18.2 Land and Water Pollution

A free GCSE Biology lesson for AQA (8461) from OBA School: how waste, sewage, fertilisers and toxic chemicals pollute land and water and reduce biodiversity. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Name the main causes of water pollution: sewage, fertiliser and toxic chemicals
• Name the main causes of land pollution: landfill and toxic chemicals
• Explain step by step how fertiliser or sewage can kill fish in a lake or river (eutrophication)
• Explain why pollution reduces biodiversity
• Analyse data on water quality, including percentage change

Key words

• Fertiliser: Chemicals containing minerals such as nitrates and phosphates, added to soil to help crops grow.
• Pesticide: A chemical used to kill insects or other pests that damage crops.
• Herbicide: A chemical used to kill weeds (unwanted plants).
• Algal bloom: A very rapid growth of algae on the surface of water, often caused by extra nitrates and phosphates.
• Eutrophi- cation: The process in which extra minerals in water lead to an algal bloom, death of plants, and a fall in oxygen that kills animals.
• Decomposer: A microorganism (bacterium or fungus) that breaks down dead material, using oxygen as it respires.

Polluting the land

As the population grows, we make more household and industrial waste. Much of it is buried in landfill sites.

• Landfill takes up land that was once a habitat for plants and animals.
• Toxic chemicals from batteries, paints and electrical goods can seep out of landfill into the soil and into streams.
• Farmers spray pesticides and herbicides. These can be washed into soil and water, and they can kill organisms other than the pests and weeds they are meant for.
• Some industrial waste, such as radioactive waste, must be stored safely for a very long time.

Polluting the water

• Sewage; Where it comes from Untreated waste from homes, or overflow from sewage works after heavy rain; How it harms life in water Adds minerals and organic matter; decomposers use up oxygen; may carry disease-causing bacteria
• Fertiliser; Where it comes from Washed (leached) off fields by rain into rivers and lakes; How it harms life in water Extra nitrates and phosphates cause an algal bloom (eutrophication)
• Toxic chemicals; Where it comes from Factories, pesticides and herbicides from farms, landfill leaks; How it harms life in water Poison and kill organisms; some build up along food chains

Key fact: Toxic chemicals that organisms cannot break down build up in their bodies. Animals at the top of a food chain eat many contaminated prey, so they can get a dangerously high dose.

How fertiliser and sewage kill fish

This chain of events is called eutrophication. In the exam you need each step in the right order, with the reason.

Exam tip: It is not the algae that use up all the oxygen in the water. The oxygen is used by bacteria respiring as they decompose the dead plants and algae.

Measuring water pollution

Scientists measure the concentration of dissolved oxygen at different points along a river. Low oxygen shows that a lot of decomposition is happening, often because of pollution.

Worked example: percentage decrease in oxygen

Upstream of the outflow (−100 m) the oxygen concentration is 9.2 mg/dm³. At 200 m downstream it is 2.6 mg/dm³.
Decrease = 9.2 − 2.6 = 6.6 mg/dm³
Percentage decrease = 6.6 ÷ 9.2 × 100 = 71.7%

Answer: about 72% decrease

Further downstream the oxygen rises again: the sewage has been broken down and diluted, and oxygen dissolves back in from the air.

Common mistakes

• Saying the fish die “because of the fertiliser”. The fertiliser is not poisonous to fish; they die from lack of oxygen.
• Saying algae or plants use up the oxygen. Explain that bacteria (decomposers) respire and use up the oxygen as they break down dead material.
• Missing out the step that light is blocked, so plants underneath cannot photosynthesise.
• Forgetting that landfill reduces biodiversity simply by taking up land, as well as by leaking chemicals.

Revision questions

Try these, then check your answers below each one.

Question 1. In eutrophication, what directly causes fish to die? (Choose from: Too much light reaching the water, A lack of dissolved oxygen in the water, Nitrates from fertiliser poisoning them, Algae eating the fish’s food.)

Answer: A lack of dissolved oxygen in the water

Working: Bacteria decomposing dead plants use up the dissolved oxygen by respiration, so fish suffocate.

Question 2. True or false? Bacteria that decompose dead plants in a lake use up oxygen in the water by respiration.

Answer: True

Working: True. Aerobic respiration by large numbers of bacteria lowers the dissolved oxygen.

Question 3. The dissolved oxygen in a river fell from 9.0 mg/dm³ above a sewage pipe to 3.6 mg/dm³ below it. Calculate the percentage decrease.

Answer: 60 %

Working: 9.0 − 3.6 = 5.4. 5.4 ÷ 9.0 × 100 = 60%.

Related lessons

• Previous lesson: 18.1 The Human Population Explosion
• Next lesson: 18.3 Air Pollution
• In this chapter: 18.4 Deforestation and Peat Destruction
• In this chapter: 18.5 Global Warming
• In this chapter: 18.6 The Impact of Change

Continue in the full course

This lesson is section 18.2 (AGCBH1802) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.