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Mi <br /> s ` <br /> V <br /> 1.3 HYDROGEOLOGY AND WATER USE <br /> The geological formations in the Stockton area cin be classified <br /> in two general groups: semiconsolidated saline-grater-bearing <br /> units and overlying unconsolidated fresh-water--bearing units. <br /> The semiconsolidated rock units are composed of materials of the <br /> Mehrten and the Valley Spring formations. These formations are <br /> of volcanic origin and contain andesitic and rhyolitic breccia, <br /> tuff, and ash. The ground water within these two formations is <br /> Isaline; it is separated from the overlying free -water-bearing <br /> i units by an impermeable bed within the upper Mehrten Formation. <br /> The unconsolidated units consist of alluvial sediments of the <br /> j victor Formation, the Arroyo Seco Gravel, and tie Laguna <br /> Formation These formations, composed of gravel,, sands, milts, <br /> and clays, have a total thickness of approxime:XIY 1,000 feet. <br /> F Good quality ground water is present in theses tirti,ts, under. <br /> generally unconfined aquifer conditions. alchough minor confined <br /> aquifers exist. A previous study (DWF,. "'_;55) had n(dicated that a <br /> _, P ,X ' aadlit ch <br /> loxida waters F <br /> -� "barrier effect existed between th Y <br /> zon the west and the good-qualit,, k:,{ �_s dowing into the tockton <br /> area from Sierran sources to the -east. R�aua.t5 of a It2°r <br /> investigation (DW <br /> 1967) indica��,a ba �rier exists; <br /> that in fact, water of poor gt?31 ' s migr& .`yng into the <br /> Stockton area from the =:-: t, at ra.ry go4.7erxted .by the laws of <br /> fluid mechanics, the constant gec�la �C`1 p 'ameters, and the <br /> variable current hydraulic parameters b which ground water is <br /> apparent <br /> control-led. There are no lithologic, structural, or app <br /> h consistent hydraulic discontinuities which would stop the <br /> F incursion of water in an easterly direction from the Delta. <br /> Th <br /> depth to ground water. was. recorded as between 47 and 50 feet <br /> a . <br /> at the time of well installations (GTI 1986, 1987) A <br /> piezometric :map prepared in June .1987 indicated that: the ground-- <br /> water gradient at this site is.toward the northeast (GTI 1987) . <br /> All ground water in the Stockton area is considered to be a <br /> 1 2 <br /> EXXON9:84L <br /> z> <br /> rss <br /> s <br /> t <br />