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• These borings further defined the general hydrogeology of the site, of a confined silty, clayey, sand aquifer, overlain by a silty clay. This aquifer contains a large amount of clay and fine grained particles and therefore has a low hydraulic conductivity which probably has slowed the migration of the contamination. The confined nature of the aquifer and the presence of the gasoline under the confining clay as evidenced by the SBI at 15 foot sample makes remediation of the contamination difficult. The lack of vertical permeability, confining clay layer, and the trapping of the gasoline beneath the clay present a number of problems : I. The lack of vertical permeability, greatly slows the rate of vertical gas exchange effecting a number of factors: A. Very low rates of oxygen exchange from the atmosphere to the aquifer. This slows the action of aerobic biodegradation which is much faster than anaerobic biodegradation. B. Very low rates of carbon dioxide exchange from the aquifer to the atmosphere, thereby causing a build up of carbon dioxide in the aquifer inhibiting the aerobic bacteria. C. Very low rate of hydrocarbon exchange from the aquifer to the atmosphere, causing a significant slowing of gasoline dissipation to the atmosphere. 2 . Confined aquifer, with gasoline trapped under the confining clay layer: A. No permeable vadose zone (dry soil) . This makes straight vapor extraction impractical because of the lack of a dry permeable section of the aquifer from which to extract the hydrocarbons. Air flow is nearly impossible through the confining clay. B. Uneven nature of base of confining clay layer trapping floating free product. The floating product trapped in high spots in the aquifer will not flow towards a pumping well until the aquifer has been dewatered to below the base of the clay. 5