The migration of harmful substances in soils takes place mainly through advection, dispersion, diffusion and evaporation and can be enhanced by complexation and colloidal sorption. These mechanisms are mainly gov-erned by the water content and structure of soil, by temperature and by the amount of complexing and colloidal compounds in soil. Migration is most effective when the water content and permeability of soil are high. The most effective sorptive compounds are organic matter, the oxyhydroxides of iron, aluminium and manganese, and clay minerals. Sorption is usually most effective in soils with high concentrations of these compounds.
Sorption also increases with an increase in the specific surface area.
The retention capacity of soil is limited and can be exceeded by excessive loading. The retention capacity may also be lowered due to a change in chemical conditions. The most critical areas for the migration of harmful substances in Finland are sand and gravel formations poor in organic matter and with high permeability. These also constitute the main exploited groundwater aquifers in the country. Soil horizons are crucial for pollutant retention.