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7hN3dweII&RoIIo <br /> 5.2 Contaminant Persistence • <br /> 5.2.1 Soil <br /> Metals, heavy fuels, and semivolatile organic contamination in soil are relatively persistent. All <br /> have a tendency to sorb onto organic material and certain clays, retarding transport in <br /> groundwater. Transport is further retarded by the fact that most soil contamination occurs in the <br /> top five feet of soil, well above the groundwater table. The low vapor pressures of the heavy fuel <br /> hydrocarbons and semivolatile PAHs minimize the potential for these contaminants to <br /> significantly volatize, except when they occur at the ground surface. Volatilization is retarded by <br /> the presence of buildings, asphalt and concrete paving, and compacted soils. Transport to <br /> groundwater is also retarded by these factors, and by the routing of stormwater to storm drains. <br /> 5.2.2 Groundwater <br /> Low groundwater flow velocities and low solubilities of the majority of the contaminants found <br /> in groundwater increase the persistence of contaminants in groundwater. Lighter-end fuel . <br /> hydrocarbons, such as gasoline and BTEX compounds, are susceptible to bioremediation when <br /> indigenous bacteria and nutrients are present. TPH-g and BTEX are found at elevated <br /> concentrations at Areas 15, 16, and 21, and it is likely that natural bioremediation of THP-g and <br /> BTEX is occurring at these sites. <br /> • <br /> 25970309.DGD 32 <br /> 24 August 2000 <br />