ABSTRACT
This chapter presents basic information on the migration of contaminants and attenuation processes of soils. Attenuation is a physical, chemical, and/or biological reaction or transformation that causes a temporary or permanent decrease in the concentrations of many contaminants of waste in a fixed time and distance travelled. The attenuation of leachate from a landfill occurs in two stages: flow through the unsaturated zone, and flow through the groundwater aquifer. Natural attenuation mechanisms may be categorized as physical, chemical and biological. The major attenuation mechanisms falling into these three categories are: physical: filtration, diffusion and dispersion, dilution, and sorption; chemical: precipitation/dissolution, adsorption/desorption, complexation, ion exchange, and redox reaction; and microbiological: aerobic, and anaerobic. Physical sorption is a function of van der Waals forces, and hydro-dynamic and electrokinetic properties of soil particles. Precipitation and dissolution are important reactions that control concentration levels and limit the total amount of contaminants in leachate when leaching through soils.
