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Harding Lawson Associates <br /> volatile hydrocarbons if the soil matrix is permeable to air and <br /> of sufficiently large hydrocarbon concentrations can be removed <br /> twith the soil gas. This remedial option is effective in removing <br /> petroleum hydrocarbons from both the vadose and the capillary <br /> ' fringe zones. Though the soil matrix has a low vapor <br /> ' permeability, vapor movement still exists through the vadose and <br /> eventually will lead to the volatilization of the hydrocarbon <br /> contaminants and removal by the vapor extraction system (VES) . <br /> This is the recommended remedial action for soil at this site. <br /> 1 <br /> 5. In-situ Bioremediation <br /> This alternative entails stimulating microbial <br /> populations to metabolize hydrocarbons, thereby producing water, <br /> carbon dioxide, and biomass. This method was not chosen because <br /> of complexities involved with supplying nutrients in a clayey <br /> soil matrix and assurance that those nutrients would be evenly <br /> distributed. However, it should be noted that vacuum extraction <br /> does tend to stimulate biological activity by supplying oxygen to <br /> the existing microbial population and typically enough nutrients <br /> ' exist to support microbial growth for a significant period of <br /> time. <br /> C. General Groundwater Remediation Alternatives <br /> ' The following remedial alternatives were considered: <br /> • No action; <br /> ' a Soil vapor extraction of hydrocarbons from the <br /> groundwater surface; <br /> 11 <br />