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' Site Background Information: Fisco Warehouse Page 2 of 5 1 the site): ' Vadose Zone - This zone extends from the surface to 54 feet below surface grade (bsg), the depth to the water table.The vadose zone consists of predominantly fine-grained material from the surface ' to 54 feet bsg, with a coarse-grained sand unit occurring between. 20 feet and 25 feet bsg across a portion of the site. This zone is characterized by low pore pressure. ' Hydrological Unit I (HUI) - Starting at the top of the current saturated zone, HUI extends from. 54 feet to 80 feet bsg and is comprised of silty sand to sandy silt. This zone is characterized by predominantly high pore pressure. IHydrological Unit 2 (HU2) - HU2 extends from 80 feet to 96 feet bsg. It is comprised of an upper I 0-foot layer of fine-grained soil underlain by a 5-foot sand and gravelly sand interval. This zone is ' characterized by low pore pressure. Hydrological Unit 3 (HU3) - HU3 extends from 96 feet bsg to, and possibly beyond, 134 feet bsg. ' ,Silt and clay are the dominant soil types in this interval. HU3 is characterized by pore pressure variations from moderate to high pressures, UNDERGROUND STORAGE TANK REMOVAL ' Two 550-gallon underground storage tanks(USTs)were removed from the site on 03 October 1986 by Stockton Service Station Equipment Company. According to correspondence from the San I Joaquin County-Environmental Health Department(EHD),both USTs were reportedly used to store gasoline.Upon removal of the USTs,one soil sample was collected from the floor of the excavation. The sample was analyzed by California Water Labs, Inc., a state-certified analytical laboratory, for ' total petroleum hydrocarbons quantified as diesel (TPH-d), benzene, toluene, and xylenes. TPH-d and xylenes were detected in the soil sample at concentrations of 3,000 milligrams per ' kilogram(mg/kg)and 0.188 mg/kg,respectively. Benzene and toluene were not detected above the analytical method reporting limit of 0.010 mg/kg.Based upon results of the soil sample analysis,the EHD required assessment of the lateral and vertical extents of hydrocarbon-impacted soil and/or ground water in the area of the former UST. PREVIOUS SUBSURFACE INVESTIGATION On 18 September 1997, five soil probe borings (BI through B5)were advanced at the site to assess the lateral and vertical extents of petroleum-impacted soil.Total petroleum hydrocarbons quantified as gasoline (TPH-g) were detected in samples from BI, B3, and B5 at concentrations as high as Advanced GeoEnvironmental,Inc