IGCSE · Practice questions · Pollution

Six steps to a dead lake.

Six original Cambridge-style questions on water pollution and eutrophication, plastics on land and in the sea, and the sources and effects of carbon dioxide and methane, including oxygen data from a river.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Ions, growth, death, decay, respiration, suffocation.

Eutrophication (Extended): ions up, producers up, producers die, decomposers up, oxygen down, animals die.
The oxygen is used by decomposers, not by the algae.
Plastics: non-biodegradable; eaten, entangling, microplastics in food chains.
Carbon dioxide: fossil fuels and deforestation. Methane: cattle, rice, landfill.

Explain

[6 marks]

Extended. Fertiliser is washed from a field into a lake. Explain how this can lead to the death of fish in the lake.

The fertiliser increases the concentration of nitrate and other ions in the water. ✓
This causes increased growth of producers, especially algae (an algal bloom). ✓
Algae block the light, so plants below die; the algae also die later. ✓
Decomposers (bacteria) increase and break down the dead producers, so there is more decomposition. ✓
The decomposers carry out more aerobic respiration, so the dissolved oxygen concentration falls. ✓
Fish and other organisms that need dissolved oxygen die. ✓

Data

[4 marks]

Dissolved oxygen was measured at points along a river below a pipe releasing untreated sewage.

Distance downstream / m	0 (above pipe)	50	200	500	1000
Dissolved oxygen / mg per dm³	9.5	3.0	2.2	5.8	9.0

(a) Describe the changes in dissolved oxygen. (b) Explain why the oxygen falls below the pipe. (c) Suggest why it recovers further downstream.

(a) It falls sharply from 9.5 to 3.0 just below the pipe, reaches its lowest (2.2) at 200 m, then rises back to 9.0 by 1000 m. ✓
(b) The sewage provides organic matter, so decomposer bacteria multiply. ✓
Their aerobic respiration uses up the dissolved oxygen. ✓
(c) The organic matter is used up so fewer decomposers respire; oxygen dissolves from the air (and is produced by photosynthesis), so levels recover. ✓

Recall

[4 marks]

State the sources of carbon dioxide and methane from human activities, and describe how an increase in these gases causes climate change.

Carbon dioxide: burning fossil fuels; deforestation (burning trees, fewer trees photosynthesising). ✓
Methane: cattle (digestion), rice paddy fields, decaying waste in landfill. ✓
These gases absorb infrared (heat) radiated by the Earth and radiate it back, so more heat is trapped: the enhanced greenhouse effect. ✓
Global temperatures rise, causing climate change: melting ice, rising sea levels, extreme weather. ✓

Explain

[4 marks]

Describe the effects of non-biodegradable plastics in (a) aquatic and (b) terrestrial ecosystems.

(a) Animals such as turtles, seabirds and fish swallow plastic, blocking the gut so they starve. ✓
(a) Animals become entangled in nets and bags and drown or are injured; microplastics enter food chains. ✓
(b) Plastic litter spoils habitats and can trap or be eaten by land animals. ✓
(b) Plastic is not decomposed, so it builds up in landfill, and toxic chemicals can leak into soil and water. ✓

Analysis

[3 marks]

Explain why untreated sewage reduces the dissolved oxygen in a river faster than the same amount of excess fertiliser.

Sewage contains organic matter that decomposers can feed on straight away. ✓
So decomposer numbers and their aerobic respiration increase immediately, using up oxygen. ✓
Fertiliser only supplies ions; oxygen falls only after producers have grown, died and begun to decompose. ✓

Explain

[3 marks]

Suggest three ways to reduce eutrophication of rivers near farmland.

Apply fertiliser at the correct amount and time, not before heavy rain. ✓
Leave strips of uncultivated land or hedges beside rivers to absorb run-off. ✓
Treat sewage fully before releasing it into rivers (or use organic fertiliser such as manure carefully). ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.