06 April 2014

#150 Overuse of fertilisers, water pollution by sewage

It is very tempting for farmers to increase the amount of fertilisers applied to crops to try and increase crop yields. However, this can lead to the eutrophication of rivers and lakes and the sequence occurs.  

1. Overuse of fertilisers
  • fertilisers (very soluble) are easily leached out of the soil
  • fertilisers are washed into a water system (river or lake)
  • algae absorb fertiliser and grow rapidly (algal bloom)
  • algae form a blanket on the surface of the water, blocking light from algae below
  • algae die without light
  • bacteria decompose dead algae, using up O2 in the water for respiration
  • animals in water die through lack of O2.
Fishermen row a boat in a algae-filled lake in China.
 Credit: Totallycoolpix.com

A second effect of overuse of fertilisers can be the death of the plants. High concentrations of the fertilizer around plant roots can cause the roots to lose water by osmosis. The plant then wilts and dies. 

2. Sewage 

Sewage can result in eutrophication in a similar way to overuse of fertilisers. This is because sewage contains high levels of nutrients such as phosphates, organic matter and bacteria. The phosphates act as fertilisers for algae, while the bacteria feed on the organic matter and reproduce rapidly, using up O2 in respiration.

If sewage is untreated before disposal it can lead to disease organisms such as cholera and typhoid being transmitted in the water. 

3. Chemical waste 

Chemical waste such as heavy metal (mercury, nickel…) and oil can cause serious pollution. Some chemicals may be dumped (or enter water systems through leaching) in low concentrations, at which levels they are not toxics. However, bioaccumulation occurs if they enter a food chain. Plankton absorb the chemical and has no mechanism for excreting it. Animals such as small fish, feeding on large number of plankton, build up the chemical because, again, they have no means of excreting it.

Animals, including humans, at the top of the food chain, eat many fish and accumulate high concentration of the chemical, which is now toxic. Poisons such as mercury damage the central nervous system and can lead to death.

Seabirds are severely affected by oil spills.

When oil is dumped into water it can form a surface layer, coating animals such as birds that feed in the water. When the birds try to clean their feathers they swallow the oil, which poisons them. Oil also disrupts food chains.

Try this

Figure below shows part of a river into which sewage is pumped. The river water flows form W to Z, with the sewage being added at X.

Some of the effects of adding sewage to the river are shown in picture below. 

Try this

1. Describe the changes in the levels from W to Z of:

i) nitrates                                                                                [2 marks]
ii) suspended solid                                                                  [2 marks]

2. Suggest why the level of O2:

i) drops at X                                                                          [1 mark]
ii) increase again toward Z                                                     [1 mark]

3. Suggest 2 reasons why levels of algae drop:
i) when sewage is added to the river                                      [2 marks]
ii) towards Z                                                                         [2 marks]
4. A farm at Z used herbicides on the field next to the river. Suggest why this could cause further problems in the river.                               [1 mark]


1. i) Constant level between W and X, or starts off quite low, or at point X it start to drop then increases toward Y before dropping again toward Z.

   ii) Level starts off quite low, then at point X it increases sharply; level returns nearly to original level between Y and Z.

2. i) One suggestion from:
- aerobic respiration by sewage fungus
- lack of algae to produce O2

 ii) One suggestion from:
- lack of sewage fungus
- photosynthesis by alge.

3. i) Two suggestions from:
- presence of suspended solids blocks ligh for algae
- lack of nitrate in the water
- possible presence of toxins in sewage
- possible increase in temperature or unsuitable temperature.

ii) Two suggestions from:
- shortage of nitrates
- grazing by aquatic herbivores
- possible drop in temperature , or unsuitable temperature.

4. One suggestion from:
- herbicides could leach into river and kill algae
- herbicides will kill algae and disrupt food chain
- herbicides may be toxic to other organisms in the river.

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