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Project No S8100-06-11A June 29, 1995 INVESTIGATIVE SUMMARY The Caltrans District 10 Stockton Yard facility located at 1604 South B Street in Stockton, California formerly contained one 5,000-gallon diesel underground storage tank (UST), and one 10,000-gallon ' unleaded gasoline UST The USTs, two pump islands, and the associated piping were removed from the site by Placer Tractor in December 1989 Soil samples collected from the UST excavation contained total petroleum hydrocarbons as gasoline (TPHg) and diesel (TPHd) indicating that releases of fuel ' hydrocarbons into the surrounding vadose zone soils had occurred The highest TPHg concentration was detected beneath the west end of the unleaded fuel UST at a concentration of 2,000 milligrams per kilogram (mg/kg) The project site encompasses the area surrounding the former refueling facilities and the associated USTs tIn March 1992, Alton Geosciences advanced two soil borings (SB-1 and SB-2) and constructed three monitoring wells (MW-1 through MW-3) to assess the extent of impacts to soil and groundwater in the vicinity of the former fuel USTs and the associated pump islands TPHg and TPHd were detected in vadose zone soil at a depth of 14 0 meters (46 feet) below ground surface (bgs) in SB-1 and MW-3 The maximum concentrations were 120 mg/kg for TPHg (SB-1) and 31 mg/kg for TPHd (MW-3) Site investigation work performed by Geocon Environmental Consultants was designed to provide additional information regarding the vertical and lateral extent of petroleum hydrocarbon impacts to soil and groundwater The subsurface investigation included the excavation and sampling of soil borings, the construction of five groundwater monitoring wells(MW-4 through MW-8), the construction of one vapor extraction well (VW-1), the chemical analysis of soil and groundwater samples, and the determination of ' groundwater elevations beneath the site A summary of the results of the site investigation work performed by Geocon are as follows ' • The UST excavation samples indicated that releases of fuel hydrocarbons into the vadose zone soils had occurred Excavation sample UST-1 collected from the west end of the unleaded fuel 1 UST excavation contained TPHg at a concentration of 2,000 mg/kg and excavation sample UST 2-1 collected from the east end of the diesel fuel UST contained TPHd at a concentration of 58 mg/kg • TPHg and TPHd were detected in vadose zone soil in SB-I and MW-3 at a depth of 14 0 meters (46 feet) bgs at concentrations up to 120 mg/kg TPHg in SB-1 and 31 mg/kg TPHd in MW-3 TPHg and TPHd were not detected in SB-2, MW-1 and MW-2 with the exception of 1 0 mg/kg ' TPHg reported at 14 0 meters (46 feet) bgs in MW-2 • TPHg and TPHd were detected in vadose zone soil at a depth of 13 7 meters (45 feet) in MW-5 at concentrations of 17 mg/kg and 12 mg/kg, respectively TPHg was detected in the capillary fringe in VW1-12 at a depth of 19 8 meters (65 feet) at a concentration of 1 3 mg/kg i