The big idea: Water quality describes how clean and usable a body of fresh water is — its levels of nutrients, oxygen, sediment, salts and chemicals.
Water pollution is the addition of substances that lower that quality and harm life or human use. The biggest freshwater threats come from agriculture, settlements and industry.
The key process you must master is eutrophication — nutrient enrichment that triggers algal blooms and oxygen loss.
Key terms for water quality
- Eutrophication — enrichment of water with nutrients (nitrate, phosphate), causing algal blooms and oxygen loss.
- Algal bloom — a rapid surface growth of algae that blocks light and later uses up oxygen as it decays.
- Dissolved oxygen — the oxygen in the water that fish and insects need; it falls as algae decay.
- Dead zone — water so low in oxygen (hypoxic) that fish and other animals cannot survive.
- Salinisation — a build-up of salts in soil and water, often from over-irrigation in dry areas.
- Point source — pollution from one identifiable outlet (a factory pipe, a sewage works).
- Diffuse (non-point) source — pollution spread across an area, such as fertiliser washing off farmland.
Human vs physical causes: A pollution problem usually has both kinds of cause. Human causes are activities — fertiliser run-off, sewage, industrial waste. Physical causes are natural conditions that concentrate the problem — shallow, warm, slow-moving or enclosed water where nutrients build up and algae thrive.
How this is tested: The eutrophication stimulus is usually a line graph or table tracking nitrate, dissolved oxygen and algae down a river or across years — sometimes a shaded map of a spreading algal bloom. Short data marks ask you to Identify a change, Estimate a value or Describe the trend. Lock onto the mirror pattern (nutrients up, oxygen down) and always quote figures with units.
Read the key first. Which lines rise toward the lake, and which one falls?
Interactive diagram
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Using the graph, describe how water quality changes between the site above the farms and the lake inlet.
Model answer plan
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| Site | Nitrate (mg/l) | Dissolved oxygen (mg/l) | Algae cover (% of surface) |
|---|---|---|---|
| 1 — above the farms | 3 | 9 | 5 |
| 2 — beside the fields | 12 | 7 | 25 |
| 3 — below the fields | 28 | 4 | 60 |
| 4 — at the lake inlet | 31 | 3 | 80 |
Follow nitrate up, oxygen down: On a eutrophication dataset the signature is a mirror image: as nitrate (and algae) rise downstream, dissolved oxygen falls. Read both columns and pair the figures — that contrast is the answer to most 'describe the change' questions.
Lake Erie's algal blooms: Lake Erie, on the USA–Canada border, is shallow and warm, so nutrients from surrounding farmland concentrate easily. Satellite maps show its green summer algal blooms spreading far wider along the southern shore in recent years than two decades ago — a textbook eutrophication data-response.
Using the river data above: (a) state the nitrate level at the lake inlet; (b) describe how dissolved oxygen changes downstream; (c) identify the relationship between nitrate and algae cover.
Model answer plan
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Eutrophication has a clear chain of cause and effect. Nutrients enter the water, algae bloom across the surface, light is blocked, the algae die and bacteria use up the oxygen as they decay — leaving a low-oxygen dead zone where fish suffocate.
| Cause type | Example | Why it raises eutrophication |
|---|---|---|
| Human | Fertiliser run-off | Nitrate and phosphate wash off fields into the water |
| Human | Sewage and detergents | Add nutrients directly from settlements |
| Human | Industrial discharge | Warm, nutrient-rich effluent feeds algae |
| Physical | Shallow, warm water | Warmth speeds algal growth; sunlight reaches the bed |
| Physical | Slow / enclosed flow | Little flushing, so nutrients accumulate |
| Physical | Inflowing drainage | Streams concentrate run-off into one part of the lake |
Environmental impacts of eutrophication
- Oxygen depletion — decaying algae starve the water of oxygen, causing fish die-offs.
- Dead zones — large areas become uninhabitable for aquatic life.
- Loss of biodiversity — sensitive species disappear; the ecosystem simplifies.
- Surface choking — thick algae and weed block light and clog waterways.
Parts of a freshwater lake have become heavily eutrophic. Explain one human reason and one physical reason for this.
Model answer plan
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Always finish the chain: Don't stop at 'fertiliser run-off'. Carry it through: nutrients → algae bloom → algae decay → bacteria use oxygen → fish die. The development marks are in the chain, not the trigger.
Salinisation and pressure on wetlands: Salinisation is a build-up of salts in soil and water. In dry areas, over-irrigation raises the water table; as that water evaporates it leaves salt concentrated near the surface, poisoning crops and the water supply.
Wetlands are squeezed by agriculture (fertiliser run-off, drainage) and by altered water flow (abstraction, dams), both of which lower their water quality and biodiversity.
Explain one way that farming activity can lead to salinisation, and one way that salinisation harms farmers.
Model answer plan
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How this is tested — the [10] Examine essay: Paper 1 Option A ends with a 10-mark Examine essay, marked on markbands. Two recurring versions: how serious the different effects of agriculture on water quality are relative to one another (run-off, eutrophication, salinisation, sediment), and how much power different stakeholders hold when managing those effects.
Top band needs: accurate terms, two or more developed effects/stakeholders with examples, a weighing of relative importance / conflict, and a clear conclusion.
Examine how serious the different effects of agriculture on freshwater quality are relative to one another.
Model answer plan
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