4.4 Water pollution

Significant idea:

  • Water pollution, both to groundwater and surface water, is a major global problem, the effects of which influence human and other biological systems.

Knowledge and understanding:

  1. There are a variety of freshwater and marine pollution sources.
  2. Types of aquatic pollutants include floating debris, organic material, inorganic plant nutrients (nitrates and phosphates), toxic metals, synthetic compounds, suspended solids, hot water, oil, radioactive pollution, pathogens, light, noise and biological pollutants (invasive species).
  3. A wide range of parameters can be used to directly test the quality of aquatic systems, including pH, temperature, suspended solids (turbidity), metals, nitrates and phosphates.
  4. Biodegradation of organic material utilizes oxygen, which can lead to anoxic conditions and subsequent anaerobic decomposition, which in turn leads to formation of methane, hydrogen sulfide and ammonia (toxic gases).
  5. Biochemical oxygen demand (BOD) is a measure of the amount of dissolved oxygen required to break down the organic material in a given volume of water through aerobic biological activity. BOD is used to indirectly measure the amount of organic matter within a sample.
  6. Some species can be indicative of polluted waters and can be used as indicator species.
  7. A biotic index indirectly measures pollution by assaying the impact on species within the community according to their tolerance, diversity and relative abundance.
  8. Eutrophication can occur when lakes, estuaries and coastal waters receive inputs of nutrients (nitrates and phosphates), which results in an excess growth of plants and phytoplankton.
  9. Dead zones in both oceans and fresh water can occur when there is not enough oxygen to support marine life.
  10. Application of figure 3 to water pollution management strategies includes:
    1. reducing human activities that produce pollutants (for example, alternatives
      to current fertilizers and detergents)
    2. reducing release of pollution into the environment (for example, treatment of waste water to remove nitrates and phosphates)
    3. removing pollutants from the environment and restoring ecosystems (for example, removal of mud from eutrophic lakes and reintroduction of plant and fish species).

Applications and skills:

  • Analyse water pollution data.
  • Explain the process and impacts of eutrophication.
  • Evaluate the uses of indicator species and biotic indices in measuring aquatic pollution.
  • Evaluate pollution management strategies with respect to water pollution.


  • Countries with limited access to clean water often have higher incidences of water- borne illnesses.

Theory of knowledge:

  • A wide range of parameters are used to test the quality of water and judgments are made about causes and effects of water quality—how can we effectively identify cause–effect relationships, given that we can only ever observe correlation?


  • Terrestrial food production systems and food choices (5.2)
  • Climate change— causes and impacts (7.2)
  • Sustainability (1.4)
  • Resource use in society (8.2)
  • Biodiversity and conservation (topic 3)
  • Solid domestic waste (8.3)