SOIL POLLUTION: CAUSES, TYPES, CONSEQUENCES - ENVIRONMENT - 2025

The pollution of soil is their chemical or physical degradation affecting their ecological and anthropogenic utility. When soils are polluted, imbalances occur that negatively affect life in the ecosystem.

Before the 70s of the 20th century, no importance was given to soil contamination. However, more and more information has accumulated regarding the environmental impact generated by this type of pollution.

Soil pollution from mining in Australia. Source: CSIRO

In 1972 the European Community released the "European Soil Charter". In this document, soil is classified as a precious resource that is easy to destroy and that must be protected.

The physical-chemical characteristics of the soil determine the mechanisms of interaction with pollutants. Its character as a porous matrix variable in composition, including a gaseous and a liquid phase, allows the retention of contaminants.

Among the causes of soil contamination is the inadequate management of solid, liquid and gaseous, urban and industrial waste. The waste thrown on the ground or the effluents discharged into it incorporate a large amount of pollutants and acid rain causes its acidification.

Mining and oil activities cause physical and chemical degradation of the soil. In this sense, one of the biggest problems is the contamination of the soil by heavy metals.

On the other hand, agricultural activities also deteriorate soils through the excessive use of agrochemicals and agricultural machinery. Fertilizers and pesticides affect populations of microorganisms in the soil as well as plants.

The most common soil pollutants are heavy metals, agrochemicals, salts, solid waste, organic waste, and biological pollutants. These pollutants cause chemical, physical, biological, thermal and visual contamination of soils.

Soil contamination causes public health problems when contaminated plants and water are consumed. In the same way, the landscape is degraded and enormous economic losses are caused.

We find examples of soil contamination in the oil industry, such as in eastern Ecuador where containment ponds were built. These lagoons were not well sealed and various toxic substances contaminated the soil through infiltration.

A very widespread situation of soil contamination are the so-called sanitary landfills. In Bucaramanga (Colombia) solid waste was deposited with poor management for more than 40 years, becoming a source of diseases.

Another example is the case of Caño Mánamo in the Orinoco river delta (Venezuela) due to a poorly conceived infrastructure and development project. Here an embankment-road was built that served as a dike and altered the water regime of the area, causing the acidification of the soils.

Solutions to the problem of soil contamination merit comprehensive management that encompasses prevention and restoration.

Prevention requires the implementation of a sustainable approach to development. It should focus on agriculture as well as the management of urban and industrial waste and emissions.

The restoration of contaminated soils includes a series of technologies aimed at removing, neutralizing, containing or immobilizing the contaminants. Chemical, biological and physical agents are used for this.

characteristics

- Soil

The soil is the product of the decomposition of the bedrock by physical, chemical and biological factors. It constitutes the surface layer of the earth's crust that has been subjected to geological processes of decomposition of the mother rock.

Erosion and sedimentation, as well as the climate (rain, wind, humidity and temperature), contribute to the formation of the soil structure. On the other hand, living beings also actively participate in the soil formation process (pedogenesis).

The action of bacteria, fungi, worms and other living beings degrades organic matter and soil particles.

An ecosystem

Soil is an ecosystem that includes a physical (soil particles), gas (air) and liquid (water) phase where abiotic (non-living) and biotic (living) elements interact.

Among the abiotic elements are various minerals, water and gases such as CO2 and O2. Within biotics there are a diversity of microorganisms (bacteria, fungi, protozoa, nematodes), worms as well as insects, reptiles and mammals.

Physicochemical characteristics

There are different types of soil, defined by their origin and physical-chemical characteristics.

The original bedrock defines the basic characteristics of the soil while the action of environmental and biological factors contribute to its evolution.

The elemental properties of a soil are given by its structure, texture, moisture content and chemical composition (mainly organic matter).

The structure is associated with the size and arrangement of the soil aggregates and their vertical distribution. In this way, layers or horizons are formed in the soil with particular characteristics and a proportion of sand, silt and clay.

The water content is essential for the chemical and biological processes that occur in the soil. Solar radiation heats the ground and that energy catalyzes various reactions, even making underground life possible.

- Mechanisms of contamination and soil-contaminant interaction

The components and properties of the soil indicated determine the soil-pollutant interaction and the impact that may exist.

Based on this, the pollution mechanisms that act on the soil are varied. These include physical processes such as the retention of pollutants in the soil structure or their infiltration, diffusion and transport.

Additionally, alterations, transformations and, in general, chemical changes due to the action of pollutants also occur in the soil. In this case, the most relevant are chemical and biological processes (biotransformation and biodegradation).

- Indicators of soil quality

Biological indicators

A very important indicator is the respiration rate that allows estimating the biological activity in the soil. It is measured from the evolution of carbon dioxide resulting from the decomposition of organic matter.

In turn, biological activity depends on factors such as humidity, temperature, oxygen content and organic matter in the soil. Other biological indicators are carbon nitrogen mineralization, nitrogen fixation, total biomass, and measurement of certain enzymes.

Moisture content

There is an optimal moisture content for the development of biological processes in the soil. This is located around 60% of the pore space since higher humidity would affect the availability of oxygen.

Soil fertility

Fertility is given both by the content and availability of essential mineral elements for plant nutrition. These include macronutrients (nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur) and micronutrients (iron, zinc, manganese, boron, copper, molybdenum, and chlorine).

Acidity

It is determined by the amount of hydrogen ions in the soil solution. The higher the amount of hydrogen ions, the more acidic a soil is and this affects the availability of certain nutrients.

Most nutrients are available at a pH of 5.8 to 6.5 (slightly acidic).

Salinity

It refers to the amount of water-soluble salts present in the soil, the dominant salt being sodium chloride. A high content of salts affects the fertility of the soils.

Causes of soil contamination

- Mining and oil activity

Soil contamination with arsenic in Denmark. Source: Bochr

Mining

Mining activity is one of the most negative for soils, since it destroys the topsoil and its structure. In addition, it adds highly toxic pollutants to the soil such as various heavy metals.

For example, gold mining contaminates soils and bodies of water with mercury and arsenic.

Petroleum

The residual sludge derived from the extraction of hydrocarbons is a serious problem of soil contamination. Its composition includes heavy metals (cadmium, mercury), hydrocarbons and other compounds.

Some modern techniques such as hydraulic fracturing (fracking) further increase the degree of contamination.

Fracking consists of breaking the rock into lower strata, releasing the trapped gas and oil. In this process, more than 600 chemical substances are added that end up contaminating the soil and water.

- Agriculture and breeding

Agrochemicals

In agriculture, especially intensive monoculture, a large amount of pesticides and fertilizers are used. Pesticides include herbicides, insecticides, fungicides, bactericides, among others.

Insecticides and herbicides in many cases pollute soils with residual active components. Fertilizers add nitrites, nitrates, and phosphate fertilizers are a source of cadmium.

Excess chemical fertilizers can cause acidity in soils and imbalances in microorganism populations.

Irrigation water

Irrigation water is the main cause of salinization of soils due to its salt content. Additionally, through irrigation water the soil can be contaminated with microorganisms or heavy metals.

Fuel spills

Agriculture pollutes the soil through the use of machinery, which represents a threat of fuel and oil spills.

- Industrial waste

Direct

Depending on the nature of the industry, there are multiple wastes that can end up contaminating the soil. In particular, effluents carry heavy metals, solvents, detergents and other dangerous chemicals to the ground.

For example, cadmium is a very common pollutant generated by the nickel-cadmium battery industry. It is also used as a stabilizer in the PVC plastics industry or in the metallurgical and electronics industry.

Indirect

Emissions of precursor gases such as nitrogen oxides, sulfur oxides, and carbon dioxide cause acid rain. These acids when reaching the soil modify its pH and produce its acidification.

Burning coal in thermoelectric plants produces CO2 (the main greenhouse gas) and other pollutants. For example, the burning of coal is an important source of mercury that by deposition contaminates the soil.

It has been estimated that 74% of heavy metal pollution comes from combustion ash.

- Urban waste

Solid waste and urban effluents are a source of all kinds of pollutants that reach the ground due to their poor management. Sanitary landfills incorporate huge amounts of plastic waste, batteries, organic waste, metals, electronic devices, among others.

Landscape degradation due to soil contamination. Source: Kenneth Allen / Rubbish site, Drumaduff

Automotive traffic

The burning of fossil fuels is a source of soil contamination by deposition, being very serious when it includes leaded gasoline.

- Inadequate engineering practices

Some engineering works alter ecosystems causing soil degradation. For example an embankment, road or railroad can cut off the flow of water to an area or increase it.

If the water runoff to the ground is cut off, it can dry out and erode or increase the concentration of salts. If the water outflow is prevented, the soil will flood and undergo anoxic and oxidation processes.

Main pollutants

- Heavy metals

A soil has a certain natural concentration of heavy metals, depending on the parent material from which it was derived (parent rock). The problem of contamination arises when the human being adds additional amounts that increase said concentration.

Most common heavy metals

The most abundant polluting heavy metals are lead, cadmium, nickel, copper, tin, mercury, arsenic, chromium and zinc. Aluminum is also included in this category despite being a lighter metal.

The concentration of these metals in some soils is double that normally found in the earth's crust. For example, in the case of cadmium it can be six times higher.

- Radioactive elements

Radioactive elements such as uranium are highly dangerous pollutants due to their serious effects on life. These are added to the ground by leaks from radioactive waste deposits or by accidents at nuclear plants.

Radioactive atoms can also be removed from lower strata by drilling activities. For example, there are still large tracts of Ukrainian and Belarusian soils contaminated today due to the 1986 Chernobyl accident.

Furthermore, when hydraulic fracturing occurs, radioactive materials such as radium, radon, uranium and thorium can be entrained.

- You go out

They are water-soluble salts that form highly concentrated solutions such as sodium, calcium, magnesium, potassium, chloride, sulfate, carbonate, and bicarbonate ions.

The combinations with the greatest salinity problems in soils are sodium chloride, magnesium sulfate and sodium sulfate.

- Agrochemicals

Fertilizers

Inorganic fertilizers when used in excess become pollutants by creating nutritional imbalances, salinity and acidity of soils. According to the FAO, more than 200 million tons of fertilizers are consumed annually in the world.

Pesticides

The indiscriminate use of pesticides causes serious contamination problems as the herbicide atrazine used to control weeds in corn is residual. These herbicides cloud biogeochemical cycles by altering microbial communities and carbon and nitrogen dynamics.

In Brazil alone, around 1,000 tons of pesticides are used per year and in Argentina more than 300 million tons of glyphosate herbicide are used per year.

Glyphosate causes serious health problems, especially affecting the nervous system.

- Slurry

This includes all types of organic waste from agricultural and livestock activities. For soil contamination purposes, animal excreta and dead animals are especially relevant.

In these cases, waste can be a vehicle for pathogenic organisms that contaminate food products via soil contamination.

- Solid waste

The main solid wastes that pollute soils are plastics, which in turn release toxic substances such as dioxins. Additionally, construction debris, discarded electronics, batteries, and other objects pollute the floors.

Lead in urban gardens

400 mg / kg of lead have been detected in the soil of the Miraflores orchard (Seville, Spain), the maximum allowed being 275 mg / kg.

Lead absorbed by the cultures was concentrated at 0.51 mg / kg (0.10 mg / kg the maximum allowed). It was established that the source of contamination was paint remains in construction debris previously deposited in the area.

- Biological

Soil contamination caused by living organisms can be expressed in various ways.

An exaggerated population increase of the organisms already existing in the soil can degrade their quality. This is because microorganisms consume oxygen and organic matter in the soil.

Another form of contamination is the incorporation into the soil of plant pathogenic organisms such as nematodes or fungi as well as human pathogens.

Types of soil contamination

- Chemical contamination

It is the main form of soil contamination, consisting of the incorporation of chemical substances that degrade its quality. The substances and ways in which they work are very diverse.

Chemicals affect the structure of the soil, for example oil spills or thinners used in fracking. Likewise, microorganisms are affected with pesticides and fertilizers that can also be toxic to plants and humans.

An example of the chemical processes involved in chemical contamination of soils is acidity. When substances with hydronium ion activity are added to the soil, the pH of the soil drops and toxic metal ions are released.

- Physical contamination

There is physical contamination of the soil when polluting materials are added or its structure is altered. In the first case we have solid waste deposits on the ground such as garbage or debris.

With regard to the alteration of the soil structure, the most evident phenomenon is compaction. This can occur by trampling animals in livestock activity or by the action of machinery.

Soil compaction in the United States. Source: United States Air Force, MSgt. Rickie D. Bickle

In the agricultural case, the excessive use of machinery alters the structure of the soil and affects its physical fertility. This happens when numerous harrow passes are made that end up pulverizing the soil.

When multiple plowing passes are made at constant depth, the so-called plow floor is produced, which consists of a compact layer of soil. In the first case, soil erosion is promoted and in the second, infiltration is reduced.

- Biological contamination

Contaminated water, whether by irrigation, urban effluents or floods, introduces various pathogens to the soil. They can affect plants, animals or humans.

For example, sewage carries fecal coliforms and other pathogens, and a flood can carry sludge contaminated with plant pathogenic nematodes.

- Thermal pollution

The increase in soil temperature affects soil organisms by influencing their humidity and oxygenation. These temperature increases can be caused by high temperature effluents or by the effect of global warming.

- Visual contamination

The accumulations of garbage and rubbish dumps on the ground cause a negative visual impact that has consequences ranging from psychological to economic.

Consequences of soil contamination

- Ecological

The Biodiversity

Soil contamination affects the survival of life in this highly biologically active ecosystem. In the soil, radical systems, bacteria, fungi, protozoa, nematodes, insects and subterranean mammals coexist in complex antagonistic and symbiotic relationships.

Nowadays, the interrelation between tree roots and fungi (mycorrhizae) has become important in mutual benefit that can communicate root systems of different trees. The mycorrhizal fungi in this complex system are highly susceptible to soil contamination.

Biogeochemical cycles and biofilter

The soil intervenes in the biogeochemical cycles of carbon, nitrogen, phosphorus and organic matter. Precisely because of this role in biogeochemical cycles, the soil fulfills a function as a biofilter, by processing and degrading different substances.

Soils have the capacity to filter tens of thousands of cubic kilometers of water each year. Pollution can alter this ability to self-purify and therefore affect the environment.

- Anthropic

Reduction of agricultural and livestock production

The loss of soil fertility or its toxicity as a result of contamination reduces agricultural and livestock production. The main problems are acidification, salinity and desertification of soils.

It is estimated that more than 70% of the world's soils are affected or threatened by these processes.

Drinking water contamination

Contaminated soils affect surface and groundwater sources by entrainment, infiltration or leaching. Heavy metals, pesticide residues, fertilizers and other chemicals end up being washed into water sources.

Agrochemicals cause eutrophication by incorporating excess nutrients into the water and promoting population explosions of algae and aquatic plants. This reduces the dissolved oxygen causing the death of a large number of aquatic species.

Heavy metals and other substances that get into the water are toxic to animals and humans.

Public health

Contaminated soils can affect public health in a number of ways such as garbage piles. These are the focus of vector-borne diseases that develop in the waste.

When there is contamination by heavy metals, these are transferred to the plants and from there to the human being. For example, cocoa soils contaminated by cadmium, transfer this heavy metal by absorption to cocoa and from there to chocolate.

Prolonged exposure to cadmium can cause serious kidney and digestive problems in humans. It can also cause demineralization of the bone system and lead to osteoporosis.

Landscape degradation

A soil degraded by the loss of its natural characteristics in turn degrades the landscape of which it is a part. In this sense, it is a factor of visual pollution, affecting tourist and recreational activities.

Loss of economic value

Soil contamination makes it lose economic value. This may be due to the loss of fertility in agricultural land or degradation of the landscape affecting tourism.

Examples of places with soil contamination

- Oil pollution in Ecuador

The pollution problem

The Petroecuador company has received harsh criticism for contamination of soil and water in the oil exploitation areas in eastern Ecuador. The contamination arose from the use of oil dams around the drilling wells.

These dams housed toxic waste covered only by a layer of earth. The pollutants mixed with the soil by gravity and overflowed the dams polluting the water sources.

Restoration

Containment and bioremediation measures have been implemented. For this, waterproof coatings and drainage networks were applied to recover polluted runoff water.

Then the dams were excavated and the extracted soil was spread in biopiles to which strains of biodegradable bacteria were inoculated. These piles were periodically aerated and watered.

In this way it was possible to successfully treat 140,000 tons of soil in 35 contaminated sites.

- El Carrasco landfill (Bucaramanga, Colombia)

The project

This sanitary landfill began to function in 1978 as an open-air landfill and in 1985 it was transformed into a sanitary landfill. Approximately 900 tons of garbage are deposited at this site per day from 13 municipalities.

Consequences

The mismanagement of the sanitary landfill caused the contamination of the soils both at the site and in nearby areas. The pollutants were displaced by runoff and leaching, affecting soils and water sources.

In addition, the existence of this landfill in the area has caused the degradation of the landscape. In 2019, the definitive closure process began as it exceeded its useful life.

- Caño Mánamo (Delta Amacuro, Venezuela)

This is an interesting case of soil contamination by counterproductive engineering work. It is one of the great water courses of the Orinoco river delta (Venezuela).

The project

In 1966 the Venezuelan Corporation of Guayana decided to build a road dike. The idea was to prevent the flooding of the soils in order to use them for agriculture.

Consequences

The results were absolutely counterproductive because they did not know the ecological dynamics of the soils in the area. By reducing the flow of fresh water through the pipe, the salty water from the sea penetrated the interior and the dry and aerated soils became acidic.

The soils in the area have expandable clays that, when drying, cracked, affecting the structure. In the deep horizons there are sulfurous pyrites that, when in contact with the air, form sulfuric acid.

All this represented an ecological and social tragedy, since the indigenous Warao traditional inhabitants of the delta were affected.

Solutions

- Preventive

Ecological agriculture

To reduce soil pollution, it is necessary to implement a more environmentally friendly agriculture. It is necessary to reduce the use of agrochemicals and a less intensive use of agricultural machinery.

Organic agriculture and minimal tillage are alternatives that reduce soil contamination. This reduces the use of chemical pesticides and inorganic fertilizers and there is little use of heavy agricultural machinery.

Waste control

To avoid soil contamination, it is essential to reduce the waste that reaches it. For this, integrated systems for solid waste management, control of urban and industrial effluents and industrial gas emissions must be implemented.

- Restoration

There are methods for degrading pollutants in the soil that vary depending on the type of soil and the nature of the pollutant. In some cases these procedures are performed on-site by applying them to the soil in its natural location.

Another approach is ex situ methods, which require moving the soil to be processed at suitable sites. However, the transfer of the soil is limited to very specific cases, due to its cost and logistical difficulties.

Chemical restoration

This consists of applying certain chemicals that degrade pollutants or neutralize their effect. For example, chemical catalysis, based on the use of oxidizing substances (hydrogen peroxide or potassium permanganate).

An example of neutralization is the use of agricultural lime to correct the acidity of the soil. Also the incorporation of organic matter to the soil favors the recovery of its structure and biological activity.

Biological restoration or bio-recovery

In cases of oil contamination, bacteria and fungi capable of degrading hydrocarbons are used.

Faced with contamination of soils with heavy metals, phytoremediation or phytocorrection is used. For this, heavy metal tolerant plant species are used, such as the species Alyssum murale, which is a hyperaccumulator of nickel.

Plants can be harvested and thus extract metals (phytoextraction), or plants degrade metals in their metabolism (phytodegradation). Phytostabilization criteria can also be used when planting plants that simply retain metals.

Physical restoration

One approach is to remove polluting residues from the ground such as picking up waste dumped or accumulated on the ground. For example, eradicating a landfill or removing debris.

In the case of heavy metals, containment barriers are used to prevent their mobility. For volatile contaminants such as light hydrocarbons or non-chlorinated solvents, aerating the soil by using a plow may be sufficient.

Another option is to build air extraction wells to facilitate the evaporation of the pollutant.

Pollutants can also be removed by electricity (electrokinetics). Electrodes are inserted into the ground and a low-intensity electrical current is generated that precipitates pollutants.

References

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2. Brookes PC (1995). The use of microbial parameters in monitoring soil pollution by heavy metals. Biology and Fertility of Soils 19: 269–279.
3. Diéz-Lázaro FJ (2008). Phyto-correction of soils contaminated with heavy metals: Evaluation of tolerant plants and optimization of the process through agronomic practices. Department of Soil Science and Agricultural Chemistry, University of Santiago de Compostela. 336 p.
4. Duxbury T (1985). Ecological Aspects of Heavy Metal Responses in Microorganisms. Advances in Microbial Ecology: 185–235.
5. Mirsal IA (2008). Soil pollution. Origin, Monitoring & Remediation. Springer, Berlin, Heidelberg, Germany. 312 p.