfingering <br /> layers of gravels,: sands; silts, and clays (USGS 1972) Fresh':" <br /> �. <br /> groundwater is abundant in unconsolidated sediments (DWR 1961 ) <br /> . and is recharged frnm the Sierra Nevada to the east. Relatively <br /> saline waters recharge and intrude on these better-quality <br /> aquifers. from the Coast ranges and the Delta in the west. The <br /> layers of unconsolidated sediments differ widely. in thickness and <br /> �. extent . <br /> . The geological formations in the Stockton area"can be classified I <br /> ��. in ,:two genera lgroups: semiconsolidated saline-water-bearing <br /> . <br /> i <br /> .units and. overlying unconsolidated freshwater-bearing units. The <br /> semiconsolidated rock units are composed of materials of the <br /> " Mehrten `and the Valley Spring formations. These formations are <br /> af , volcanic origin and contain andesitic .and rhyolitic breccia, <br /> tuff, and ash: The groundwater within these two formations is <br /> spline, it is-separated from the overlying freshwater-bearing <br /> € units by an impermeable layer within :the upper Mehrten Formation. <br /> .m The`° reconsolidated units .consist :of .alluvial sediments of the <br /> Victor Formati.:on, Arroyo, Seco Gravel, and..Laguna Formation.. . <br /> 0 <br /> e35/71050/2-90/tx 15 <br /> E <br />