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i Previous Investigations SSB Environmental Consultants, Inc performed the initial investigation at the site in July of 1991 The preliminary investigation consisted of the installation of four (4) 2-inch diameter PVC groundwater monitonng wells to about 75 feet below grade at the locations shown on Figure 2 Results of analyses indicated the highest concentrations of gasoline range hydrocarbons (TPH) in soil were found between depths of 20 and 45 feet in well MW2 (37 mg/kg to 550 mg/kg) TPH concentrations in sods from well MW1 ranged from 0 4 to 37 mg/kg The other bonngs had TPH concentrations less than 7 mg/kg Gasoline range hydrocarbons were detected in groundwater from each of the wells, with benzene concentrations ranging from 36 4 to 5,270 ug/l The details of this investigation are presented in the SSB report titled "Preliminary Assessment of Soil and Groundwater Quality at Thrifty Oil Co Station No 172, 7647 Pacific Avenue, Stockton, California", dated July 17, 1991 GEOLOGY The site is located in the San Joaquin Valley of the Central Valley geomorphic province Holocene age flood-basin deposits extend from the ground surface to a maximum depth of about 100 feet below the site Oligocene to Holocene age alluvial deposits he below the younger flood-basin deposits to a maximum depth of about 3,500 feet below grade 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 Based on boring logs recorded during drilling activities, the sediment section beneath the site generally consists of dark, organic silty clay to a depth of 5 feet, underlain by brown, silty sand to 10 feet, poorly sorted sand to 20 feet, silt and clayey silt to 45 feet Poorly sorted sands and silty sands underlie these units to at least 75 feet below grade Descriptions of these sediments • Page 2