Soil contamination


Soil contamination, soil pollution, or land pollution as a part of land degradation is caused by the presence of xenobiotic chemicals or other alteration in the natural soil environment. It is typically caused by industrial activity, agricultural chemicals or improper disposal of waste. The most common chemicals involved are petroleum hydrocarbons, polynuclear aromatic hydrocarbons, solvents, pesticides, and heavy metals. The concern over soil contamination stems primarily from health risks, from direct contact with contaminated soil or consumption of plants growing in contaminated soil, vapour inhalation from the contaminants, or from secondary contamination of water supplies within and underlying the soil. Mapping of contaminated soil sites and the resulting cleanups are time-consuming and expensive tasks, and require expertise in geology, hydrology, chemistry, computer modelling, and GIS in Environmental Contamination, as well as an appreciation of the history of site pollution. It has been suggested that the examination of humus forms, which necessitates only a cursory glance upon ground floor thickness and structure of the underlying mineral horizon, could used at low cost for the early detection and mapping of potential soil contamination.
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 :
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 may indirectly worsen the contamination effects in that soil remediation becomes more tedious.
File:Agbogbloshie, Ghana 2019.jpg|left|thumb|263x263px|E-waste processing in Agbogbloshie, Ghana. Improper disposal of manufactured goods and industrial wastes, often means that communities in the global south have to process goods. Especially without proper protections, heavy metals and other contaminates can seep into the soil, and create water pollution and air pollution.
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. 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 anthracene, benzofluoranthene, benzofluoranthene, benzopyrene, indeno. 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 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 but not that of heavy metals.

Pesticides and herbicides

A pesticide is a substance used to kill a pest. A pesticide may be a chemical substance, biological agent, antimicrobial, disinfectant or device used against any pest. Pests include insects, plant pathogens, weeds, mollusks, birds, mammals, fish, nematodes 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. They are similar to auxins and most are biodegradable by soil bacteria. However, one group derived from trinitrotoluene have the impurity dioxin, which is very toxic and causes fatality even in low concentrations. Another herbicide is Paraquat. It is highly toxic but it rapidly degrades in soil due to the action of bacteria and does not kill soil fauna.
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.
There are now two main groups of synthetic insecticides:
1. Organochlorines include DDT, Aldrin, Dieldrin and BHC. They are cheap to produce, potent and persistent. DDT was used on a massive scale from the 1930s, with a peak of 72,000 tonnes used 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. 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 the numbers of birds of prey like ospreys and peregrine falcons in the 1950s – they are 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. As it has low water solubility, it tends to stay at the water surface, so organisms that live there are most affected. DDT found in fish that formed part of the human food chain caused concern, but the levels found in the liver, kidney and brain tissues was less than 1 ppm and in fat was 10 ppm, which was below the level likely to cause harm. However, DDT was banned in the UK and the United States to stop the further buildup of it in the food chain. 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 is less so and Malathion is generally considered safe as it has low toxicity and is rapidly broken down in the mammalian liver. This group works by preventing normal nerve transmission as cholinesterase is prevented from breaking down the transmitter substance acetylcholine, resulting in uncontrolled muscle movements.

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. 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.

Human health

Exposure pathways

Contaminated or polluted soil directly affects human health through direct contact with soil or via inhalation of soil contaminants that have vaporized; potentially greater threats are posed by the infiltration of soil contamination into groundwater aquifers used for human consumption, sometimes in areas apparently far removed from any apparent source of above-ground contamination. Toxic metals can also make their way up the food chain through plants that reside in soils containing high concentrations of heavy metals. This tends to result in the development of pollution-related diseases.
Most exposure is accidental, and exposure can happen through:
  • Ingesting dust or soil directly
  • Ingesting food or vegetables grown in contaminated soil or with foods in contact with contaminants
  • Skin contact with dust or soil
  • Vapors from the soil
  • Inhaling clouds of dust while working in soils or windy environments
However, some studies estimate that 90% of exposure is through eating contaminated food.