Did you notice the sand the last time you went to the beach? And the ground on your last visit to the football field? The soil in both these places is distinctively different. Have you ever wondered why soil is a different texture and colour at different places? This is due to the different minerals in soil. Let us learn more about this.
Soil is the thin layer of loose material covering the earth’s surface. It is composed of both organic and inorganic materials. The formation of soil is an extremely long process taking up to an average of a thousand years. Soil is formed when surface rocks break into several smaller particles, called regolith, which then gets mixed with varied organic matter. This whole process is called Weathering. So weathering happens through three major ways, namely
Physical Weathering: This is the geological process when rocks get fragmented into smaller particles, without changing the chemical composition of the rocks. This primarily happens due to fluctuating temperatures causing the rocks to break apart. Chemical Weathering: This is the erosion of rocks and other surface materials caused due to chemical reactions. The rocks react with substances in the atmosphere, such as moisture, air, water etc. The resulting substance has a different chemical composition than the rock from which it formed. Hydration, Hydrolysis, Oxidation, Carbonation and Reduction are the chemical processes involved in chemical weathering. Biological Weathering: This is the process of disintegration of rocks due to actions of living organisms (animals, plants, microbes etc), like when a plant grows in the fissure of a rock and its roots exert pressure on the rock forcing in to break apart. Even microbes produce organic material that causes weathering.
Soil is essential a mix of various particles and substances. So let us have a look at all the things that make up soil,
Minerals: A very important substance found in soil. Minerals basically formed by the break down of large rocks. Some of the most common minerals found in soil are, Iron, Potassium, Magnesium, Calcium, Sulphur etc. Humus: Humus is the organic substances that are formed due to decomposition of dead and decomposing plants and animals. It lends the soil its fertility. Living Organisms: These are mostly microbes and other organisms (such as worms, bacteria, fungi etc.) that live in the soil and perform the decomposition of animals and plants that gives the soil humus. Water and Air: Water and air form a part of the soil and allow living organisms to perform their functions. They also help in the process of photosynthesis.
Soil is a mixture of various components. Now let us explore the importance of the minerals present in the soil. Minerals are the largest component in the soil, making up almost 40% to 45% of the total components. The minerals in the soil are classified into two categories, namely
Primary Minerals in soil: These are minerals which have not been chemically altered since deposition. They are same or similar to their parent materials. Often these materials are bigger in size and irregular in shape. These primary minerals are usually found in sand and silt. Secondary Minerals in soil: These are minerals formed as a result of weathering of Primary Minerals. Secondary minerals are mainly found in fine silt or clay. The particle size of these minerals is much smaller, due to the weathering process. These minerals have a large surface area that helps them retain moisture.
Soil pollution refers to the contamination of soil with anomalous concentrations of toxic substances. It is a serious environmental concern since it harbours many health hazards. For example, exposure to soil containing high concentrations of benzene increases the risk of contracting leukaemia. An image detailing the discolouration of soil due to soil pollution is provided below.
It is important to understand that all soils contain compounds that are harmful/toxic to human beings and other living organisms. However, the concentration of such substances in unpolluted soil is low enough that they do not pose any threat to the surrounding ecosystem. When the concentration of one or more such toxic substances is high enough to cause damage to living organisms, the soil is said to be contaminated.
The root cause of soil pollution is often one of the following:
Agriculture (excessive/improper use of pesticides)
Excessive industrial activity
Poor management or inefficient disposal of waste
The challenges faced in soil remediation (decontamination of soil) are closely related to the extent of soil pollution. The greater the contamination, the greater the requirement of resources for remediation.
What are the Pollutants that Contaminate Soil?
Some of the most hazardous soil pollutants are xenobiotics – substances that are not naturally found in nature and are synthesized by human beings. The term ‘xenobiotic’ has Greek roots – ‘Xenos’ (foreigner), and ‘Bios’ (life). Several xenobiotics are known to be carcinogens. An illustration detailing major soil pollutants is provided below.
The different types of pollutants that are found in contaminated soil are listed in this subsection.
Heavy Metals
The presence of heavy metals (such as lead and mercury, in abnormally high concentrations) in soils can cause it to become highly toxic to human beings. Some metals that can be classified as soil pollutants are tabulated below.
Toxic Metals that Cause Soil Pollution
Arsenic
Mercury
Lead
Antimony
Zinc
Nickel
Cadmium
Selenium
Beryllium
Thallium
Chromium
Copper
These metals can originate from several sources such as mining activities, agricultural activities, electronic waste (e-waste), and medical waste.
Polycyclic Aromatic Hydrocarbons
Polycyclic aromatic hydrocarbons (often abbreviated to PAHs) are organic compounds that:
Contain only carbon and hydrogen atoms.
Contain more than one aromatic ring in their chemical structures.
Common examples of PAHs include naphthalene, anthracene, and phenalene. Exposure to polycyclic aromatic hydrocarbons has been linked to several forms of cancer. These organic compounds can also cause cardiovascular diseases in humans.
Soil pollution due to PAHs can be sourced to coke (coal) processing, vehicle emissions, cigarette smoke, and the extraction of shale oil.
Industrial Waste
The discharge of industrial waste into soils can result in soil pollution. Some common soil pollutants that can be sourced to industrial waste are listed below.
Chlorinated industrial solvents
Dioxins produced from the manufacture of pesticides and the incineration of waste.
Plasticizers/dispersants
Polychlorinated biphenyls (PCBs)
The petroleum industry creates many petroleum hydrocarbon waste products. Some of these wastes, such as benzene and methylbenzene, are known to be carcinogenic in nature.
Pesticides
Pesticides are substances (or mixtures of substances) that are used to kill or inhibit the growth of pests. Common types of pesticides used in agriculture include:
Herbicides – used to kill/control weeds and other unwanted plants.
Insecticides – used to kill insects.
Fungicides – used to kill parasitic fungi or inhibit their growth.
However, the unintentional diffusion of pesticides into the environment (commonly known as ‘pesticide drift’) poses a variety of environmental concerns such as water pollution and soil pollution. Some important soil contaminants found in pesticides are listed below.
These chemicals pose several health risks to humans. Examples of health hazards related to pesticides include diseases of the central nervous system, immune system diseases, cancer, and birth defects.
What are the Processes that Cause Soil Pollution?
Soil pollution can be broadly classified into two categories –
Naturally caused soil pollution
Anthropogenic soil pollution (caused by human activity)
Natural Pollution of Soil
In some extremely rare processes, some pollutants are naturally accumulated in soils. This can occur due to the differential deposition of soil by the atmosphere. Another manner in which this type of soil pollution can occur is via the transportation of soil pollutants with precipitation water.
An example of natural soil pollution is the accumulation of compounds containing the perchlorate anion (ClO4–) in some dry, arid ecosystems. It is important to note that some contaminants can be naturally produced in the soil under the effect of certain environmental conditions. For example, perchlorates can be formed in soils containing chlorine and certain metals during a thunderstorm.
Anthropogenic Soil Pollution
Almost all cases of soil pollution are anthropogenic in nature. A variety of human activities can lead to the contamination of soil. Some such processes are listed below.
The demolition of old buildings can involve the contamination of nearby soil with asbestos.
Usage of lead-based paint during construction activities can also pollute the soil with hazardous concentrations of lead.
Spillage of petrol and diesel during transportation can contaminate soils with the hydrocarbons found in petroleum.
Activities associated with metal casting factories (foundries) often cause the dispersion of metallic contaminants into the nearby soils.
Underground mining activities can cause the contamination of land with heavy metals.
Improper disposal of highly toxic industrial/chemical waste can severely pollute the soil. For example, the storage of toxic wastes in landfills can result in the seepage of the waste into the soil. This waste can go on to pollute groundwater as well.
Chemical pesticides contain several hazardous substances. Excessive and inefficient use of chemical pesticides can result in severe soil pollution.
Sewage produced in urbanized areas can also contaminate soil (if not disposed of correctly). These wastes may also contain several carcinogenic substances.
Other forms of waste that can pollute soil include nuclear waste, e-waste, and coal ash.
What are the Negative Consequences of Soil Pollution?
Soil pollution harbours a broad spectrum of negative consequences that affect plants, animals, humans, and the ecosystem as a whole. Since children are more susceptible to diseases, polluted soil poses a greater threat to them. Some important effects of soil pollution are detailed in this subsection.
Effects on Human Beings
Soil contaminants can exist in all three phases (solid, liquid, and gaseous). Therefore, these contaminants can find their way into the human body via several channels such as direct contact with the skin or through the inhalation of contaminated soil dust.
The short term effects of human exposure to polluted soil include:
Headaches, nausea, and vomiting.
Coughing, pain in the chest, and wheezing.
Irritation of the skin and the eyes.
Fatigue and weakness.
A variety of long-term ailments have been linked to soil pollution. Some such diseases are listed below.
Exposure to high levels of lead can result in permanent damage to the nervous system. Children are particularly vulnerable to lead.
Depression of the CNS (Central Nervous System).
Damage to vital organs such as the kidney and the liver.
Higher risk of developing cancer.
It can be noted that many soil pollutants such as petroleum hydrocarbons and industrial solvents have been linked to congenital disorders in humans. Thus, soil pollution can have several negative effects on human health.
Effects on Plants and Animals
Since soil pollution is often accompanied by a decrease in the availability of nutrients, plant life ceases to thrive in such soils. Soils contaminated with inorganic aluminium can prove toxic to plants. Also, this type of pollution often increases the salinity of the soil, making it inhospitable for the growth of plant life.
Plants that are grown in polluted soil may accumulate high concentrations of soil pollutants through a process known as bioaccumulation. When these plants are consumed by herbivores, all the accumulated pollutants are passed up the food chain. This can result in the loss/extinction of many desirable animal species. Also, these pollutants can eventually make their way to the top of the food chain and manifest as diseases in human beings.
Effects on the Ecosystem
Since the volatile contaminants in the soil can be carried away into the atmosphere by winds or can seep into underground water reserves, soil pollution can be a direct contributor to air and water pollution.
It can also contribute towards acid rain (by releasing huge quantities of ammonia into the atmosphere).
Acidic soils are inhospitable to several microorganisms that improve soil texture and help in the decomposition of organic matter. Thus, the negative effects of soil pollution also impact soil quality and texture.
Crop yield is greatly affected by this form of pollution. In China, over 12 million tons of grain (worth approximately 2.6 billion USD) is found to be unfit for human consumption due to contamination with heavy metals (as per studies conducted by the China Dialogue).
How can Soil Pollution be Controlled?
Several technologies have been developed to tackle soil remediation. Some important strategies followed for the decontamination of polluted soil are listed below.
Excavation and subsequent transportation of polluted soils to remote, uninhabited locations.
Extraction of pollutants via thermal remediation – the temperature is raised in order to force the contaminants into the vapour phase, after which they can be collected through vapour extraction.
Bioremediation or phytoremediation involves the use of microorganisms and plants for the decontamination of soil.
Mycoremediation involves the use of fungi for the accumulation of heavy metal contaminants.
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