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4.0 GEOLOGY AND HYDROGEOLOGY The site is located in the San Joaquin Valley of the Central Valley geomorphic province Recently published geologic maps indicate that Holocene age flood-basin deposits extend from ' ground surface to a maximum depth of approximately 100 feet below the site (Thrifty 1995) Oligocene to Holocene age alluvial deposits he below the younger flood-basin deposits to a maximum depth of approximately 3,500 feet below ground surface Continental and marine ' rocks of pre-Tertiary to Oligocene age underlie the alluvial deposits, and are composed of shale, sandstone and conglomerate Crystalline bedrock underlies the sedimentary formations (Thrifty 1995) ' The soil at the site consisted predominantly of clay, silt, clayey silt, and silty clay, interbedded with sand layers consisting of silty sand, fine- to medium-grained sand, and gravelly sand at ' depths of between 65 and 75 feet bgs, 90 and 100 feet bgs, and 110 and 115 feet bgs The thickest fine-grained interval within the sand layers was encountered between 75 and 95 feet bgs Geologic cross sections are provided on Figures 3A (A-A'), 3B (B-B'), and 3C (C-C') ' At the time of WCC's subsurface investigation in 1988, groundwater was encountered beneath g the site at approximately 50 feet bgs within the silty clay sediments During a subsequent site assessment investigation conducted by Thrifty in November 1992, groundwater was encountered during drilling at 80 feet bgs, indicating a significant drop had occurred in the local water table The static groundwater level has fluctuated from 71 08 to 48 50 feet bgs (about 22 feet) between 1992 and 2002 with a slight drop to 49 87 feet bgs in February 2004 As measured on August 17, 2004, the depth to groundwater ranged from 50 56 to 52 72 feet below top of casing ' Historically, groundwater has flowed to the northeast at gradients ranging from 0 003 to 0 007 feet/foot There have been some variations to the range of gradients, with steeper gradients (May 2002 and August 2003) but they appear to be the exception As of August 17, 2004, the groundwater flow direction is to the northeast at an approximate gradient of 0 0054 feet/foot (Figure 4) It should be noted that silt has accumulated in each groundwater monitoring well at thicknesses ranging from 3 54 feet in well MW-6 to 18 30 feet in well MW-4 The likely cause of the silt ' build-up is the fact that most of the soil underlying the site are silts and clays and the slot size in the wells 1s 0 020 inch with Monterey 43 sand pack ' 50 SENSITIVE RECEPTOR In September 1992, at the request of Thrifty, SSB Environmental Consultants, Inc performed a ' well survey to locate wells within 2,000 feet of former Thrifty Station #171 and in April 1996, Fugro West, Inc also performed a drinking water well survey to locate wells within 0 5 miles of the site Copies of these well surveys are provided in Appendix E ' On May 18 and 19, 2004, Thrifty conducted a well search of the potential downgradient receptor wells for both surveys Thrifty was accompanied by the SJCEHD on May 18, 2004 Following the search activities, Thrifty was able to confirm only the continued presence of one well which ' 5