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i On July 11, 1995, Thnfty installed three offsite groundwater monitoring wells (MW5, MW6 and MW9) to a depth of approximately 60 feet bgs Laboratory analysis of all soil samples collected from each monitoring well did not detect TPH and BTEX at concentrations which exceeded laboratory detection limits TPH was detected in groundwater samples collected from these wells at concentrations ranging up to 1,000 ug/l and benzene was detected concentrations ranging up to 3.4 ug/l (Thrifty, 1995) On July 11, 1995, three vadose wells (VWA, VWB and VWC) and three groundwater monitoring wells (MW 1, MW2 and MW$) were properly abandoned prior to station remodeling activities (Thnfty, 1995) On July 21, 1995, five single-walled, steel USTs, all dispensers and their associated piping were removed from the site Approximately 900 cubic yards of soil were removed from the UST excavation and transported to Forward Incorporated, of Stockton, California for disposal. Nine soil samples were collected beneath the five USTs and seven soil samples were collected were collected at a depth of 2 to 4 feet beneath the product lines TPH and benzene were detected in soil samples collected from beneath the USTs at concentrations ranging up to 12,000 mg/kg and 33 mg/kg, respectively (Alisto, 1995). Groundwater monitoring and sampling has occurred at the site since June 6, 1991 3.0 GEOLOGY AND HYDROGEOLOGY 3.1 Geology The site is located in the San Joaquin Valley of the Central Valley (sometimes referred to as the "Great Valley") geomorphic province Geologically, the Central Valley is a large elongate northwest-trending asymmetric structural trough that has been filled with a tremendously thick sequence sediments ranging in age from Jurassic to Recent The basun has a regional southward tilt, which is interrupted by two significant cross-valley faults The northernmost fault, the Stockton fault, is the boundary used by most geologists to separate the Central Valley into two sub-basins the Sacramento and San Joaquin, (Hackell, 1966) Holocene age flood-plam deposits extend from ground surface to a maximum depth of approximately 100 feet below the site Oligocene to Holocene age alluvial deposits he below the younger flood-basin deposits to a maximum depth of approximately 3,500 feet bgs. 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 (Thnfty, 1995). 2