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p.e <br /> The geological formations in the Stockton area can be classified <br /> in two general groups: semiconsolidated saline-water-bearing <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 /> of volcanic origin and contain andesitic and rhyolitic breccia, <br /> tuff, and ash. The <br /> groundwater within these two formations is <br /> saline; it is separated from the overlying freshwater-bearing <br /> units by an impermeable bed within the upper Mehrten Formation. <br /> The unconsolidated units consist of alluvial sediments of the <br /> Victor Formation, Arroyo Seco Gravel, and Laguna Formation. <br /> These formations, composed of. gravels, sands, silts, and clays, <br /> •-� have a total thickness of approximately 1 ,000 feet. Good quality. <br /> groundwater is present in these units, under generally unconfined <br /> -aquifer conditions, although minor confined aquifers exist. A <br /> previous study .(DWR.. 1955) .had indicated that a "barrier effect', ' <br /> # existed between the poor-quality chloride waters on the west and i <br /> i W the good-quality waters flowing into the Stockton area from <br /> Sierran sources to the east. Results of a -later investigation <br /> (DWR 1967 ) indicate that no such barrier exists; that in fact, <br /> h water of poor quality. is migrating into the Stockton area from . i <br /> the west, at`a rate .governed by the laws `of. fluid .mechanics, the. <br /> constant geological + <br /> .� ' parameters, .and .he variable current <br /> hydraulic, parameters by which groundwater is controlled.. There <br /> A. are.-.no .lithologic, structural, or apparent consistent hydraulic j <br /> discontinuities that would stop the incursion of water in an <br /> easterly direction from-_the_Delta. <br /> 1 .3.2 Site H dro Bolo <br /> ! <br /> Five monitoring wells were installed .at the site .during previous <br /> investigations. Initial depth. to groundwater. was recorded <br /> t between 47 and 50 feet, and theroundwater <br /> g gradient was calcu- <br /> lated to be toward the northeast (GTI 1.987 ) . The northeast <br /> � � f <br /> e25/71050/8-89/txt 3 <br />