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(YVATC v A S S O C I A T E S I N C 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 ~" The former USTs were removed from the site and replaced by three 12,000-gallon gasoline USTs and one 550-gallon waste oil UST in 1988. 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. 4 Vapor Extraction Vapor extraction at the site could be useful to reduce the highest concentrations of TPH and benzene. The effectiveness of vapor extraction is directly related to the permeability of the contaminated soil. The 40W subsurface conditions at the site generally consist of variable layers of silty and sandy clay, silty sand, clay, and silt from the surface to approximately 26.5 feet bgs. A soil vapor extraction pilot test would need to be conducted to evaluate the potential effectiveness of vapor extraction at the site. u DTw y �_ 7"ag 6W Excavation 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 6w replacement material. Approximately 450 to 500 cubic yards of soil was excavated in 1988. Laboratory analytical data collected since 1988 indicate petroleum impacted soil is present along the western property boundary and southeast of the fuel dispenser island. The location of the existing USTs overlaps a large portion of the location of the former USTs. REMEDIAL ALTERNATIVES — GROUNDWATER ATC evaluated four remedial alternatives to further reduce petroleum hydrocarbons in the groundwater beneath the subject property. The alternatives include pump and treat, air sparging, natural attenuation and enhanced natural attenuation. Pump and Treat This remedial approach involves pumping of ground water to the surface for treatment. It would require the installation of additional four-inch diameter extraction wells. Prior to installation of the extraction wells, aquifer testing using existing monitoring wells would be conducted to determine aquifer characteristics. The results would be used to determine the most effective pumping rate, capture zone, draw down, and design of the extraction wells. A variable speed electrical pump would be placed in each extraction well. Groundwater would be placed in a surface holding tank prior to treatment. The groundwater would then be pumped through dual 1,000-pound Former Cheaper#37 January 29 2004 ATC Project Number 5425847.0037 Page 7