Soil contamination occurs when harmful substances, mainly resulting from human activity, find their way into the earth and alter the balance of the soil.
The composition of soils can be affected by the impact of human activities. Harmful substances can be released directly when toxic product spills occur or indirectly through atmospheric pollution (smog, industrial dust, volcanism, acid rain, or radioactive rain). These substances, called contaminants, are likely to infiltrate deep into the soil, to be carried by rainwater towards streams and groundwater, as well as to be transported through the air by the wind. Soil contamination has a simultaneous impact on the hydrosphere and the atmosphere.
Depending on their nature, some contaminants in soils can be degraded by microorganisms. Another part is absorbed by plants, then spread through food chains, contaminating all living organisms, including humans. The effects of contaminants on soils depend not only on their chemical composition, but also on the nature of the soils.
Contaminants can be separated into three broad categories.
-
Organic contaminants: these are microorganisms (bacteria, viruses, parasites), dead organic matter (manure), hydrocarbons (petroleum and derivatives), and persistent organic pollutants (POPs) such as solvents, pesticides, insecticides, and fertilizers. In small quantities, these elements are easily incorporated into the environment. However, large-scale use of these contaminants, such as in mass agriculture, leads to saturation of the soil and the remaining contaminants are leached by rainwater and carried into the waterways. Residues of these contaminants are generally toxic to soil flora and fauna. In addition, the eutrophication of natural water bodies is accelerated by the presence of these organic contaminants.
-
Inorganic contaminants: these are mainly heavy metals (lead, mercury, zinc, cadmium, nickel, arsenic). They come from landfills, domestic and industrial waste, mineral and oil extraction sites, and air pollution. When they are found in large quantities in the environment, they become harmful and can alter soil fertility. They can also contaminate streams and groundwater reserves.
-
Radioactive contaminants: nuclear waste that comes from nuclear power plants or nuclear accidents (Chernobyl, Fukushima). A large part of these elements is rapidly absorbed by plants. If living organisms ingest these plants, the contaminants are likely to spread through the food chain, leading to deformities, genetic mutations, and cancer. The remaining contaminants spread into the soil horizons and can remain there for a very long period of time.
