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3 . DISCUSSION OF RESULTS <br /> The boundary between the Tulare formations (characteristic of the <br /> western part of the San Joaquin Valley) , the flood basin deposit <br /> in the central and Delta part, and the Laguna Foundation (eastern <br /> part ) are not clearly recognized in the Stockton area, but <br /> differences in lithology and static water leaks suggest that the <br /> ` ground water directly under the site and that pumped from public <br /> wells located nearby are not in direct hydraulic connection and <br /> belong to different geologic formations. The aquifer underlying <br /> the site appears to be isolated from the deeper confined aquifer, <br /> probably by an impervious layer similar to the Corcoran clay that <br /> is widespread within the Tulare Formation. <br /> The source of water for the unconfined aquifer in the area is <br /> mainly infiltration from the Calaveras River, irrigation water, <br /> and precipitation. This water is usually of poor quality in the <br /> western part of Stockton. The recharge zone of the deeper <br /> aquifer is in the Sierra Nevada through streams traversing the <br /> ` outcrop area, from direct rainfall, and from irrigation through <br /> infiltration. <br /> The lateral and vertical heterogeneity of water-bearing forma- <br /> tions has created locally confined or semi-confined aquifers and <br /> occasional minor perched-water zones. Ground water underlying <br /> the investigated site appears to be an example of a perched- <br /> water-bearing zone. As shown in Figure 7 , the perched water <br /> flows north, which suggest that the recharge zone for this <br /> ` aquifer might be the Calaveras River. <br /> L The hydraulic gradient based on water level measurements in the <br /> monitoring wells is 0. 005 . The estimated ground-water velocity <br /> ` in the sandy lenses is <br /> L <br /> 23 <br />