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to the groundwater reservoir is by infiltration of rainfall, infiltration from stream, 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 the 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 (0400 feet thick) and the Mehrten Formation (75-400 feet thick) (Poland and Evenson, 1966) ' In the Central Valley, extensive agriculture development has caused major 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 the 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 Mancopa, 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 imgation water (Poland and Evenson, 1996) SSB conducted a well survey within a half-mile radius of the Site during their preliminary investigation Their findings indicated that a City of Stockton well (No 11) is located at the corner on 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 in 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 utilize 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) In March 1999, groundwater (the first water-bearing zone) was encountered at a depth of approximately 33 feet bgs during the installation of groundwater monitoring well MW-12 The hydraulic gradient was measured to be 0 0013 feet per foot to the southwest-southeast on ' March 31, 1999 A.1152831 ASA