Table of Contents
Types and Sources of Soil Pollution
Soil is not just “dirt”; it is a complex, living system that stores water and nutrients and houses countless organisms. When harmful substances accumulate in soil or its structure is severely damaged, we speak of soil pollution or soil degradation. In this chapter, the focus is on how human activities specifically impair soils, what substances are involved, and what consequences and countermeasures exist.
Chemical Contamination
Heavy Metals and Other Inorganic Pollutants
Heavy metals are elements with relatively high atomic mass that can be toxic even in small amounts. They do not break down and therefore accumulate in soils over long periods.
Important examples:
- Lead (Pb):
• From old leaded gasoline residues along roads, lead-based paints, shooting ranges, certain industrial emissions and waste.
• Accumulates in upper soil layers; can be taken up by plants or cling to dust particles.
• Neurotoxic, especially harmful for children. - Cadmium (Cd):
• From phosphate fertilizers, metal smelting, waste incineration, sewage sludge.
• Enters plants (e.g., cereals, leafy vegetables, tobacco); in the human body it can damage kidneys and bones. - Mercury (Hg):
• From coal combustion, certain industrial processes, historical use in pesticides and mining.
• Can be converted by microorganisms into highly toxic methylmercury, which moves through food chains. - Arsenic (As), chromium (Cr), copper (Cu), zinc (Zn), nickel (Ni):
• Some are essential in trace amounts but become harmful at higher concentrations.
• Sources include wood preservatives, tanneries, metal processing, antifouling paints, and manure or slurry (for Cu, Zn).
Other inorganic contaminants:
- Excess salts (NaCl and others) from irrigation with saline water or road salt:
• Cause soil salinization, harming plants and soil organisms, and leading to structural damage (dispersion of clay particles). - Nitrate ($\text{NO}_3^-$):
• From overuse of nitrogen fertilizers and intensive livestock farming.
• May leach to groundwater (water pollution topic), but high nitrate levels in soil also alter species composition and favor nitrophilous (nitrogen-loving) species.
Organic Pollutants
Organic pollutants are carbon-containing chemicals, often synthetic. Many are persistent (hard to degrade) and can accumulate in food chains.
Key groups:
- Pesticides (insecticides, herbicides, fungicides):
• Used in agriculture, forestry, and gardening.
• Some modern pesticides degrade relatively quickly, but others are very persistent.
• Example groups:
– Organochlorine compounds (such as old DDT-type pesticides): long-lived, fat-soluble; often banned but still detectable in many soils.
– Organophosphates and carbamates: typically more degradable but acutely toxic to insects and sometimes to humans and beneficial organisms. - Persistent Organic Pollutants (POPs):
• Include some pesticides and industrial chemicals such as PCBs (polychlorinated biphenyls) and dioxins.
• Very stable, bioaccumulative, toxic; can be transported far from their source via air and dust. - Petroleum hydrocarbons:
• From leaking fuel tanks, accidents with oil products, industrial spills.
• Mineral oils and fuels can form hydrophobic films, displacing water and oxygen from soil pores and harming soil fauna and flora. - Polycyclic aromatic hydrocarbons (PAHs):
• Produced by incomplete combustion (e.g., from traffic, heating, industry, wood burning).
• Deposit from the air onto soils, especially near roads and cities.
• Many PAHs are carcinogenic.
Physical and Structural Damage
Not all soil pollution is strictly “chemical.” Changes to soil structure and physical properties can also be considered forms of degradation and are closely linked to contamination.
Soil Compaction
- Causes:
• Heavy machinery in agriculture and forestry (tractors, harvesters) especially when soils are wet.
• Construction work and traffic on unpaved surfaces. - Effects:
• Pore spaces collapse; air and water circulation are reduced.
• Root growth is impeded, groundwater recharge decreases, and surface runoff increases, which can promote erosion.
• Soil organisms lose habitat and oxygen.
Erosion and Loss of Topsoil
The topsoil (rich in humus) is by far the most fertile layer. Once it is removed by water or wind, it takes many decades or centuries to rebuild.
- Human contributions:
• Overgrazing, deforestation, poor cultivation practices (e.g., plowing along, not across, slopes).
• Construction that leaves bare soil exposed. - Links to pollution:
• Pollutants bound to fine soil particles can be transported with eroded material into streams, lakes, and distant areas.
• Degraded soils are more prone to receiving and retaining contaminants in concentrated hotspots because natural buffering capacity (humus, clay) is lost.
Sealing of Soil Surfaces
“Sealing” means covering soil with impermeable materials.
- Examples:
• Asphalt and concrete in cities, parking lots, industrial areas, large commercial zones.
• Buildings and infrastructure that completely separate soil from the atmosphere. - Consequences:
• Loss of soil functions (no water infiltration, no plant growth, no habitat for soil organisms).
• Increased risk of flooding due to faster surface runoff.
• Local climate effects (urban heat islands) because natural cooling by evaporation and plant transpiration is lost.
Littering and Solid Waste
- Mismanaged waste and illegal dumping sites can introduce plastics, metals, glass, and other materials into soil.
- Plastics may fragment into microplastics, which can be ingested by soil fauna and alter physical soil properties (water retention, pore structure) and transport attached pollutants.
How Soil Pollution Occurs: Main Pathways
Agriculture and Forestry
- Fertilizers and Manure:
• Excess application of mineral fertilizers and slurry leads to nutrient surpluses (especially nitrogen and phosphorus), salinization, and contamination with accompanying substances (e.g., cadmium in phosphate fertilizers). - Pesticides:
• Repeated or inappropriate use accumulates residues and affects non-target organisms (e.g., earthworms, beneficial insects, microorganisms). - Sewage Sludge as Fertilizer:
• Sewage sludge contains plant nutrients but also heavy metals, pharmaceuticals, hormones, and microplastics.
• If insufficiently treated and regulated, it can introduce a mixture of contaminants into soils.
Industry, Mining, and Energy Production
- Emissions and Deposition:
• Exhaust gases and dust from smelters, power stations, cement factories, and other plants deposit heavy metals and other pollutants onto surrounding soils. - Direct Spills and Leaks:
• From storage tanks (fuel, chemicals), transport accidents, or production failures. - Mining and Tailings:
• Mining residues (tailings) can contain high concentrations of metals and sulfides, which acidify soils and release heavy metals as they oxidize.
Urbanization and Infrastructure
- Traffic:
• Metals from tire and brake abrasion, old leaded gasoline residues, PAHs from exhaust, and road salt accumulate in roadside soils.
• Noise and vibration are physical stresses that indirectly affect soil via vegetation and compaction. - Construction Sites:
• Mix and move soils from different locations, sometimes spreading pollutants.
• Can leave behind rubble, asbestos, and other harmful building materials in the ground.
Atmospheric Transport and Deposition
Even remote areas can be affected:
- Volatile or fine particulate pollutants travel long distances in the atmosphere.
- They settle with dust, rain, and snow, leading to diffuse contamination (e.g., low-level heavy metal or POP loads in mountain or polar soils far from the original source).
Effects of Soil Pollution
Impacts on Soil Organisms and Soil Functions
Soil is an ecosystem with immense biodiversity (bacteria, fungi, protozoa, nematodes, mites, springtails, earthworms, and more).
- Toxic effects:
• Many pollutants (e.g., heavy metals, pesticides) are directly toxic to soil organisms, reducing their abundance and diversity.
• Sensitive species disappear, more tolerant species may dominate. - Disturbed decomposition and nutrient cycling:
• If decomposers (bacteria, fungi, earthworms) are impaired, organic matter breaks down more slowly or differently.
• This alters humus formation, nutrient availability, and soil structure. - Reduced self-purification capacity:
• Healthy soils can degrade many organic pollutants biologically or bind some harmful substances securely.
• If soil life is degraded, this natural “filter function” weakens, and pollutants persist longer or migrate more easily.
Effects on Plants and Vegetation
Polluted soils affect plant growth and the composition of plant communities.
- Uptake of toxic substances:
• Some pollutants enter roots and accumulate in leaves, fruits, or seeds.
• High concentrations can stunt growth, cause chlorosis (yellowing leaves), leaf deformities, or plant death. - Nutrient imbalances:
• Pollutants can interfere with nutrient uptake (e.g., excess heavy metals blocking essential nutrients).
• Salinization prevents plants from taking up water properly, leading to wilting despite moist soil. - Changes in species composition:
• Pollution and over-fertilization favor a few tolerant or fast-growing species.
• Specialized or sensitive species disappear, reducing overall biodiversity.
Risks to Animals and Humans
Soil pollutants can reach animals and humans indirectly.
- Food chain transfer:
• Plants or soil animals (e.g., worms, insects) that contain heavy metals or persistent organic pollutants are eaten by herbivores, which in turn are eaten by predators, including humans.
• Concentrations can increase along food chains (bioaccumulation and biomagnification). - Direct exposure:
• Children playing on contaminated ground may ingest or inhale contaminated dust.
• Workers on polluted sites are exposed through skin contact and inhalation. - Long-term health effects:
• Some pollutants (e.g., certain heavy metals, POPs) are carcinogenic, neurotoxic, or hormone-disrupting.
• Effects can be chronic and appear only after long exposure at low doses.
Assessment and Monitoring of Soil Pollution
Sampling and Chemical Analyses
- Soil samples are taken at defined depths and positions and analyzed in laboratories for:
• Heavy metals, nutrients, pH, organic pollutants, salinity, and more. - Comparing measured values with guideline or threshold values helps assess risk and need for action.
Bioindicators and Ecotoxicological Tests
- Bioindicator organisms:
• Certain plant species or lichens that are particularly sensitive or tolerant indicate pollution levels (e.g., heavy metal indicator plants).
• Soil invertebrates (earthworms, springtails) and microorganisms are used in standardized toxicity tests. - Biological parameters:
• Enzyme activities (e.g., dehydrogenase, phosphatase) or respiration rates in soil can indicate overall biological activity and stress.
Mapping and Historical Investigation
- Many contaminated sites are linked to past industrial or military uses, landfills, or old waste deposits.
- Historical maps, company records, and aerial photographs help identify potential problem areas for detailed investigation.
Protection and Remediation of Soils
Preventive Measures
Preventing pollution is more effective and cheaper than cleaning up later.
- Regulatory instruments:
• Limits for pollutants in fertilizers, sewage sludge, composts.
• Strict approval procedures and restrictions for pesticides and industrial chemicals.
• Emission standards for industrial plants and vehicles. - Good agricultural practice:
• Precise fertilization according to plant needs (soil testing, nutrient balances).
• Reduced or targeted pesticide use (e.g., integrated pest management, biological control).
• Avoidance of driving heavy machinery on wet soils, use of lighter equipment or special tires to reduce compaction.
• Protective cultivation methods that minimize erosion (ground cover, contour plowing, strips of vegetation). - Urban and spatial planning:
• Limiting soil sealing, promoting green spaces and permeable surfaces.
• Reusing already built-up spaces (“brownfield redevelopment”) instead of converting new land (“greenfield development”).
Remediation of Contaminated Soils
When soils are already polluted, various remediation strategies exist, often combined:
- Excavation and disposal:
• Contaminated soil is dug up and taken to landfills or special treatment plants.
• Effective but expensive; destroys the original soil structure. - Containment and immobilization:
• Polluted areas may be capped with clean material or sealed to prevent contact and spread.
• Chemical binders can immobilize some contaminants in situ, reducing mobility but not removing them. - Thermal and chemical treatments:
• High temperatures (thermal desorption, incineration) can destroy or evaporate organic pollutants.
• Certain chemicals can be used to wash pollutants out of the soil or transform them – methods that require careful control. - Biological methods (bioremediation):
• Microorganisms degrade organic pollutants such as some hydrocarbons and pesticides.
• Conditions (oxygen, nutrients, moisture) are optimized to support these microbes. - Phytoremediation:
• Some plant species can accumulate metals or break down certain organic pollutants.
• Types:
– Phytoextraction: Plants take up contaminants and are then harvested and disposed of.
– Phytostabilization: Plants stabilize pollutants in the soil and reduce erosion and leaching.
– Phytodegradation: Plant-associated microbes degrade pollutants in the root zone.
Sustainable Soil Management
Long-term soil protection is part of sustainable land use:
- Maintain and build humus (organic matter) to improve structure, fertility, and pollutant-buffering capacity.
- Promote diverse crop rotations and agroecosystems to stabilize soil functions.
- Integrate soil protection into climate strategies, as soils also act as important carbon stores.
Summary
Soil pollution and degradation arise from many human activities: intensive agriculture, industry, traffic, construction, and improper waste handling. Contaminants include heavy metals, synthetic organic compounds, excess nutrients, and salts, while physical impacts such as compaction, erosion, and sealing further impair soil functions. The consequences affect soil organisms, plants, animals, and humans, and they weaken soil’s roles in water regulation, nutrient cycling, and climate buffering. Protecting and rehabilitating soils requires preventive regulation, careful land use, and, where necessary, technical and biological remediation methods to restore or preserve this vital but vulnerable component of the biosphere.