Water Pollution (Cambridge (CIE) O Level Biology) : Revision Note
Untreated Sewage & Excess Fertiliser
Human activities have led to the pollution of land, water and air
Pollution comes from a variety of sources, including industry and manufacturing processes, waste and discarded rubbish, chemicals from farming practices, nuclear fall-out, and untreated sewage
Effects of Pollution Table
Pollutant | Source/cause | Effect |
|---|---|---|
Untreated sewage | Lack of sewage treatment plants in inhabited areas (due to poor infrastructure/lack of money) means sewage runs/is pumped into streams or rivers | Provides a good source of food for bacteria which increase rapidly, depleting the oxygen dissolved in the water (as they respire aerobically) and causing the death of aquatic organisms such as fish; this is known as eutrophication. |
Chemical waste | Chemicals such as heavy metals like mercury can be released from factories into rivers and oceans or leach into land surrounding the factories. | Many heavy metals and other chemicals are persistent - they do not break down and so can build up in food chains (known as bioaccumulation), poisoning the top carnivores. |
Discarded rubbish (trash) | Much rubbish consists of plastic that is either discarded or buried in landfills | Much rubbish, such as that made from plastic, is non-biodegradable and remains in the environment for hundreds of years Animals also eat the plastic as it breaks into smaller pieces (especially in the ocean) and it can get into food chains this way |
Fertilisers | Runoff from agricultural land if applied in too high a concentration | Causes algal blooms which then die and provide a good source of food for decomposing bacteria which increase rapidly, depleting the oxygen dissolved in the water (as they respire aerobically) and causing the death of aquatic organisms such as fish; this is eutrophication. |
Nuclear fallout | Radioactive particles get into the environment from accidental leakage from nuclear power plants or the explosion of a nuclear bomb | Some radioactive particles have long half-lives and can remain in the environment for many years They can cause increased risks of cancer and smaller particles can be carried by winds hundreds of miles from the original site of exposure |
Methane | Cattle farming, rice fields, landfills | Methane is a powerful greenhouse gas which contributes to the enhanced greenhouse effect that is causing climate change |
Carbon dioxide | Produced when fossil fuels are burnt, and also released when trees are burnt to clear land for human use. | Carbon dioxide is a greenhouse gas which contributes to the enhanced greenhouse effect that is causing climate change. |
Eutrophication
Runoff of fertiliser (containing nitrates and other mineral ions) from farmland enters the water and causes increased growth of algae and water plants
The resulting ‘algal bloom’ blocks sunlight so water plants on the bottom start to die, as does the algae when competition for nutrients becomes too intense
As water plants and algae die in greater numbers, decomposing bacteria increase in number and use up the dissolved oxygen whilst respiring aerobically
As a result, there is less oxygen dissolved in water, so aquatic organisms such as fish and insects may be unable to breathe and may die
Eutrophication Diagram
The sequence of events causing eutrophication in lakes and rivers
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