Causes of Soil Pollution

Soil naturally contains a variety of compounds, which could be classified as pollutants, whether it is contaminated or not. These include metals, inorganic ions, and salts such as phosphates, carbonates, sulfates, and nitrates, as well as organic compounds like lipids, proteins, DNA, fatty acids, hydrocarbons, and alcohols. Soil pollution is identified when the concentration of these pollutants exceeds their normal acceptable levels.

These compounds in the soil arise from various factors, including microbial activity, the decomposition of living organisms such as animals and plants, and the deposition of certain compounds from the atmosphere through precipitation. Additionally, they may be transported to the soil via wind, surface water, or shallow groundwater that flows through the soil. Generally, the sources of soil pollution can be categorized into two main types: anthropogenic and natural sources.

Human Factors Contributing to Soil Pollution

Chemicals resulting from human activities are the primary source of soil pollution. These chemicals may be used directly in manufacturing processes or may be produced as by-products in various sectors, including industrial activities, livestock farming, household waste, agricultural chemicals, and petroleum-derived products. Typically, these chemicals reach the soil either accidentally through oil spills, leaching from landfills, or through deliberate applications such as fertilizers and pesticides, or irrigation with untreated sewage. Below are key human-related factors significantly contributing to soil pollution:

Industrial Sector

The industrial sector is a major contributor of diverse pollutants, each affecting the environment in different ways. Pollutants can enter the atmosphere, water, or soil through various pathways. Gaseous pollutants and radioactive nuclides may enter the atmosphere and subsequently reach the soil through acid rain or regular precipitation. Pollutants can also be transferred to soil due to improper practices in the industrial sector, such as incorrect chemical storage on agricultural land and the direct disposal of waste. In the past, factories allocated specific areas for waste disposal, which were considered safe; however, these have now become significant pollution sources. The challenge with soil pollution from industrial activities is that the area designated for this sector is often extensive, rendering remediation costly and difficult.

Improper practices can also lead to a new type of pollution, known as thermal pollution. Some industries discharge water and other liquids used for cooling machinery in thermal power plants into oceans, rivers, and other water sources, resulting in thermal pollution combined with elevated concentrations of heavy metals and chlorides, ultimately devastating aquatic life. Other anthropogenic sources, such as dust, waste, raw material leakage, fires, and fuel ash, may also increase heavy metal levels in specific industrial sites.

According to European directives on pollution prevention and control, human activities causing soil pollution in the industrial sector are classified into six main categories:

Energy production.
Metal production and processing.
Metal industries.
Chemical industries and facilities.
Waste management sector.
Other sectors, including paper production, textiles, leather tanning, concentrated poultry farming, carbon and graphite production, among others.

The repercussions of soil pollution caused by industrial activities include:

Contamination of groundwater with hazardous chemicals.
Disruption of ecological balance.
Release of harmful gases into the environment.
Emittance of harmful radiations affecting health.
Decrease in plant production on agricultural lands.
Increase in soil salinity, known as salinization, prominently affecting areas near industrial sites, especially those related to caustic chlorine production, textile manufacturing, glass, rubber, leather tanning, metal fabrication, pharmaceuticals, oil and gas extraction, dye manufacturing, ceramics, and the production of soaps and detergents.

Mining Sector

Historically, mining has had a profound impact on soil, water, and vegetation. Documentation indicates significant damage caused to soil by mining activities. The smelting of metals to separate them generates considerable amounts of heavy metals and toxic elements. The problem is compounded by these contaminants remaining in the environment long after mining operations cease. For example, high and toxic concentrations of chromium and nickel have been found in agricultural lands near abandoned chrome and asbestos mines, posing severe threats to human and livestock health in the vicinity.

Additionally, mining activities contribute to the dispersion of numerous radioactively active substances, posing major risks to surrounding ecosystems. Fertilizer production, for instance, heavily relies on mining phosphate-rich rocks, yielding a secondary product called phosphogypsum, which retains approximately 80% of its original radioactivity despite the decay of some radioactive elements like radon. During oil and gas extraction, commonly known sources of stable pollution, spills of oil and saline solutions may occur, containing high concentrations of salt and trace amounts of toxic elements, as well as some naturally radiative materials.

Agricultural Sector

Modern agriculture employs various innovative and technological methods to enhance crop yield, which often involves substantial usage of fertilizers, pesticides, and herbicides, significantly contributing to soil pollution. Furthermore, agricultural waste, sludge, fertilizers, debris, and inorganic chemicals resulting from soil erosion amplify soil contamination.

The following highlights some of the most polluting agricultural practices affecting soil quality:

Fertilizers

Fertilizers are a primary contributor to soil pollution. The fertilizer industry relies on phosphate rock and fly ash, which increase the levels of heavy metals like copper, zinc, lead, cadmium, and persistent, bio-accumulative, and toxic chemicals (PBTs) in the soil. Both animal and chemical fertilizers often yield elevated pollution levels. Phosphate fertilizers inherently contain cadmium from phosphate rock sources, alongside high concentrations of copper and zinc.

Pesticides

Pesticides, including insecticides, herbicides, and fungicides, also play a significant role in soil pollution. Pesticides applied to seeds and plant leaves inevitably reach the soil, leading to the accumulation of these residues over time, which subsequently pollutes water, soil, and food. Chemicals found in fertilizers can be exceedingly harmful to human health. Furthermore, these chemicals may bioaccumulate in herbivorous animals, hindering their growth, and may return to higher trophic levels through carnivorous consumption, a process known as biomagnification, which describes the increased concentration of non-biodegradable substances across the food chain.

Livestock Production

Livestock farming contributes to soil pollution when animal waste is not managed and disposed of correctly. Animal excrement often contains parasites and residual medications, which can persist in the soil for years. Many substances used in veterinary medicine are fat-soluble and non-biodegradable, affecting countless beneficial microorganisms within the soil.

Urban Life

Waste produced from commercial and residential buildings also contributes to soil pollution. This waste typically comprises trash, plastics, glass, metal containers, fibers, paper, rubber, fuel waste, tree leaves, abandoned vehicles, and other products. Although urban waste differs in composition from industrial waste, it still poses a significant threat to soil due to its non-biodegradable nature. Urban practices that lead to soil pollution include:

Human activities causing direct and indirect soil pollution.
Improper drainage methods and surface runoff contaminated with pollutants near land and waterways.
Inadequate waste disposal practices leading to the breakdown of waste components within the soil and accumulation of arising chemicals and pollutants, which may seep into groundwater or household water systems.
Significant waste accumulation in a localized area, increasing bacteria presence in the soil and leading to methane emissions from decomposing waste, which exacerbates the greenhouse effect and diminishes the surrounding air quality. Waste accumulation can also release foul odors impacting living organisms.

Transportation Sector

Activities related to transportation are major contributors to soil pollution due to emissions from fuel combustion in vehicle engines, which settle on soil through atmospheric deposition. Additionally, runoff created by vehicles traveling on wet roads during rain can transport polycyclic aromatic hydrocarbons, rubber, and particles enriched with heavy metals from the erosion of metal components and tires. This issue can be mitigated by establishing a proper sewage system.

The problem of soil pollution close to roads and highways is a serious concern in urban and suburban areas. Soil at road edges undergoes numerous processes related to heavy metals, as these metals can settle on leaves, being absorbed and subsequently transported to the plant’s upper tissues. When livestock graze on roadside lands, pollutants travel from plants to animals, and eventually to humans, posing health risks. One of the most significant pollutants linked to the transportation sector is lead from leaded gasoline.

Wastewater

Wastewater contributes to soil pollution as a result of improper management of sewage and liquids from residential water usage, alongside uncontrolled disposal of industrial waste containing a wide array of pollutants, and agricultural runoff resulting from livestock farming. Irrigation with wastewater causes drastic changes in the treated soils, including physical alterations like filtration capacity, organic content, and porosity, alongside numerous chemical changes like soil reactions, alkalinity, salinity, and nutrient levels such as nitrogen, potassium, and phosphorus. Additionally, soil may become contaminated through sludge from sewage treatment, leading to the accumulation of metals like lead, zinc, cadmium, and nickel, potentially resulting in plant toxicity.

Weapons and Mines Industry

Wars during the twentieth century have resulted in significant soil pollution. Weapons manufactured from non-biodegradable materials and long-lasting chemicals can remain contaminating the soil for centuries after warfare has ended. This issue poses a grave threat to current soil health. Presently, vast areas of land remain impacted by military activities, such as those designated for shooting tests, where restoration efforts are often time-consuming, sometimes spanning centuries.

Both World Wars left behind severe legacies on soil, including land mines, leftover munitions, chemical remnants, radioactive materials, and toxic biological agents—leaving large expanses dilapidated and unsuitable for any form of investment. Numerous reports confirm the existence of unexploded ordnance in various countries, alongside studies indicating that soil often suffered from pollution during warfare, due to the lack of regulations on weapon production at that time.

Natural Causes of Soil Pollution

Natural events also serve as sources of soil pollution. Volcanic eruptions and wildfires release several toxic elements into the atmosphere, including dioxin-like compounds and polycyclic aromatic hydrocarbons. Additionally, soils affected by volcanic activities are notably rich in heavy metals. Heavy metal pollutants can arise from natural weathering processes of rocks or from minerals like copper and chromium. While these natural pollutants may not typically pose significant environmental challenges, issues arise when ecosystems face external stressors that compromise their resilience and resistance.

Heavy metals and radioactive nuclides found in the soil can endanger human health and the environment if their levels exceed normal ranges. For example, arsenic is a potent contaminant in soil, stemming from volcanic activities or the weathering of certain minerals, and is present in mineral ores. Moreover, radon gas occurs naturally in rocks and soil, with its distribution being influenced by soil structure and porosity. Notably, natural radioactive emissions occur in acidic igneous rocks; however, studies show that carbonate-rich soils or sedimentary rocks emit more radon than any other soil or rock types.

For more information on soil pollution, you can read the article on Soil Pollution Research.

To learn more about environmental pollution, you can read the article on Environmental Pollution Research.