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Former Discovery Chevrolet June 5, 1998 Tracy,California Corrective Action Plan diversions and redistribution of surface water throughout the area. Groundwater quality in the water-bearing units of the San Joaquin River Basin is generally fair to good (meets recommended secondary or primary standards for drinking water). 3.3 Local Geology The site is located in the northern portion of the San Joaquin Valley in western San Joaquin County, California. Unconsolidated continental and lacustrine deposits of Holocene and Pleistocene age underlie the northern portion of the San Joaquin Valley. These deposits consist predominantly of unconsolidated fine-grained materials, with buried channels of sand and gravel. These deposits are underlain at depth by coarser-grained water-bearing continental sediments. Reported subsurface materials encountered during drilling at the site consisted primarily of fine-grained soils such as silty to sandy clay, clayey silt, and clayey sand. Intersecting cross-sections through the area of investigation were included as Figures 4 and 5 in Horizon's Problem Assessment Report. 3.4 Local Hydrogeology The most productive water wells in the Tracy area are those wells completed within the Lower Aquifer Zone of the valley alluvium. The valley alluvium consists of permeable continental and lacustrine deposits interlaced by numerous gravelly stream. According to DWR records, the nearest well to the site is 700 feet north-northwest of the site and is completed to 427 feet bsg. An apparent water well discovered outside of the northwest corner of the service bay area at the site was measured to a depth of approximately 145 feet. The shallow groundwater being monitored beneath the site is within the uppermost water- bearing or water table zone. Shallow groundwater generally occurs in the uppermost water- bearing zones in either unconfined or semiconfined conditions. Because of the generally fine-grained nature of the shallow water-bearing materials and poor historical land-use practices, shallow groundwater in the area is no longer utilized for domestic purposes, but ' .; may still be used for agricultural irrigation purposes. The shallow groundwater beneath the 3001-21.cap 6 HORIZON ENVIRONMENTAL INC.