The main sources of water pollution are agriculture, domestic sewage, industry, mining, urban runoff, oil pollution and solid waste. In IB ESS Topic 4.4, core for SL and HL, link each source to its pollutants, processes and ecological and societal impacts.
Water pollution occurs when substances or energy enter water and cause harmful ecological or societal effects. Sources may be point sources, released from an identifiable location, or non-point sources, dispersed through runoff across an area.
| Source | Typical pollutants and effects |
|---|---|
| Agriculture | Nutrients cause eutrophication; pesticides may be toxic; animal waste adds pathogens and organic matter. |
| Domestic sewage | Organic matter raises biochemical oxygen demand (BOD) as decomposers consume oxygen; untreated sewage spreads pathogens. |
| Industry | Heavy metals, acids, toxic chemicals and heated water poison organisms or alter habitats. |
| Mining | Acid drainage lowers pH; metals poison organisms; sediment reduces light and damages habitats. |
| Urban runoff | Rain carries oil, metals, road salt, litter and sewage from impermeable surfaces. |
| Oil extraction and transport | Spills coat organisms, damage feathers and gills, and contaminate coasts. |
| Solid waste | Plastics entangle organisms and form microplastics that transport pollutants through food chains. |
A named example is Minamata Bay, Japan, where industrial mercury entered the food chain and biomagnified, causing severe neurological illness. It also illustrates environmental justice: fishing communities suffered health and livelihood losses while receiving little benefit from the industry.
A common misconception is that a source and pollutant are identical. Agriculture is the source; nitrate-rich fertilizer runoff is the pollutant input; eutrophication is the resulting process.
Exam technique: For an “explain” question, write a complete causal chain: source, pollutant, process and impact. For example, fertilizer runoff adds nitrates, stimulating algal growth; decomposition then raises BOD, reduces dissolved oxygen and may kill fish and aquatic invertebrates.