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Cicologkal Technks lnc. Page 2 Sinclair Trucking Feasibility Study Addendum Project No. 1030.3 April 26,2004 ' The process of air sparging and extraction is a powerful combination that works well if the site conditions are compatible. However, the tight soils, shallow groundwater, uncapped site, and lack of POTW sewer treatment makes this method unproductive. ' Air Sparging Air sparging is the process of injecting air into the aquifer to volatilize contaminants from ' the groundwater and to increase oxygen content in the subsurface to enhance natural attenuation processes. By distributing air into the aquifer, the contaminants are captured in the vadose zone and removed with the use of a vapor extraction recovery system (described ' below). The addition of air into the aquifer also aids in increasing the bacteria metabolic rate by activating aerobic bacteria and thus increasing natural degradation. Air sparging presents a couple of concerns at the site: ' • Shallow depth to groundwater. Since the vadose zone at the site is approximately 9 to 12 feet thick, there is very little unsaturated soil zone where the vertically migrating vapors could be captured. Since the site is uncapped by either asphalt or concrete, there ' would be a high possibility that short-circuiting of the vapors could occur, petroleum vapor could be pushed under buildings or into utility trenches, or uncontrolled emissions to the atmosphere of petroleum hydrocarbon vapors could occur. ' • Low permeability of materials. The vadose zone soil is predominately clays and silts, which do not conduct airflow very well through the subsurface. • Capture and control of sparged vapors requires an expensive inventory of equipment, including extraction wells and effluent treatment by thermal oxidizers or units of granular activated carbon. Vapor/Groundwater Extraction (Dual Phase Extraction) Vapor extraction is the process of drawing a vacuum on the vadose zone to produce an air flow through the subsurface, which removes volatile compounds from the soil and soil pore spaces through subsurface convective air flow, evaporation and concentration gradient ' diffusion. Typically, the extracted air is treated using thermal destruction (a thennal oxidizer or internal combustion engine) or granular activated carbon. ' A number of factors keep vapor extraction from being a promising technology at the site: • Shallow depth to groundwater— 9 to 13 feet bgs. Thin vadose zones are problematic to vapor extraction due to the tendency of the air to short circuit to the ground surface. ' The shallow groundwater would also cause a problem by mounding in and around the vapor extraction wells. A9 ' • Low permeability of vadose zone materials. The vado.se_zone materials_at_the_site_are ,rimarily' composed o__ f silts and clays, which have low penneability. Low permeability soils create problems for vapor-extraction systems by limiting the radial influence of the extraction wells. A blower with high vacuum potential must be used which increases energy consumption. High vacuum would also cause a problem with mounding of the groundwater... _. • The majority of the contamination is tied up in the smear zone that ranges in depth from about 8.5 to near 13 feet. It would be very difficult to target this zone as much of the ' year it is under the water table and the rest of the year it is within the capillary fringe zone. 1