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B17.2 Land and Water Pollution

GCSE Combined Science · AQA · Chapter B17: Biodiversity and Ecosystems

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: how sewage, fertiliser, toxic chemicals and landfill pollute water and land, and how waste can be managed. 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, toxic chemicals) and land pollution (landfill, toxic chemicals)
  • Explain step by step how fertiliser or sewage can kill fish in a lake (eutrophication)
  • Describe the problems caused by landfill and the advantages and disadvantages of incineration
  • Explain how reducing, reusing and recycling waste helps to protect biodiversity
  • Analyse data on water quality, including percentage change

Key words

  • Sewage: Waste water from toilets, homes and factories containing human waste, dissolved chemicals and often pathogens.
  • Fertiliser: Chemicals containing minerals such as nitrates and phosphates, added to soil to help crops grow.
  • Pesticide / herbicide: Chemicals sprayed on crops: pesticides kill insects and other pests; herbicides kill weeds.
  • Algal bloom: A very rapid growth of algae on the surface of water, often caused by extra nitrates and phosphates.
  • Landfill: A large hole in the ground where waste is buried.
  • Incinerator: A furnace in which waste is burned; the heat released can be used to generate electricity.

Polluting the water

  • Sewage; Where it comes from Open sewers, untreated waste from homes, overflow from sewage works after heavy rain; How it harms life in water Carries pathogens (e.g. Salmonella bacteria, norovirus) that spread disease; adds organic matter and minerals, so decomposers use up oxygen
  • Fertiliser; Where it comes from Washed off fields by rain into ponds, lakes and slow rivers; 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
  • Oil; Where it comes from Spills from tankers and leaks from oil wells; How it harms life in water Coats birds’ feathers and animals’ fur; kills shoreline and sea life

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 a lake that was stable and full of species, it can kill most animal life within a few weeks. Learn each step in order, with its reason.

Exam tip: It is not the fertiliser that poisons the fish, and it is not the algae that use up the oxygen. The oxygen is used by bacteria respiring as they decompose the dead plants and algae.

Worked example: percentage decrease in oxygen

200 m upstream of the outflow the oxygen concentration is 9.4 mg/dm³. At 250 m downstream it is 3.0 mg/dm³.
Decrease = 9.4 − 3.0 = 6.4 mg/dm³
Percentage decrease = 6.4 ÷ 9.4 × 100 = 68.1%

Answer: about 68% decrease

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

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 a habitat for plants and animals.
  • Rotting waste releases methane, a greenhouse gas, and attracts vermin such as rats that can spread disease.
  • Toxic liquids and heavy metals from batteries and electrical goods can seep into soil and streams. Batteries should always be recycled.
  • Pesticides and herbicides can kill organisms other than the pests and weeds they are meant for. Litter is also land pollution.

Managing waste

  • Landfill; Advantages Cheap; easy for waste that cannot be recycled; Disadvantages Uses land; methane; toxic leaks; vermin
  • Incineration; Advantages Less land needed for landfill; heat can generate electricity; Disadvantages Releases carbon dioxide and other air pollutants near the incinerator
  • Reduce, reuse, recycle; Advantages Less waste to landfill; fewer raw materials dug up or cut down, so fewer habitats destroyed; Disadvantages Needs collection and sorting; recycling uses some energy

Reduce means buying and using less. Reuse means using an item again, e.g. refilling a glass jar or giving clothes to a charity shop. Recycle means changing waste into raw materials for new products, e.g. plastic bottles into fleece fabric. Reusing is usually better than recycling, because it needs no energy to reprocess the material.

Worked example: recycling saves landfill

A town sends 16 500 tonnes of waste a year to landfill. A food-waste composting scheme cuts this by 24%.
Saving = 0.24 × 16 500 = 3960 tonnes
New mass to landfill = 16 500 − 3960 = 12 540 tonnes per year

Answer: 12 540 tonnes per year

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 and releasing methane.

Revision questions

Try these, then check your answers below each one.

Question 1. The graph shows dissolved oxygen in a river above and below a sewage outflow. At which point, A, B, C or D, are bacteria using up oxygen fastest? (Choose from: A, B, C, D.)

Answer: C

Working: Oxygen is lowest at C (about 2 mg/dm³), where bacteria are respiring fastest as they decompose the sewage. Further downstream (D) the sewage has been broken down and oxygen dissolves back in.

Question 2. True or false? After fertiliser washes into a pond, fish die mainly because the fertiliser is poisonous to them.

Answer: False

Working: False. The fish suffocate: bacteria decomposing dead plants and algae use up the dissolved oxygen.

Question 3. The dissolved oxygen in a lake fell from 8.0 mg/dm³ to 2.8 mg/dm³ during an algal bloom. Calculate the percentage decrease.

Answer: 65 %

Working: 8.0 − 2.8 = 5.2. 5.2 ÷ 8.0 × 100 = 65%.

Related lessons

Continue in the full course

This lesson is section B17.2 (AGCSC1702) of our free GCSE Combined Science 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 Combined Science course.

 

 

 

 

 

 

 

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