Adsorption is the bonding of an organic chemical to the soil mineral surfaces (clay) or to the organic matter surfaces. The bonding is usually temporary and is accomplished by ionic, ligand, dipole, hydrogen, or Van der Waal's bonds. Adsorption is important in the movement of organic chemicals in groundwater because it decreases the mobility and retards the migration of an organic chemical in groundwa-ter. Furthermore, the adsorbed portion of an organic chemical may not be available in solution for other chemical reactions such as hydrolysis and biodegradation.

The degree and extent of adsorption of an organic chemical to soil is determined by the chemical's structure and the soil's physical and chemical characteristics. Organic chemicals with large molecular structures, such as PCBs, PAHs, toluene, and dichlorodiphenyl trichloro-ethane (DDT), tend to be extensively adsorbed onto soil (Landrum et al. 1984). Organic chemicals with positive charges, such as the herbicides paraquat and diquat, are readily adsorbed onto the cation exchange sites (clay min-

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