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Soil contamination

Pollution of land by human-made chemicals or other alteration

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  • 5 chapters

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In North America and Europe the extent of contaminated land is best known for as many of the countries in these areas have a legal framework to identify and deal with this environmental problem. Other countries tend to be less tightly regulated despite some of them have undergone significant industrialization and are searching for more regulation.

Causes

Soil pollution can be caused by the following (non-exhaustive list):

The most common chemicals involved are petroleum hydrocarbons, solvents, pesticides, lead, and other heavy metals.

Any activity that leads to other forms of soil degradation (erosion, compaction, etc.) may indirectly worsen the contamination effects in that soil remediation becomes more tedious.

Historical deposition of coal ash used for residential, commercial, and industrial heating, as well as for industrial processes such as ore smelting, were a common source of contamination in areas that were industrialized before about 1960. Coal naturally concentrates arsenic, cadmium lead and zinc during its formation, as well as other heavy metals to a lesser degree. When the coal is burned, most of these metals become concentrated in the ash (the principal exception being mercury, which evaporates). Coal ash and slag may contain sufficient lead to qualify as a "characteristic hazardous waste", defined in the US as containing more than 5 mg/L, further revised to 1.5 mg/L of extractable lead using the TCLP procedure. In addition to lead, coal ash typically contains variable but significant concentrations of polynuclear aromatic hydrocarbons (PAHs; e.g., benzo(a)anthracene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, indeno(1,2,3-cd)pyrene, phenanthrene, anthracene, and others). These PAHs are known human carcinogens and the acceptable concentrations of them in soil are typically from 0.1 mg/kg to 10 mg/kg, with a strong variation from a PAH to another. Coal ash and slag can be recognised by the presence of off-white grains in soil, gray heterogeneous soil, or (coal slag) bubbly, vesicular pebble-sized grains.

Treated sewage sludge, known in the industry as biosolids, has become controversial as a "fertilizer". As it is the byproduct of sewage treatment, it generally contains more contaminants such as organisms, pesticides, and heavy metals than other soil. In the European Union, the Urban Waste Water Treatment Directive allows sewage sludge to be sprayed onto land, although several European countries have introduced more stringent requirements in comparison with the directive. 10 million tons dry matter of sewage sludge have been produced in Europe every year over the period 2003–2006. This has good agricultural properties due to the high nitrogen, phosphate and potassium content. However, there is a need to control sewage sludge application to agricultural land so that pathogenic microorganisms do not get into water courses and to ensure that there is no accumulation of heavy metals in the topsoil. Composting of sewage sludge allows to decrease its content in pathogens and organic pollutants (bioremediation, to the exception of persistent organic pollutants) but not that of heavy metals, although these are in a less bioavailable form.

Pesticides and herbicides

A pesticide is a substance used to kill a pest. A pesticide may be a chemical substance, biological agent (such as a virus or bacteria), antimicrobial, disinfectant or device used against any pest. Pests include harmful insects, plant pathogens, weeds, mollusks, birds, mammals, fish, nematodes (roundworms) and microbes that compete with humans for food, destroy property, spread or are a vector for disease or cause a nuisance. Although there are benefits to the use of pesticides, there are also drawbacks, such as potential toxicity to humans and other organisms.

Herbicides are used to kill weeds, especially on pavements and railways, but also in agricultural crops either for destructing the total vegetation (e.g. glyphosate) or only a class of undesired plants (e.g. 2,4-D). The so-called auxin herbicides are similar to auxins and are selective to dicots. Glyphosate is a non-selective (broad-spectrum) systemic herbicide which competes with enzymes used in the synthesis of key plant amino acids. Most herbicides are biodegradable by soil bacteria. However, one group derived from trinitrotoluene (2,4-D and 2,4,5-T) have the impurity dioxin, which is very toxic and causes fatality even in low concentrations. Another common herbicide is Paraquat, banned in the European Union but still frequently used in agricultural areas of the United States and Asia. It is highly toxic to humans and other animals and cannot rapidly degrade in the soil where it is adsorbed and thus protected in clay lattices. Glyphosate is rapidly transformed in AMPA by soil bacteria but its residues are detected in drinking water, agriculture, and food products and have major effects on the health of reproductive systems. Glyphosate is used in genetically modified crops to kill all vegetation except the target crop, more especially in developing countries where it offers yield and profit gains despite growing concerns about environment and human health.

Insecticides are used to rid farms of pests which damage crops. The insects damage not only standing crops but also stored ones and in the tropics it is reckoned that one third of the total production is lost during food storage. As with fungicides, the first insecticides used in the nineteenth century were inorganic e.g. Paris Green and other compounds of arsenic. Nicotine has also been used since 1690. Neonicotinoids, i.e. synthetic insecticides derived from nicotin are the last generation of insecticides. They have been scheduled to be highly selective to insect pests, although it appeared that acetamiprid, IMI, and thiacloprid were toxic to birds, thiacloprid to fish, and several neonicotinoids were harmful to honeybees, either by direct contact or ingestion.

There are now three main groups of synthetic insecticides:

1. Organochlorines include DDT, Aldrin, Dieldrin and benzene hexachloride (BHC). They are cheap to produce, potent and persistent but have harmful effects on a lot of beneficial organisms, from microbes to a wide range of plants and animals, humans included, hence their banishment in many (but not all) countries, inasmuch as resistance occurred in a lot of target insect pests. DDT was used on a massive scale from the 1930s, with a peak of 72,000 tonnes used in 1970. Then usage fell as the harmful environmental effects were realized. It was found worldwide in fish and birds and was even discovered in the snow in the Antarctic. It is only slightly soluble in water but is very soluble in the bloodstream, and in fats. It affects the nervous and endocrine systems and causes the eggshells of birds to lack calcium causing them to be easily breakable. It is thought to be responsible for the decline of numbers of birds of prey like ospreys and peregrine falcons in the 1950s, now recovering. As well as increased concentration via the food chain, it is known to enter via permeable membranes, so fish get it through their gills and then it accumulates in fatty organs. As it has low water solubility and a high affinity to the air-water interface, DDT tends to stay at the water surface, so organisms that live there are most affected, in particular mosquito larvae, the target organisms of malaria control. DDT and its breakdown product DDE found in fish that formed part of the human food chain caused concern, with levels found in human liver, kidney and brain tissues around 13 ppm in 1970, with a general decrease since DDT was banished from developing countries but with still high levels in Asia and Africa where DDT is used against malaria. DDT was banned by the Stockholm convention in 2001 to stop its further buildup in the food chain. However, the World Health Organization allowed its reintroduction only for control of vector-borne diseases in some tropical countries in 2006. U.S. manufacturers continued to sell DDT to developing countries, who could not afford the expensive replacement chemicals and who did not have such stringent regulations governing the use of pesticides.

2. Organophosphates, e.g. parathion, methyl parathion and about 40 other insecticides are available nationally. Parathion is highly toxic, methyl-parathion less so but health concerns have resulted in cancellation of the use of methyl-parathion in most food crops in the United States. There is no evidence that malathion affects the ability of humans to reproduce. There is also no conclusive proof that malathion causes cancer in humans, although some studies have found increased incidence of some cancers in people who are regularly exposed to pesticides, such as farmers and pesticide applicators. This group of insecticides works by preventing normal nerve transmission as acetylcholinesterase is prevented from breaking down the transmitter substance acetylcholine, resulting in uncontrolled muscle movements.

3. Neonicotinoids, e.g. acetamiprid, imidacloprid, are the last generation of insecticides and are now largely used for crop protection. They affect the central nervous system of insects, with higher selectivity for insects than organophosphates and organochlorines. However, their biocidal action includes both pests and beneficial organisms, e.g. pollinators, predatory insects, parasitoids. The ay also act as endocrine disruptors on juvenile bees. The dramatic decline of honey bee colonies, for example, could be linked to, or at least exacerbated by the use of neonicotinoids. Like nicotine, their molecular base, they are degraded in the soil but the environmental residues of neonicotinoids have enormously increased due to large-scale applications.

Agents of war

The disposal of munitions, and a lack of care in manufacture of munitions caused by the urgency of production, can contaminate soil for extended periods. There is little published evidence on this type of contamination largely because of restrictions placed by governments of many countries on the publication of material related to war effort, in particular under ongoing conflict scenarios. However, mustard gas stored during World War II has contaminated some sites for up to 50 years and the testing of Anthrax as a potential biological weapon contaminated the whole island of Gruinard, leaving it in quarantine for 48 years. There are abandoned delaboration sites around the former World War I front in Belgium and also in France which are still contaminated by arsenic and lead.

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