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-WWCVvAT4L %W90. 1" ASSOCIATES ! NC , i,. seven wells identified by the sensitive receptor survey have been threatened by the contaminant plume at the site. r INHALATION OF VAPORS--The potential for inhalation of gasoline vapors at the site involves the ability of the gasoline components to partition from the groundwater into the soil and from the soil into the ambient air. Gasoline contaminated soil beneath the site exists beneath an asphalt paved surface and ground water was LW recently measured at an average depth of 56.4 feet below the site in the seven on site wells. No direct contact with the impacted soil or water and ambient air is expected without removing the paving and excavating the soil. REMEDIAL ALTERNATIVES - SOIL ATC evaluated three remedial alternatives to further reduce petroleum hydrocarbons in soil. The alternatives include natural attenuation,vapor extraction and removal. Natural Attenuation LW The former UST was removed from the site in 1987. Degradation of the gasoline is expected to continue over time without artificially modifying the subsurface conditions.Natural attenuation is advantageous because of its �. low cost and because potential exposure of contaminated soil or soil vapors is very low. It is considered a good method of remediation if the contaminant plume is stable in the vadose zone. Vapor Extraction Vapor extraction at the site could be useful to reduce the highest concentrations of TPHg and benzene. The effectiveness of vapor extraction is directly related to the permeability of the contaminated soil. The subsurface �. conditions at the site generally consist of variable layers of clayey silt and silty clay from the surface to approximately 18 feet bgs. Silty sand and sandy silt extend from approximately 18 to 33 feet bgs and are underlain by clayey silt and silty clay to approximately 40 feet bgs. A clay layer was encountered from approximately 50 to 55 feet bgs. A soil vapor extraction pilot test was conducted in 2001 which yielded a ROI of approximately 30 feet. Approximately 2.64 pounds of TPHg and BTEX were removed during the vapor extraction pilot test. Soil vapor extraction appeared to be a viable method for remediating TPHg and BTEX impacted soil at the site. y�_ uLW " w ° f Excavation Le Excavation in general is considered a good method of quickly removing impacted soil from a site. Excavation is generally not cost effective in removing soils deeper than approximately 20 feet because of the depth restrictions of most excavating techniques and due to the cost of disposal or ex-situ remediation and LW replacement material. Laboratory analytical data indicate petroleum impacted soil extends from 15 to 54.5 feet bgs in the vicinity of the former UST location. Concentrations of petroleum hydrocarbons have been detected in } soil samples collected from the borehole of MW-2 at depths of 40.5 and 50.5 feet bgs and from the borehole of %W VEW3 at 36.5 and 46.5 feet bgs. a.. Diesel Performance January 5,2004 } ATC Project Number 5462577 Page 5