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i ! \ <br /> G <br /> I..r <br /> 1.3 HYDROGEOLAGY .PI;I) WATER USE <br /> The geological forioations in the Stockton area can be classified <br /> f _ in two .general groups: semiconsolidated saline-water-bearing <br /> { units and overlying unconsolidated fresh-water-bearing units. <br /> i The send consolidated rock units are composed of materials of the <br /> 1 Mehrten and the Valley Spring formations. These formations - <br /> are <br /> of. volcanic origin and contain andesitic and rhyolitic breccia, <br /> l tuff, and ash. The ground water within these two formations is <br /> -I <br /> �J saline; it is separated from the overlying fresh-water-bearing <br /> units by an impermeable bed within the upper Mehrten Formation. <br /> s The unconsolidated units consist of alluvial sediments of the <br /> r. Victor Formation, the Arroyo Seco Gravel, and the Laguna ` <br /> Formation. These formations, composed of gravels, sands, silts, <br /> n clays, have a total thickness of approximately 1,000 feet. <br /> I . <br /> and Y , <br /> --; � Good quality ground water is present in these units, under <br /> Y7. <br /> enerall unconfined a q uifer conditions, although minor confined i <br /> U <br /> I: <br /> aquifers exist. A previous study (DWR 1955) had indicated that a <br /> LI } <br /> a 'A "barrier. effe.ct" existed between the poor-quality chloride. waters ; <br /> on the west. and the good-quality waters flowing. into the Stockton r <br /> LI <br /> area from Sierran sources to the east. Results of a later ; <br /> investigation (DWR 1967) indicate that no such barrier exists; <br /> that in fact, water of .poor quality is migrating into the i <br /> Stockton area from thewest, at a rate governed by the laws of <br /> i <br /> fluid mechanics, the constant geological parameters, and. the <br /> variable current hydraulicparameters by which ground water is <br /> controlled There are no lithologic, structural, oor. appar <br /> ent <br /> consistent hydraulic discontinuities which would st p the <br /> !�1 incursion of water in an easterly. direction from the Delta. <br /> ! i <br /> The depth to ground water was recorded as between 47 and 50 feet r <br /> at the time of well installations (GTI 1986,. 1987) A <br /> piezometric map prepared in June 1987 indicated that the ground- <br /> t ` water gradient at. this site is toward the northeast (GTI 1987) . ` <br /> „ r x All ground water in the Stockton area is considered to be a <br /> EXXON9:84L 2 <br /> a <br /> q4,di� <br />