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3.2 Hydrogeology The deposits containing fresh groundwater are principally unconsolidated continental deposits of Pliocene to Recent age that extend to depths ranging from less than 100 to more than 3,500 feet bgs. Groundwater occurs under both confined and unconfined conditions in the Central Valley. Recharge to the groundwater reservoir is by infiltration of rainfall, infiltration from streams, canals and ditches, by infiltration of excess irrigation water and by underflow entering the valley from tributary stream canyons (Poland and Evenson, 1966) The hydrogeologically significant geologic units encountered in the area in order of shallowest to deepest, are the river-channel and flood-plain deposits (0-50 feet thick), the Victor Formation and related deposits (0-150 feet thick), the Laguna Formation (0-400 feet thick) and the Mehrten Formation (75-400 feet thick, Poland and Evensen, 1966) In the Central Valley, great agricultural development has caused mayor subsidence of the land surface Maximum subsidence ranges from 8 feet south of Bakersfield to 23 feet southeast of Los Banos The subsidence is of three types. In the lowlands of the Delta at the confluence of the Sacramento and the San Joaquin Rivers, subsidence has been caused chiefly by the oxidation of peat lands accompanying drainage and cultivation In the largest area, between Los Banos and Wasco, and at the south end of the valley between Arvin and Maricopa, most of the subsidence has been caused by lowering of the artesian head in confined aquifer systems, due to the intensive pumping of groundwater Locally, on the west and south flanks of the valley, a third type of subsidence has been caused by near- surface compaction of moisture-deficient alluvial fan deposits above the water table, after initial wetting by percolating irrigation water (Poland and Evenson, 1966) SSB conducted a survey of nearby wells during their preliminary investigation. Their findings indicated a City of Stockton well (No 11) is located at the corner of Inglewood and Leslie Avenues (approximately 2,100 feet southeast of the site) A number of private wells are located within 1,000 feet to the west of the site in a county residential area and one to the northwest at the Carden School. In addition, a number of wells are located in a small commercial center south of the site, however, they are no longer used, as the occupants currently uhhze the City of Stockton water system (Thrifty, 1995). In May, 1991, SSB encountered groundwater at a depth of approximately 58 feet bgs during the installation of groundwater monitoring wells The hydraulic gradient was measured to be 0 0017 feet per foot towards the east-southeast Subsequent measurements have confirmed this flow direction although the static groundwater level has gradually risen since this time (Thrifty, 1995) 3