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E ~ , conductivity.of the soil is the single most important parameter in determining the . <br /> groundwater flow rate. Because no soil samples were tested for hydraulic conductivity <br /> during the site investigation, it is possible that this parameter was substantially <br /> overestimated. <br /> Additionally, the dry bulk density,effective porosity, and organic carbon contezit are other <br /> parameters that were not measured during the initial site investigation. Overestimation of. <br /> the values for.these parameters would also lead to overestimating the rate of migration of <br /> the hydrocarbons. <br /> In summary, computer-modeling programs that attempt to predict the fate and transport of <br /> organic contaminants in{the subsurface should be used with caution,because they,are <br /> based on idealized conditions that rarely attain in nature, and.,they depend on numerous <br /> assumptions and a variety of data that can rarely be quantified. At the SME Railroad <br /> site, our simulations based on the limited data that are available resulted in unrealistic ' <br /> predictions that do not accurately portray_ the present distribution of contaminants. The <br /> simulations should not be used.to determinehether the case meets Re 'onal Water <br /> ali Control Board criteria for site closure. Rather, decisions should be based on the <br /> solid laboratory data t at Were o #ained in the original site investigation. These'data- <br /> demonstrate that the impacted area is of very limited extent, and the contaminants are } <br /> almost entirely low-solubility compounds whose movement in the subsurface is strongly <br /> impeded by the fine-grained nature of the soil.. More mobile contaminants, such as r <br /> MTBE, benzene, or other gasoline'range hydrocarbons, are not present in the soil. <br /> a <br /> 9 <br />