In nearly all developed and developing countries, there are thousands of contamination sites within soils which are presenting high risks for the environment and human health. Soil contamination is caused by the presence of toxic and hazardous chemicals and organic compounds on the surface of the soil. The contaminants are physically and/or chemically attached to soil particles, or are trapped within the small spaces between soil particles. The most common chemicals are hydrocarbons and aromatic hydrocarbons from the production and use of oil products as well as from organic solvents developed through chemical production processes. Mercury and other heavy metals used in the chemical and mining industries are contaminating large areas. The improper storage of pesticides has the potential to contaminate sites for decades. Soil contamination can also occur through accidents during road and rail transportation.
The main high-risk contaminants belong to the so called ''Persistent Organic Chemicals'' ( [Öffnet internen Link im aktuellen Fenster] POPs), mentioned in the Stockholm Convention of United Nations Environment Programme (UNEP). These chemicals (e.g. DDT, Lindane, PCBs, Dioxins and Furans) are persistent and tend to accumulate in soil and bio-accumulate in organisms.
Obsolete pesticides are often stored improperly. Liquid-based pesticides can leak out of corroded drums into the soil and groundwater which may end up polluting other local water bodies such as lakes and rivers. The wind can transport pesticide-contaminated soil over wide areas. Pesticide contaminated sites are normally found at agrochemical storage facilities and mixing or loading areas. Soils are often contaminated as a result of accidental spillage with mixtures of pesticides at concentrations well above field-application rates.
Once pesticides enter the soil, they spread at rates that depend on the soil type and nature of the contaminants. A relatively small amount of spilled pesticides can therefore create a much larger volume of contaminated soil. For example, during the Tuareg rebellion in Northern Mali in 1996, they destroyed a depot that was storing approximately 50 tonnes of pesticides which have contaminated several thousand tonnes of soil around the storage and the only freshwater well in the area.
Obviously, contaminated sites can pose serious health and environmental threats among nearby communities. This can directly lead to health risks through physical contact with highly contaminated soil or vapours from volatile organic contaminates. Contaminants may accumulate and bio-magnify within food chains via crops and animals. Pesticides, fertilizers, and toxic substances (such as used motor oil) may also leach from the surface into groundwater over time and spread through water systems into areas which are far away from the contaminated area. Contaminated drinking water could cause serious health effects such as cancer and acute poisoning.
Cleaning up contaminated sites and water is a costly, technically complex and difficult task. Environmental remediation deals with the removal of contaminants from the environmental medium such as soil and ground water. Mapping of contaminated soil sites and the resulting clean-up activities are time consuming and are extremely expensive. Risk assessments are essential in order to understand the dimension of risks. Geology, hydrology and chemistry skills are necessary to develop adequate technical and economical proposals for solving specific contamination problems.
Every site is different. The first priority should be determining and eliminating the source of contamination and its impact on the environment. This requires a broad understanding of the contaminant’s chemical properties. Quite often, the contaminants are unknown and samples must be analysed. Depending on the results of the chemical analysis and risk assessment, there are three basic ways of dealing with contaminated soil:
- Removal of the source of contamination;
- Containing the contaminated sites by covering the contaminated soil with concrete or impermeable layers,
- Preventing human contact with the contamination by covering the contaminated sites with clean soil, fencing-off contaminated areas, and closing contaminated wells.
- Removing contamination is much more expensive than containing or preventing it.