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Information for the former Wheel Country/Service Station, California Highway Patrol, and former Shell Station(7-11 Store) can be found in the Site Assessment Report(Treadwell & Rollo, 2003a) ' 3.4 Hydrogeologic Setting The 541 Site and surrounding area are situated on alluvial and continental deposits of Miocene to Holocene age and typically consist of poorly sorted clay, silt, sand and gravel (USGS, 1982) ' The regional groundwater flow direction is generally to the north toward the Old River branch of the San Joaquin River (SJC, 1999) The local depth to groundwater is reported to range from approximately 4 to 9 feet below ground ' surface(bgs) and the local gradient ranges from 0 02 to 0 002 (Treadwell & Rollo, 2003a) The top of groundwater at the 541 Site was reported to be at approximately 10 feet bgs in August 2000(AGE, 2000) However, during the construction of the Sherwin Williams building and storm water basin in 2001-02, the depth to groundwater was noted to be at approximately 6 to 6 5 feet bgs (Bird, 2003) ' Currently, the local groundwater flow direction is to the north-northwest based on monitoring well data from the adjacent LUST sites(Secor, 2003) Historically, groundwater flowed to the ' north-northeast from at least 1986 (first monitoring wells installed in the area) to 1988 This is based on monitoring reports from the Mobil and Shell Station facilities (KE, 1986, 1987 and ' 1988 and HLA, 1986) Therefore, the commingled plumes from the Chevron, Mobil, and Scotties Stations would have been located upgradient of the 541 Site prior to 1988 (Figure 4) Chevron installed their first monitoring wells in 1995 and Scotties initially installed wells in 2000, after the flow direction had changed to the north-northwest The reason for the flow ' direction change is unknown Field Investigation Report 7 8 December 2003 ' TREADWLLL&ROLLO,INC