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Project No S8100-06-1 l A June 29, 1995 70 CONCLUSIONS AND RECOMMENDATIONS t71 CONCLUSIONS ' Based on Geocon's review of all soil and groundwater data acquired through June 1995, summarized below are conclusions regarding the environmental investigations performed at the site • 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 • TPHg and TPHd were detected in vadose zone sort in SB-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 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 1 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 VWI-12 at a depth of 19 8 meters (65 feet) at a concentration of 1 3 mg/kg • BTEX exists in vadose zone and capillary fringe soil in VW-I at concentrations ranging up to ' 0 13 mg/kg benzene, 0 061 mg/kg toluene, and 0 034 mg/kg total xylenes • The Iow concentrations of TPHg in vadose zone soils 1n SB-1, SB-2, MW-1, MW-2, MW-3 and VW-1 that surround the former USTs location, combined with the existence of TPHg impacted groundwater beneath the former USTs indicate 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 • A minimum of one additional soil boring within the former UST excavation would be required ' in order to attempt to better estimate the volume of vadose zone soil impacted with TPHg • Initial evaluation of soil remediation options indicates that soil vapor extraction is the most cost ' effective soil remediation technology available to reduce the mass of TPHg in vadose zone and capillary fringe soil The performance of a vapor extraction field pilot test would be required to obtain design criteria for a soil vapor extraction system 12