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Project No 58100-06-11A June 29, 1995 20 INTRODUCTION ' 2 1 GENERAL OBJECTIVES ' The objective of this Task Order was to provide additional information regarding the vertical and lateral extent of petroleum hydrocarbon soil and groundwater impacts in the vicinity of the former fuel USTs ' 22 PREVIOUS INVESTIGATIONS The results of the tank removal soil sampling activities and subsequent subsurface investigations performed by others are summarized below The USTs at the site and associated refueling facilities were removed by Placer Tractor in December 1989 The results of laboratory tests performed on soil samples obtained from the UST excavations indicated ' the presence of benzene, toluene, ethylbenzene, and total xylenes (BTEX) at concentrations up to 120 milligrams per kilogram (mg/kg) and maximum total petroleum hydrocarbons as gasoline (TPHg) and diesel (TPHd) concentrations of 2,000 and 58 mg/kg, respectively The TPHg concentration of 2,000 ' mg/kg was reported for a soil sample obtained at a depth of 4 3 meters (14 feet) below ground surface (bgs) from the fill riser end (west end) of the former gasoline UST (sample UST 1-1) The remaining UST and product piping excavation sample locations are not known In March 1992, Alton Geosciences advanced two soil borings (SB-1 and SB-2) to a depth of 20 3 meters (66 5 feet) bgs,constructed three monitoring wells(MW-I through MW-3)to a depth of 24 2 meters(79 5 ' feet) bgs, and collected soil and groundwater samples for chemical analyses The approximate locations of the borings and wells are depicted on Figure 2 The results of the analyses indicated that TPHg existed in soil in SB-1 and MW-3 at a depth of 14 0 meters (46 feet) bgs at concentrations of 120 and 30 mg/kg, respectively TPHd was also reported at the 14 0 meter (46 foot) depth in SB-1 and MW-3 at concentrations of 3 8 and 31 mg/kg, respectively The ' laboratory reports indicate that the TPHd results "do not appear to contain diesel " TPHg and TPHd were not detected in SB-2, and 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 ' Groundwater sampled from MW-1 through MW-3 in April 1992 was reported by Alton Geosciences to be impacted with TPHg at concentrations between 2,400 and 4,900 micrograms per Inter (ug/L), TPHd between 73 and 180 ug/L, and benzene between 260 and 1,100 ug/L Groundwater was encountered at a depth of approximately 20 1 meters (66 feet) bgs and the gradient was reported to be 0 002 to the west In October 1993, Environmental Assessors obtained groundwater samples from MW-1 through MW-3 for 1 TPHg and BTEX analyses TPHg was reported at concentrations between 4,500 and 27,000 ug/L, and benzene between 520 and 11,000 ug/L Groundwater was encountered at a depth of approximately 20 7 meters (68 feet) bgs and the gradient was determined to be 0 0006 to the northeast 2