Water Pollution and Eutrophication

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 4.16 — Biological consequences of water pollution by sewage
• 4.17 — Eutrophication caused by leached minerals from fertiliser

Specification section 4(d) Human influences on the environment — also taught in The Carbon and Nitrogen Cycles, Air Pollution and the Greenhouse Gases, Global Warming and Deforestation.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Here is the fact that makes this whole topic counter-intuitive: neither sewage nor fertiliser poisons fish. Both are broadly harmless to them directly. What kills the fish in both cases is a shortage of oxygen, and the shortage is produced by bacteria — organisms nobody was thinking about, respiring so vigorously that they strip the dissolved oxygen out of the water.

SEWAGE — the chain from discharge to dead fish

1. Untreated or partly treated sewage is discharged into a river. It contains a great deal of organic material.
2. That organic material is food for DECOMPOSER BACTERIA, whose population increases rapidly.
3. The bacteria respire AEROBICALLY as they break the material down, using the oxygen dissolved in the water.
4. Because there are now so many of them, they use the dissolved oxygen faster than it can be replaced from the air.
5. The concentration of dissolved oxygen falls sharply.
6. Fish and other aerobic organisms cannot obtain enough oxygen for respiration, so they die or move away, and the number of species present falls.

Sewage brings a second and separate hazard, which is worth naming because it is a different mechanism entirely. It contains PATHOGENS — bacteria and viruses from human waste, causing diseases such as cholera and typhoid. If the water is used for drinking or washing, those diseases spread. That harm is direct infection and has nothing to do with oxygen, so an answer about sewage should keep the two apart.

EUTROPHICATION — the same ending, reached from fertiliser

1. Fertiliser containing nitrates is applied to farmland. If more is applied than the crop can absorb, or if heavy rain follows, the soluble nitrates are LEACHED — washed down through the soil and into a river or lake.
2. In the water, those nitrates are a mineral supply for algae, which had previously been limited by exactly that shortage.
3. The algae grow and reproduce rapidly, forming an ALGAL BLOOM that covers the surface.
4. The bloom BLOCKS THE LIGHT reaching the plants below, so those plants cannot photosynthesise and they die.
5. The dead plants — and later the algae themselves, which are short-lived — are food for decomposer bacteria, whose population rises rapidly.
6. The bacteria respire aerobically and use up the dissolved oxygen faster than it is replaced, so the concentration falls.
7. Fish and other aerobic organisms cannot obtain enough oxygen and die.

Compare the two chains and the structure becomes obvious. They begin differently — one adds food for bacteria directly, the other adds a nutrient that eventually produces food for bacteria — and they end identically, in the last three steps. Learn that shared ending once and you have the second half of both answers.

Think of it like a crowd rushing into a room with one small window

A room with a single window is comfortable for a handful of people. Let a hundred in and nobody has done anything wrong, nothing toxic has been introduced, and yet within minutes the air is unbreathable — because the rate at which the occupants use the oxygen has overtaken the rate at which the window can replace it. Both halves matter: the demand rose, and the supply could not follow. That is exactly the arithmetic of a polluted river, where the water surface is the window.

Two details make that supply side worse than it first appears. Oxygen dissolves into water only at the surface, and it diffuses downwards very slowly, so a deep or still body of water replaces its oxygen much less readily than a fast-flowing shallow one. And warm water holds less dissolved oxygen than cold water, so the same discharge does far more damage in summer. Both explain why the effects of pollution are worst in a slow, warm river in August.

A useful thing to know about rivers, because data questions rely on it: a river RECOVERS downstream of a single discharge. The bacteria eventually exhaust the organic material, their numbers fall, oxygen dissolves back in from the air, and species return in order — the most oxygen-demanding, such as mayfly nymphs and trout, being the last to reappear. Plotting oxygen concentration against distance downstream gives a curve that dips sharply at the outfall and climbs back over some kilometres, and the presence of particular INDICATOR SPECIES tells you how far along that recovery a given stretch is.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Eutrophication

The process in which mineral ions, especially nitrates leached from fertiliser, enrich a body of water, causing algae to grow rapidly and ultimately leading to a fall in dissolved oxygen and the death of aerobic organisms.

Statement 4.16 — the effects of sewage, the chain to write

1. Sewage discharged into a river adds a large amount of organic material.
2. Decomposer bacteria feed on it and their population increases rapidly.
3. The bacteria respire aerobically, using the oxygen dissolved in the water.
4. They use it faster than it can be replaced from the air, so the concentration of dissolved oxygen falls.
5. Fish and other aerobic organisms cannot obtain enough oxygen and die or move away, reducing the number of species present.
6. Separately, sewage also contains pathogens which cause diseases such as cholera and typhoid if the water is used for drinking or washing.

Statement 4.17 — eutrophication, the chain to write

1. Nitrates from fertiliser are leached from farmland into a river or lake.
2. The nitrates allow algae to grow and reproduce rapidly, forming an algal bloom on the surface.
3. The bloom blocks light from reaching the plants below, so they cannot photosynthesise and they die.
4. Decomposer bacteria feed on the dead plants and algae, and their population increases rapidly.
5. The bacteria respire aerobically and use up the dissolved oxygen faster than it is replaced.
6. The dissolved oxygen concentration falls, so fish and other aerobic organisms die.

The two routes compared — different beginnings, identical endings

Why the oxygen supply cannot keep up — the factors that make it worse

• Oxygen dissolves into water only at the SURFACE, and diffuses downwards slowly
• A deep or still body of water therefore replaces its oxygen far more slowly than a shallow, fast-flowing one
• WARM water holds less dissolved oxygen than cold water, so the same discharge does more damage in summer
• The bacterial population rises rapidly, so demand increases much faster than supply can respond
• A river RECOVERS downstream once the organic material is exhausted: the bacterial population falls while oxygen continues to dissolve in from the air, and species return in order of how much oxygen they need

What actually kills the fish — the point almost everyone gets wrong

A. NOT the sewage or the fertiliser, neither of which is poisonous to fish at these concentrations.

B. The DECOMPOSER BACTERIA, whose population rises and whose aerobic respiration removes the dissolved oxygen faster than it can be replaced. The fish suffocate.

Not this: Algae in an algal bloom use up all the oxygen, which is what kills the fish.

This: Algae photosynthesise and produce oxygen while they are alive. The oxygen is used by the BACTERIA that decompose them and the plants they shaded out, after the bloom has blocked the light and those plants have died.

Model answer [5 marks]

Explain how the discharge of untreated sewage into a river can kill the fish living in it. [5]

Sewage contains a large amount of organic material, which is food for decomposer bacteria, so their population increases rapidly. These bacteria respire aerobically, using the oxygen dissolved in the water. Because there are so many of them, they use the dissolved oxygen faster than it can be replaced from the air. The concentration of dissolved oxygen in the water therefore falls. Fish cannot obtain enough oxygen for aerobic respiration and so they die or move away.

Model answer [5 marks]

Explain how fertiliser applied to a field can cause the death of fish in a nearby lake. [5]

Soluble nitrates from the fertiliser are leached out of the soil by rain and washed into the lake. The nitrates allow algae to grow and reproduce rapidly, producing an algal bloom that covers the surface. The bloom blocks the light reaching the plants beneath, so they cannot photosynthesise and they die. Decomposer bacteria feed on the dead plants and algae and their population rises rapidly, and they respire aerobically, using up the dissolved oxygen faster than it can be replaced. The oxygen concentration falls and the fish cannot obtain enough for respiration, so they die.

Model answer [2 marks]

Suggest two ways in which a farmer could reduce the risk of eutrophication in a nearby river. [2]

The farmer could apply less fertiliser, or apply only as much as the crop can absorb, so that fewer nitrates remain in the soil to be leached. They could also avoid applying fertiliser just before heavy rain or when the ground is already waterlogged, and leave an unfertilised strip of land between the field and the river to intercept the run-off.

Mark-losing trap. The BACTERIA use up the oxygen, not the sewage and not the algae. Say which organism is respiring.

Mark-losing trap. Algae PRODUCE oxygen while alive. They cause harm by blocking light and then by dying and feeding bacteria.

Mark-losing trap. Say the bacteria respire AEROBICALLY — that is what connects their numbers to the falling oxygen.

Mark-losing trap. Sewage causes TWO separate harms: oxygen depletion, and disease from pathogens. Keep them apart.

Mark-losing trap. Nitrates are leached because they are SOLUBLE. That is why rain carries them down and out of the field.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · Identify [1 mark] — In a river polluted by sewage, which organisms use up the dissolved oxygen?
2. Grade 7 · Explain [2 marks] — In eutrophication, explain why the plants growing on the bed of a lake die after an algal bloom forms.
3. Grade 8 · Explain [5 marks] — Explain how nitrates leaching from a fertilised field can lead to the death of fish in a nearby lake. Select every statement that belongs in a full-mark explanation.
4. Grade 9 · Calculate [3 marks] — Upstream of a sewage outfall a river contains 9.6 mg of dissolved oxygen per dm³. Two kilometres downstream the concentration is 2.4 mg/dm³. Calculate the percentage decrease in dissolved oxygen concentration.
5. 9+ · Analyse [5 marks] — A river is sampled at intervals downstream of a single sewage outfall. Dissolved oxygen falls sharply just below the outfall, stays low for several kilometres and then rises steadily back to its original level. Bloodworms are abundant in the low-oxygen stretch; mayfly nymphs are absent there but present upstream and far downstream. Select every statement that belongs in a full-mark explanation.

The people behind this science

Two ways into the same idea — the one who proved dirty water was killing people, and the one who traced what runs off a field. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Florence Nightingale — the one who proved dirty water was killing people

Half of statement 4.16 is about pathogens in sewage causing disease, and Nightingale is the person who forced that connection into public policy. At Scutari she found that far more soldiers were dying of infectious disease than of wounds, and she proved it with data rather than assertion — inventing a diagram to display the causes of death month by month so that ministers who would not read a table could not miss the point. Her argument was about drains, water supply and ventilation, and it changed British hospitals permanently.

• “What was actually killing the soldiers at Scutari?”
• “Why did you present your evidence as a diagram rather than a table?”
• “How do you persuade people who do not want to be persuaded?”
• “What did you think was wrong with the water and the drains?”
• “How much of medicine is really about sanitation?”

Rachel Carson — the one who traced what runs off a field

This page's central mechanism is that something applied to a field arrives somewhere it was never intended to be, and Carson built her whole case on that idea. She traced pesticides from crops into streams, into the organisms living in them and up the food chains that followed — establishing that land and water are not separate systems and that whatever is spread on one arrives in the other. Leaching of nitrates is the same journey by a different substance.

• “How does something sprayed on a field end up in a river?”
• “Why did people assume that what was applied to land stayed there?”
• “What happens to a stream when the land around it is farmed intensively?”
• “How would you convince a farmer that the run-off matters?”

Carry on through the course

• Previous lesson: Global Warming and Deforestation
• Next lesson: Increasing Crop Yield
• Populations, Ecosystems and Sampling with Quadrats — Four nested words for where organisms live — and how to count a population you could never count in full.
• Biodiversity, Distribution and Limiting Factors — Why organisms are where they are and not somewhere else — and how to measure the variety of life in a place rather than just its quantity.
• Food Chains, Webs and Pyramids — Who eats whom, drawn three different ways — and why only one of the three can come out upside down.
• Energy Transfer Along a Food Chain — About nine tenths of the energy disappears at every step. Here is exactly where it goes — and what that means for how the world is fed.
• The Carbon and Nitrogen Cycles — The same atoms, used again and again — and the four bacteria that do the work nothing else can.
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification