The hydrodynamic dispersion discussed in the previous section affects all the contaminant concentrations equally; however, adsorption, precipitation, and chemical reactions with other groundwater constituents retard the rate of advance of the affected contaminants. This effect is described by the retardation factor (Rd), which can range from a value of 1 to 50 for organics often encountered at field sites. The lowest values are for conservative substances, such as chlorides, which are not removed in the groundwater system. Chlorides move with the same velocity as the adjacent water in the system, and any change in observed chloride concentration is due to dispersion only, not retardation. Retardation is a function of soil and groundwater characteristics and is not necessarily constant for all locations. The Rd for some metals might be close to 1 if the aquifer is flowing through clean sandy soils with a low pH but close to 50 for clayey soils. The Rd for organic compounds depends on sorption of the compounds to soil organic matter plus volatilization and biodegradation. The sorptive reactions depend on the quantity of organic matter in the soil and on the solubility of the organic material in the groundwater. Insoluble compounds such as dichloro-diphenyl-trichloroethane (DDT), benzo[a]pyrenes, and some polychlorinated biphenyls
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