Table of Contents
Overview: Human-Caused Pollution of Air, Water, and Soil
In this chapter, the focus is on how human activities contaminate the three main environmental compartments—air, water, and soil—and what this means for living organisms and ecosystems. General aspects of human–biosphere interactions and resource use are treated elsewhere; here we concentrate specifically on pollution: its main sources, types, and biological consequences.
Pollutants can be:
- Gaseous (e.g. sulfur dioxide, nitrogen oxides, ozone)
- Particulate (solid or liquid particles suspended in air or water)
- Dissolved substances (e.g. heavy metals, nitrates, organic toxins)
- Solid wastes (plastics, rubble, contaminated sediments)
A central feature of pollution is that substances are introduced into the environment in amounts, forms, or locations where natural processes can no longer neutralize or recycle them without damage.
Because air, water, and soil are strongly interconnected, pollution in one compartment often spreads to the others (for example, air pollutants are washed down by rain, enter soil, and are carried into water bodies).
Air Pollution
Air pollution refers to the presence of substances in the atmosphere that are harmful to humans, other organisms, or ecosystems, or that alter climate and atmospheric chemistry.
Main Sources of Air Pollution
Typical anthropogenic (human-made) sources include:
- Combustion of fossil fuels (coal, oil, gas)
- Power plants, industry, heating systems
- Road, air, and ship traffic
- Industrial processes
- Metal smelting, cement production, chemical industry
- Agriculture
- Ammonia from intensive livestock farming
- Methane from ruminants and rice paddies
- Waste management and biomass burning
- Waste incineration, open burning, forest and peat fires
Natural sources (e.g. volcanoes, wildfires, dust storms) also contribute, but the rapid, large-scale increase of pollutants is largely due to human activity.
Important Air Pollutants and Their Effects
Particulate Matter (Aerosols, Fine Dust)
- Coarse particles: $> 10 \,\mu\text{m}$, often trapped in the upper respiratory tract.
- Fine particles: $< 2.5 \,\mu\text{m}$ (PM$_{2.5}$) and ultrafine particles: $< 0.1 \,\mu\text{m}$ can reach deep into the lungs and even enter the bloodstream.
Sources:
- Diesel engines, coal burning, biomass burning, industrial processes, tire and brake abrasion.
Biological and ecological effects:
- Irritation and inflammation of the respiratory tract
- Increased risk of asthma, cardiovascular disease, and lung cancer
- Reduced photosynthesis when dust coats leaf surfaces
- Transport carriers for toxic substances (e.g. heavy metals, organic toxins) that settle on soil and water
Sulfur Dioxide (SO$_2$) and Nitrogen Oxides (NO and NO$_2$)
Sources:
- SO$_2$: coal/oil power plants, industry, some volcanoes
- NO$_x$: traffic, combustion engines, power plants, agriculture (nitrogen fertilizers → NO → NO$_2$)
Consequences:
- Formation of Acid Rain
In the atmosphere, SO$_2$ and NO$_x$ are oxidized and dissolved in water droplets, forming sulfuric and nitric acid:
$$\text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3 \rightarrow \text{H}_2\text{SO}_4$$
$$2\,\text{NO}_2 + \text{H}_2\text{O} \rightarrow \text{HNO}_3 + \text{HNO}_2$$
These acids fall with rain or snow and lower the pH of soils and waters.
Biological effects:
- Leaching of essential nutrients (Ca$^{2+}$, Mg$^{2+}$, K$^{+}$) from soils
- Mobilization of toxic metals like aluminum, harming plant roots
- Damage to forest canopies (needle/leaf damage, reduced growth)
- Acidification of lakes and streams, harming fish, amphibians, and invertebrates
- Formation of Ground-Level Ozone and Photochemical Smog
NO$_x$ and volatile organic compounds (VOCs) from solvents and fuels can be converted under sunlight into ozone (O$_3$) close to the ground.
- Damages lung tissue, exacerbates asthma
- Reduces crop yields and forest growth by damaging leaf tissues and reducing photosynthesis
Climate-Relevant Gases (Greenhouse Gases)
While detailed climate change is treated elsewhere, some greenhouse gases are important pollutants in the context of air quality:
- Carbon dioxide (CO$_2$) from burning fossil fuels and deforestation
- Methane (CH$_4$) from livestock, landfills, and fossil fuel extraction
- Nitrous oxide (N$_2$O) from nitrogen fertilizers and industry
These gases absorb infrared radiation and influence the Earth’s heat balance. Climate change, in turn, alters ecological conditions (e.g. heat stress, changed rainfall patterns, ocean warming and acidification).
Secondary Transport: From Air to Water and Soil
Airborne pollutants do not stay where they are emitted:
- Dry deposition: particles and gases settle directly onto surfaces.
- Wet deposition: pollutants are captured by rain or snow and washed out of the atmosphere.
Consequences:
- Remote ecosystems can be affected (e.g. pollutants from industrial regions deposited in mountain lakes or the Arctic).
- Persistent pollutants can enter soils and surface waters and accumulate in food webs (bioaccumulation and biomagnification).
Water Pollution
Water pollution is the contamination of surface waters (rivers, lakes, coastal oceans) and groundwater with substances or energy forms (e.g. heat) that impair their ecological functions and use for drinking, irrigation, or recreation.
Main Sources of Water Pollution
- Domestic wastewater (untreated or insufficiently treated sewage)
- Industrial effluents
- Chemical, textile, paper, and metal-processing industries
- Agriculture
- Fertilizers (nitrates, phosphates)
- Pesticides and veterinary medicines
- Runoff from sealed surfaces
- Streets, parking lots (oil residues, heavy metals, road salt)
- Mining and resource extraction
- Acid mine drainage, heavy metals
- Accidents
- Oil spills, chemical accidents
- Waste disposal
- Dumping of solid waste in or near water bodies, plastic pollution
Types of Water Pollution
1. Organic Pollution and Eutrophication
Organic pollution involves carbon-containing, biodegradable substances entering water:
- Domestic sewage, food industry waste, manure, plant residues
Microorganisms break down this material using oxygen:
$$\text{Organic matter} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} + \text{biomass}$$
If the input is large, the biochemical oxygen demand (BOD) rises:
- Dissolved oxygen is rapidly consumed.
- Fish and invertebrates dependent on high oxygen levels may suffocate.
- Anaerobic conditions develop in sediments, producing toxic gases (e.g. H$_2$S).
Eutrophication is enrichment of water with nutrients, especially nitrogen and phosphorus:
Sources:
- Fertilizer runoff
- Detergents (where phosphates are still used)
- Sewage and manure
Consequences:
- Strong increase of phytoplankton and algae (algal blooms)
- Reduced light penetration, death of submerged plants
- After blooms collapse, decomposition consumes large amounts of oxygen
- Development of hypoxic (low oxygen) or anoxic (no oxygen) zones; many organisms die or flee
Long-term changes:
- Species-poor communities dominated by a few tolerant species
- Shifts in food webs and loss of biodiversity
2. Chemical Pollution: Heavy Metals and Organic Toxins
Examples:
- Heavy metals: mercury (Hg), lead (Pb), cadmium (Cd), arsenic (As)
- Sources: mining, batteries, pigments, industrial wastewater
- Some bind strongly to sediments and organic matter, accumulating in organisms.
- Persistent organic pollutants (POPs):
- Pesticides (e.g. DDT), industrial chemicals (PCBs), some flame retardants
- Long-lived, fat-soluble, travel long distances, accumulate in food chains.
Biological consequences:
- Toxic effects on nervous system, liver, kidneys, reproduction
- Eggshell thinning in birds (for some pesticides)
- Endocrine disruption (interference with hormone systems)
- Bioaccumulation: pollutants accumulate in organisms’ tissues over time
- Biomagnification: concentrations increase at higher trophic levels (e.g. top predators, including humans)
3. Pathogens in Water
Untreated sewage, manure, and animal carcasses can introduce:
- Bacteria, viruses, protozoa, and parasitic worms
Consequences:
- Waterborne diseases (e.g. diarrhea, cholera, typhoid, hepatitis A)
- High health risk where drinking water and wastewater are not strictly separated
4. Thermal Pollution
Power plants and industry use water as a coolant and then discharge it at higher temperatures.
Effects:
- Warm water holds less dissolved oxygen.
- Changes in species composition: warm-adapted species benefit; cold-adapted species may disappear.
- Timing of biological processes (e.g. spawning, plankton blooms) can be disrupted.
5. Plastic Pollution and Microplastics
- Large plastic items can cause entanglement and ingestion by animals.
- Microplastics ($< 5 \,\text{mm}$), from fragmented plastics or cosmetic products and textiles, are ingested by many aquatic organisms.
Potential effects:
- Physical blockage of digestive tracts
- Transport of attached pollutants and microorganisms into organisms
- Unknown long-term ecological impacts, but clear global spread.
Pathways from Water Pollution to Other Compartments
- Pollutants can be deposited with sediments, turning river and lake bottoms into long-term pollutant reservoirs.
- Polluted irrigation water transfers contaminants to agricultural soils.
- Evaporation and spray from polluted waters may transport volatile substances into the air.
Soil Pollution
Soils are complex systems of mineral particles, organic matter, organisms, water, and air. They provide essential functions: plant nutrition, water storage, habitat for numerous organisms, and filtering of substances before they reach groundwater. Soil pollution occurs when toxic or excessive substances are introduced and interfere with these functions.
Sources of Soil Pollution
- Agricultural inputs
- Excess fertilizers (nitrates, phosphates)
- Pesticides and herbicides
- Sludge and manure contaminated with heavy metals or pharmaceuticals
- Industrial and mining activities
- Deposition of dust and gases
- Waste dumps and tailings
- Leakages from tanks and pipelines
- Traffic
- Deposition of heavy metals, tire abrasion, oil residues
- Improper waste disposal
- Illegal dumping of household, construction, and hazardous waste
- Atmospheric deposition
- Air pollutants (e.g. acid rain, heavy metals, POPs) settling into soils
Types and Effects of Soil Pollution
1. Nutrient Overload and Salinization
Over-fertilization:
- Nitrate (NO$_3^-$) and phosphate (PO$_4^{3-}$) accumulate.
- Plants initially grow strongly, but excess nitrogen can:
- Disturb nutrient balance
- Make plants more susceptible to pests and frost
- Nitrate is mobile and can leach into groundwater, contributing to water pollution.
Salinization:
- Accumulation of soluble salts (NaCl, Na$_2$SO$_4$ etc.), often from irrigation in dry regions or use of saline water.
- High salt concentrations:
- Make it harder for plant roots to take up water (osmotic stress)
- Can be directly toxic to many plants and soil organisms
- Leads to loss of fertile soil and desertification tendencies.
2. Heavy Metals in Soils
Heavy metals originate from:
- Industry, mining, traffic, sewage sludge, phosphate fertilizers
They bind strongly to soil particles and are often poorly degradable.
Effects:
- Toxic to microorganisms, reducing decomposition and nutrient cycling
- Disturbed root growth and plant metabolism
- Uptake into crops, entering food chains
- Long-term contamination that is difficult and costly to remediate
3. Organic Pollutants in Soils
Organic pollutants include:
- Pesticides, herbicides, fungicides
- Industrial chemicals and solvents
- Petroleum hydrocarbons (oil, fuel leaks)
Consequences:
- Toxicity to soil fauna (earthworms, insects, nematodes) and flora
- Disruption of microbial communities and processes such as nitrification and decomposition
- Contamination of groundwater through leaching
- Potential uptake by plants and transfer to animals and humans via food
4. Physical Degradation and Sealing
Although not always called “pollution,” physical changes strongly affect soil functions:
- Soil compaction from heavy machinery:
- Reduces pore space, air exchange, and root penetration
- Increases erosion by water and wind
- Soil sealing (asphalt, buildings):
- Eliminates habitat and filtration functions
- Alters local water balance (enhanced runoff, reduced groundwater recharge)
Interactions with Air and Water
Soil is a critical interface:
- Pollutants can migrate from soil to:
- Plants (root uptake)
- Groundwater (leaching)
- Air (evaporation of volatile compounds, dust)
- Air and water pollution continuously influence soils through deposition and irrigation.
Biological Consequences and Ecological Perspective
Pollution of air, water, and soil has several common ecological patterns:
- Reduction of biodiversity
- Sensitive species disappear; tolerant or opportunistic species dominate.
- Simplification of food webs
- Loss of key species (e.g. top predators, specialist pollinators) leads to cascading effects.
- Disruption of ecosystem services
- Reduced water purification, soil fertility, pollination, and climate regulation.
- Health risks for humans
- Contaminated air, drinking water, and food; higher incidence of respiratory, cardiovascular, and cancer diseases.
Because pollutants cross boundaries:
- Local emissions can have global effects, and
- Pollution control often requires regional and international cooperation.
Measures to reduce pollution (e.g. filters, wastewater treatment, careful chemical use, recycling, and cleaner technologies) are discussed in other chapters, along with broader strategies for protecting nature and the environment.