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' Harding Lawson Associates <br /> wells to facilitate movement of vadose vapor towards the <br /> extraction trench. <br /> B. Effectiveness of Selected Remedial Action <br /> ' The selected remedial action is expected to be capable of <br /> removing most of the gasoline constituents from the soil and <br /> 1 groundwater. Even in a clayey soil matrix such as this, soil <br /> ' vapor extraction will remove volatile petroleum hydrocarbon <br /> compounds from the vadose and capillary fringe soil. Soil vapor <br /> extraction will also remove volatile petroleum hydrocarbons from <br /> the shallow groundwater. Gasoline constituents are very <br /> susceptible to soil vapor extraction and transport in the vapor <br /> phase. This technique greatly enhances the effectiveness of <br /> hydrocarbon removal and should be effective in long term <br /> ' remediation of groundwater quality. <br /> Groundwater extraction will remove most dissolved gasoline <br /> constituents and prevent further migration of hydrocarbons. This <br /> ' is a relatively slow method for complete removal because <br /> hydrocarbons tend to adsorb to soil surfaces and move slowly back <br /> 111 into solution as clean groundwater contacts the hydrocarbons. <br /> Groundwater extraction from the same trench utilizing vapor <br /> ' extraction will depress the water levels and expose more of the <br /> capillary fringe soil to the VES. Eventual dehydration of these <br /> ' soils will facilitate the movement of volatile hydrocarbons from <br /> ' the dissolved state to the vapor phase for removal by the VES. <br />� ' 21 <br />