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Protect No S8100-06-11A June 29, 1995 60 DATA EVALUATION AND DISCUSSION 6 1 PETROLEUM HYDROCARBONS IN SOIL Soil samples collected from the UST excavation indicated that releases of fuel hydrocarbons into the surrounding vadose zone soils had occurred Excavation sample UST-1 collected from the west end of the unleaded fuel 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 The distribution of presently detected petroleum hydrocarbons in soil is primarily limited to vadose zone ' soil above the capillary fringe Residual BTEX concentrations remain in vadose zone soil in the pen meter wells MW-5, MW-6, MW-7, and MW-8, however, the soil plume or source area is limited to the vadose zone in the vicinity of the former fuel USTs location bounded by SB-1, S13-2, MW-1, MW-2, MW-3, and ' VW-1 The highest levels of TPHg and TPHd were detected in vadose zone soil in S13-1 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 was detected in the capillary fringe in VW-1 at a depth of 19 8 meters (65 feet) at a concentration of 1 3 mg/kg 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 The former fuel UST locations are bounded by low concentrations of petroleum hydrocarbons in vadose ' soil detected in SB-1, S13-2, MW-1 through MW-3, and VW-1 indicating that a vadose zone soil plume exits beneath the former USTs The vadose zone soil plume bounded by SB-1, SB-2, MW-1, MW-2, MW-3 and VW-1 has an approximate volume of 350 cubic yards, however, the location of the central portion of the soil plume has not been identified, consequently,the concentration of impacted soil and the resulting mass of TPHd and TPHg cannot be estimated ' 62 DISSOLVED PETROLEUM HYDROCARBONS IN GROUNDWATER Evaluation of groundwater quality data collected from the onsite monitoring wells indicates that the former refueling operations has impacted the groundwater The distribution of dissolved phase hydrocarbons in groundwater is consistent with the groundwater flow directions depicted on Figure 5 The highest petroleum hydrocarbon concentrations were detected within the former UST excavation,and in the vicinity of the former UST excavation (MW-1 through MW-3 and VW-1) and the downgradient perimeter wells ' (MW-6 and MW-7) at maximum concentrations of 600 ug/L TP14d, 28,000 ug/L TPHg and 5,300 ug/L benzene Concentrations of dissolved phase hydrocarbons in the upgradient perimeter wells(MW-4, MW- 5,and MW-8) range from 130 to 550 ug/L TPHg and <0 5 to 3 1 ug/L benzene, respectively The lateral ' limits of TPHg have not been established, therefore, additional investigation will be required to further delineate the extent of TPHg impacts 11