4.4 Water pollutionIB Environmental Systems and Societies HL: Revision notes

Water pollution has multiple sources: agricultural run-off (fertilisers, pesticides, manure), untreated or poorly treated sewage, industrial discharges, mining, oil spills and plastics. It has major impacts on marine and freshwater systems: harm to organisms, loss of biodiversity, risks to human health and loss of ecosystem services.

Plastic debris is accumulating in marine environments, e.g. in ocean gyres such as the Great Pacific Garbage Patch, and breaks into microplastics that enter food webs. Management is needed to remove plastics from the supply chain (reduce, reuse and recycle, ban single-use items, producer responsibility) and to clear up existing pollution (river barriers, beach and sea clean-ups).

Water quality is the measurement of chemical, physical and biological characteristics of water (e.g. dissolved oxygen, pH, nitrate, temperature, turbidity, bacteria). It is variable and is often measured using a water quality index.

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

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

The impacts follow a sequence:

1. Nutrient input (fertiliser, sewage) increases.
2. Phytoplankton bloom; the water turns green.
3. Light is blocked, so submerged plants die.
4. Algae and plants die and sink; decomposers multiply and BOD rises.
5. Dissolved oxygen falls; fish and invertebrates die and biodiversity is lost.

Eutrophication can substantially impact ecosystem services: fish and drinking water (provisioning), water purification (regulating), and tourism and recreation (cultural) are all reduced.

Eutrophication can be addressed at three levels:

• Reducing the release of pollutants at source: limit fertiliser and manure use, phosphate-free detergents, better sewage treatment, laws and education.
• Intercepting pollutants before they reach the water: buffer strips, constructed wetlands, tertiary treatment.
• Restoring damaged systems: removing nutrient-rich sediment, harvesting algae, aerating water, biomanipulation.

Source control is usually the most effective, but a combination works best. Example: the Baltic Sea Action Plan (HELCOM, 2007).

(HL) A wide range of pollutants can be found in water: nutrients, sewage and pathogens, organic waste, heavy metals, pesticides, oil, plastics, thermal pollution and pharmaceuticals.

(HL) Algal blooms may produce toxins (e.g. from cyanobacteria) that threaten the health of humans and other animals, through drinking water, swimming or eating contaminated shellfish.

(HL) The frequency of anoxic/hypoxic waters is likely to increase because of the combined effects of global warming (warm water holds less oxygen), freshwater stratification, sewage disposal and eutrophication.

(HL) Sewage is treated in stages so the effluent can be safely released:

• Primary: screening and settlement remove solids and grit; sludge settles.
• Secondary: aeration and aerobic microorganisms break down organic matter, lowering BOD.
• Tertiary: removes nitrates and phosphates (and other pollutants), and may disinfect the water, e.g. by chlorine or UV, reducing eutrophication.

(HL) Indicator species are sensitive to pollutants (e.g. mayfly and stonefly nymphs, needing clean oxygen-rich water) or adapted to polluted waters (e.g. sludge worms, rat-tailed maggots).

(HL) A biotic index gives an indirect measure of water quality from the tolerance to pollution, relative abundance and diversity of species in the community.

(HL) Overall water quality can be assessed by calculating a water quality index (WQI), combining measurements such as dissolved oxygen, pH, BOD and nitrate into one score.

(HL) Drinking water guidelines are set by the World Health Organization (WHO).

(HL) Actions by individuals or citizen groups, e.g. not pouring chemicals down drains, using less fertiliser, river clean-ups and monitoring, can help reduce water pollution.