Almost nobody sets out to pollute a river. Pollution is mostly a side effect — of growing food, making things, driving cars and flushing toilets. If you know where it comes in from, you already know most of what an exam can ask you about stopping it.
📚 What you need to know
A pollutant is something humans add to the environment in an amount, or at a rate, that causes harm. The same substance can be harmless at low levels.
A point source enters the water at one identifiable place, such as a pipe. A diffuse (non-point) source enters over a wide area, such as run-off from farmland.
Point sources are easier to find, measure, regulate and treat. Diffuse sources are the hard ones.
Aquatic pollutants include organic matter, nitrates and phosphates, industrial effluent, urban run-off, solid waste, toxic metals, synthetic compounds, suspended solids, heat, oil, radioactive material, pathogens, light, noise and invasive species.
Some pollutants harm life directly (they are toxic). Others harm it indirectly, usually by taking oxygen out of the water.
Rivers carry pollution from the land to the sea, so freshwater and marine problems are linked.
What actually counts as a pollutant?
It is tempting to think of pollutants as a fixed list of nasty chemicals. They are not. What matters is the amount and the rate. Nitrate is a plant nutrient that occurs naturally in every river; at low levels it does nothing but feed the ecosystem. Pour in enough of it and you get algal blooms and dead fish. The nitrate did not change — the quantity did.
That gives you a useful working definition to write in an exam:
Definition to learn
A pollutant is a substance or form of energy added by human activity in amounts the system cannot absorb without harm.
Notice the phrase “or form of energy”. Heat, light and noise are all pollutants in ESS even though you cannot put them in a bottle. Students forget this and lose easy marks.
If a question asks you to “identify a pollutant”, do not just say “chemicals”. Name it — nitrate from fertiliser, warm water from a power station, sediment from a construction site. Specific always scores better than vague.
Point sources and diffuse sources
This is the single most useful split in the whole sub-topic, because it decides how a pollution problem can be managed.
The volume of pollution may be similar in both panels. What differs is whether there is a single place to point at, which is exactly what a regulator needs.
Think about how you would police each one. For the factory you can take a sample from the pipe, compare it with the licence, and fine the company if it fails. For the farmland there is no pipe. The nitrate arrives with the rain, from hundreds of fields, and no single farmer broke a rule. That is why diffuse pollution is usually tackled by changing farming practice rather than by prosecution.
Exam-ready line: point sources are easier to regulate because the pollution can be traced to one identifiable discharge, sampled and legally controlled. Diffuse sources cannot, so management has to target land use across a whole catchment.
The main types of aquatic pollutant
You do not need to memorise all fifteen. You do need to be able to give a source and an effect for any one you are shown.
Bacteria decompose it and use up dissolved oxygen, so fish suffocate
Nitrates and phosphates
Fertiliser run-off, sewage, some detergents
Feed algae and water plants, driving eutrophication
Industrial effluent
Discharge pipes from factories and processing plants
Mixed load of metals, chemicals and warm water; often directly toxic
Urban run-off
Rain washing over roads, roofs and car parks
Carries oil, grit, rubber, pesticides and animal waste in one dirty pulse
Toxic metals
Mining, smelting, old landfill, industry
Build up inside organisms and rise up the food chain
Suspended solids
Soil erosion, dredging, construction sites
Block light, clog fish gills and bury the riverbed habitat
Heat
Cooling water from power stations and factories
Warm water holds less oxygen and shifts which species can survive
Pathogens
Sewage and animal waste
Spread waterborne disease such as cholera and typhoid
Light and noise
Coastal development, shipping, sonar, drilling
Disrupt feeding, navigation, breeding and migration in marine species
Direct harm and indirect harm
This distinction is worth more marks than it looks, because it explains why sewage kills fish even though sewage is not poisonous.
🧩 Two ways a pollutant does damage
Directly — it is toxic. Mercury, pesticides and some industrial chemicals damage organisms on contact or once eaten. Effects can appear quickly, and can build up over years in tissues.
Indirectly — it changes conditions. Organic matter and nutrients are not poisons. They trigger huge growth of bacteria or algae, and it is the oxygen those organisms use that kills everything else. Suspended solids and heat also work this way.
So when a data question shows fish disappearing below a sewage outfall, the answer is almost never “the sewage poisoned them”. It is: organic matter → bacteria multiply → aerobic respiration uses dissolved oxygen → oxygen falls below what fish can tolerate → fish die or move away. Write the chain and you get the marks.
EXAM PRACTICE
A river has low nitrate upstream of a farming valley and much higher nitrate 8 km downstream, but no discharge pipe was found. Explain the likely source and why it is difficult to control. [4]
Identify the source type
No pipe + rise along a farmed stretch = a diffuse (non-point) source, most likely fertiliser leaching and run-off from fields.
Explain the mechanism
Rain washes dissolved nitrate off and through the soil, so it enters along the whole bank rather than at one spot.
Explain the management problem
There is no single discharge to sample, licence or prosecute, and the input comes from many separate landowners.
Control needs catchment-wide changes: buffer strips, timing of fertiliser use, cover crops4 marks = 4 separate points. Source, mechanism, why it is hard, what would work.
💡 Exam tip
Always name the pollutant and the human activity behind it. “Nitrate from fertiliser applied to arable fields” beats “farming”.
If a question says “suggest”, you are allowed to use sensible reasoning — you do not need a fact from the syllabus, but you do need a clear chain of cause and effect.
Heat, light and noise count. Include one of them if a question asks for a range of pollutant types; examiners like to see you did not stop at chemicals.
Link freshwater to marine. Most ocean plastic and most coastal nutrient pollution arrives down a river.
For any “why is this hard to manage” question, check first whether the source is point or diffuse. That one word usually contains the answer.
⚠️ Common mix-up
“Pollutant” and “contaminant” used loosely. In ESS, harm is part of the definition. Something present but harmless at that level is not yet a pollutant.
Saying organic matter is toxic. It is not. It removes oxygen, and the lack of oxygen does the killing.
Calling farm run-off a point source because it comes “from a farm”. The test is whether it enters at one identifiable place, not whether you can name the industry.
Confusing the source with the effect. “Eutrophication” is not a source of pollution — excess nitrate and phosphate are, and eutrophication is what follows.
Forgetting invasive species. A living organism moved by ballast water counts as pollution in this syllabus.
Up next: Plastics in Aquatic Systems — the pollutant that does not break down, does not dissolve, and ends up in the middle of the ocean.
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