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.
Retention is most effective in fine-grained tills that have some permeability but in which the flow of water is slow and dissolved substances have time to react with soil components.