4.4 Water pollutionIB Environmental Systems and Societies SL: Revision notes

Water pollution has multiple sources: sewage and wastewater, agricultural run-off (fertilisers, pesticides, manure), industrial discharges, oil, plastics and litter. Sources can be point (a pipe) or non-point (diffuse run-off). It has major impacts on marine and freshwater systems: oxygen loss, poisoning, habitat damage, loss of biodiversity and risks to human health and water supplies.

Plastic debris is accumulating in marine environments, with floating material concentrated by currents in gyres, such as the Great Pacific Garbage Patch. Plastics persist for a long time and break into microplastics. Animals become entangled in or swallow plastic.

Management is needed at two stages:

• Remove plastics from the supply chain: reduce single-use plastics, redesign packaging, deposit-return schemes, bans on items such as bags.
• Clear up existing pollution: river barriers, beach clean-ups, ocean collection systems.

Water quality is the measurement of chemical (dissolved oxygen, nitrate, phosphate, pH), physical (temperature, turbidity) and biological (e.g. faecal bacteria) characteristics. It varies in time and place and is often summarised in a water quality index (WQI), a single score combining several measurements.

Biochemical oxygen demand (BOD) is the amount of dissolved oxygen required by microorganisms to decompose organic material in the water. High BOD means a lot of organic pollution and usually low dissolved oxygen.

Eutrophication occurs when lakes, estuaries and coastal waters receive inputs of mineral nutrients, especially nitrates and phosphates, often causing excessive growth of phytoplankton.

Sequence of impacts:

1. Nutrient input from fertilisers, sewage or detergents.
2. Algal bloom.
3. Light is blocked, so submerged plants die.
4. Dead material is decomposed by bacteria.
5. BOD rises, dissolved oxygen falls.
6. Fish and other aerobic organisms die; biodiversity falls.

Eutrophication harms ecosystem services: provisioning (fish, drinking water), cultural (tourism, recreation, amenity) and regulating (water purification) services are reduced.

Management works at three levels:

• Reduce production of pollutants: less fertiliser, phosphate-free detergents.
• Intercept pollutants before they reach the water: buffer strips, wetlands, sewage treatment.
• Restore the water body: dredging, aeration, removing algae.

Prevention is usually most sustainable; clean-up is quick but costly.