The Eutrophication Disaster
Follow the journey of a nitrate ion — from a farmer's field to a city's water supply.
Ecosystem Overview
🌱 The Farm — Year of Good Harvest
It's spring. A farmer spreads NPK fertiliser across the fields. The crops grow tall and green. Everyone is happy. But something dangerous is brewing underground...
Chemistry Note
NPK fertiliser contains Nitrates (NO₃⁻), Phosphates (PO₄³⁻), and Potassium (K⁺). Nitrates are highly soluble in water — they don't stay in soil.
🔗 The Full Chain of Events
This is the complete sequence you must know for Paper 4. Click each step to reveal the chemistry.
🏭 Water Treatment: The Two-Stage Process
Stage 1: Filtration
Water passes through beds of sand and gravel. Solid particles (dead algae, debris, sediment) are physically trapped in the gaps between sand particles. This removes insoluble solids but does NOT kill bacteria.
Removes: Solid particles / Does NOT remove: dissolved chemicals or bacteria
Stage 2: Chlorination
Chlorine gas (Cl₂) or sodium hypochlorite is added to the filtered water. Chlorine kills microorganisms (bacteria, viruses) by disrupting their cell chemistry.
Removes: Bacteria and pathogens / Does NOT remove: dissolved nitrates
⚠️ Too much chlorine produces harmful chlorinated compounds (carcinogens). Careful dosing is required.
⚠️ EXAMINER TRAPS — These lose marks every year
✗ WRONG: "Fish die because of the fertiliser/nitrates in the water."
✓ CORRECT: Fish die because bacteria decompose dead algae, using up all dissolved oxygen. The nitrates are not directly toxic to fish at these concentrations.
✗ WRONG: "Eutrophication means the water becomes poisonous."
✓ CORRECT: Eutrophication is the process of water enrichment with nutrients, causing excessive algae growth, leading to deoxygenation. The water is not 'poisonous' — it simply runs out of oxygen.
✗ WRONG: "Filtration removes bacteria from water."
✓ CORRECT: Filtration removes solid particles only. Bacteria are removed/killed by chlorination. These are two separate processes.
✗ WRONG: "Phosphates cause eutrophication more than nitrates because they stay in soil."
✓ CORRECT: Actually this is partially correct — nitrates are the primary leaching problem precisely because ALL nitrates are soluble. Phosphates bind to soil and leach less. In rivers, nitrogen (nitrate) is usually the limiting nutrient.
🌍 The Big Picture: Everything Is Connected
This topic is Cambridge's favourite example of unintended consequences. A chemical that solves one problem (feeding crops) creates another (killing rivers). The examiner wants you to see the chain of cause and effect — from the field to the food chain to the water treatment plant.
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