3.4 Task 3: Science

The following extract is from The Open University course S154 Science starts here. Please read the text in the box below then answer the questions that follow it.

Keeping our water clean

Pollution from domestic sewage can lead to more than one sort of problem. Most obviously, sewage is a source of water-borne diseases such as cholera and typhoid fever. A second problem with sewage is that the bacteria that break it down, during a process called respiration, use dissolved oxygen from the water to do so. This same dissolved oxygen also supports the other aquatic life. The more sewage there is in the water, the more bacteria are required to break it down and the more dissolved oxygen they use, leaving less oxygen for fish and other aquatic animals. Once the oxygen in the water is used up, animals that need oxygen die. It isn't long before the water begins to smell distinctly unpleasant due to the gases released when the sewage begins to be broken down by bacteria that can live without oxygen. If you've ever stirred up the mud at the bottom of a stagnant pond, you will know exactly what we mean.

Agricultural activities pose problems through both crop spraying and the use of fertilisers. Crop spraying is carried out to prevent damage to crops from weeds and various forms of pests. However, such spraying can disperse herbicides and pesticides over a wide area so that they end up in water that drains into rivers and lakes. In high enough doses these pollutants may be toxic and, like lead and mercury, they can accumulate in the tissues of animals and so end up in our bodies too. Fertilisers contain nutrients, substances essential for the healthy growth of all plants and animals. However, when fertiliser seeps into lakes and rivers by drainage off the land it can lead to excessive plant growth. As a lake surface becomes covered with water weed, oxygen can no longer be dissolved in the water to replenish what is being used by fish and other aquatic animals. Once again, starved of oxygen, the animal life soon begins to die, and the water becomes stagnant and foul-smelling.

Having read the passage above, which of the following statements are true?

Please select either option (a) or (b) in each case, then compare your answer with those below.

The first one has been done for you as an example.

Question 1

  • (a) Cholera and Typhoid fever can be carried in water polluted by sewage.

  • (b) Oxygen released from sewage smells unpleasant.

  • (a) is correct.

  • (b) is incorrect, because the smell is caused by gases given off by bacteria which survive when there is no oxygen present.

Question 2

  • (a) Some bacteria use oxygen to break down sewage.

  • (b) Sewage accumulates in river mud, where it helps plant growth.


  • The correct answer is (a).

  • (b) is incorrect because the article does not suggest that sewage accumulates (builds up) in mud, nor that it helps plant growth.

Question 3

  • (a) Aquatic animals need nutrients from fertiliser for healthy growth.

  • (b) Drainage of fertilizer into lakes may pollute the water.


  • The correct answer is (b).

  • (a) is incorrect because nutrients from fertiliser actually cause aquatic animals to be starved of oxygen.

Question 4

  • (a) Fertiliser from farmland will break down sewage in rivers.

  • (b) Herbicide and pesticide concentration can build up in humans.


  • The correct answer is (b).

  • (a) is incorrect because the article does not describe any connection between fertiliser and sewage.

Question 5

  • (a) Oxygen can be dissolved in lake water.

  • (b) Fish and other aquatic animals cause water in a lake to stagnate.


  • The correct answer is (a).

  • (b) is incorrect because it is bacteria in sewage which use up the oxygen in the water.

Question 6

The first paragraph is best summarised by the sentence:

  • (a) Crop spraying, agricultural fertiliser, industrial waste and domestic sewage can all contaminate water.

  • (b) Sewage can pollute water both by introducing water-borne diseases and by reducing the amount of oxygen available for aquatic animals.


  • The correct answer is (b).

  • Although the statement in (a) is correct, paragraph one only discusses sewage; industrial waste is not mentioned in the extract.

3.3.2 Advice on reading and understanding academic texts

3.4.1 Self-assessment