Water pollution is not one substance. It is a long list of very different things — sewage, fertiliser, hot water, plastic, light, noise, even living species — that all end up in the same rivers, lakes and seas. If you can say where something came from and how it does its damage, you can answer almost any question on this topic.
📚 What you need to know
Pollution is anything added to the environment faster than it can be made harmless — a substance or a form of energy.
Pollutants reach water from point sources (one identifiable place, like a pipe) or diffuse sources (spread over a wide area, like farmland run-off).
The main aquatic pollutants: organic matter, inorganic nutrients, industrial effluent, urban run-off, solid waste, heavy metals, synthetic compounds, suspended solids, hot water, oil, radioactive material, pathogens, light, noise and invasive species.
Some pollutants are directly toxic. Others do their damage indirectly — by removing oxygen, blocking light or smothering habitats.
Point sources are easier to find, measure and control. Diffuse sources are the harder half of the problem.
The same pollutant can have very different effects depending on how much of it arrives and how quickly the water can dilute it.
What actually counts as a pollutant?
Most students picture chemicals. That is only part of it. A pollutant is anything released into a system at a rate the system cannot cope with, and that includes energy — heat from a power station, light from a coastal town, noise from a ship’s engine. It also includes living things: a species carried into a lake where it does not belong is a pollutant in exactly the same sense.
Definition to learn
pollution = matter or energy added to a system faster than the system can make it harmless
That last part matters. A river can break down a small amount of sewage without any lasting harm, because bacteria decompose it and the water re-oxygenates. Double the sewage and the same river turns anoxic. Nothing changed about the substance — only the rate at which it arrived.
If a question asks you to “explain why X is a pollutant”, do not just say it is bad. Say what it does to the organisms or the abiotic conditions, and say why the system cannot absorb it.
Point sources and diffuse sources
This is the single most useful split in the whole topic, and the source PDF barely mentions it. Ask one question of any pollutant: could you stand at the spot where it enters the water?
Point source — yes. A factory outfall pipe, a sewage works discharge, a leaking oil tanker. One location, one owner, one place to put a monitoring probe.
Diffuse (non-point) source — no. Nitrate washing off a hundred fields, oil and rubber rinsed off a whole city’s roads by rain, acid deposition falling on an entire catchment. No single entry point at all.
Monitoring a point source is a matter of putting a probe in the pipe. Measuring a diffuse source means sampling the river and working backwards.
Why examiners like this split: it explains why nitrate pollution is so stubborn. You can fine one factory. You cannot fine the rain.
The main types of aquatic pollutant
Learn these as a list of source and effect pairs. Almost every data-based question hands you a source and asks for the effect, or hands you an effect and asks you to suggest a source.
Pollutant
Where it comes from
What it does in the water
Organic matter
Untreated sewage, slurry from livestock, food processing waste, dead plant material
Feeds decomposer bacteria, which strip oxygen out of the water as they respire
Inorganic nutrients
Nitrate and phosphate from fertiliser run-off, sewage and detergents
Trigger algal blooms and eutrophication, which end in oxygen loss
Industrial effluent
Wastewater from manufacturing, processing and cooling
Mixed load of metals, solvents and warm water; often directly toxic to fish and invertebrates
Urban run-off
Rain washing over roads, car parks and roofs
Carries oil, brake dust, road salt, pet waste and litter straight into drains and rivers
Solid waste
Landfill leachate and litter dropped near waterways
Leaches chemicals into groundwater; traps and injures wildlife
Heavy metals
Mining, smelting, old paint, batteries, poor waste disposal
Mercury, lead, cadmium and arsenic build up in tissue and pass up food chains
Synthetic compounds
Pesticides, herbicides, medicines and industrial chemicals
Persist for years, disrupt hormones and reproduction, hard to remove once dispersed
Suspended solids
Soil erosion, construction sites, dredging
Cloud the water, clog fish gills and settle over gravel beds where eggs are laid
Hot water
Cooling water from power stations and heavy industry
Warm water holds less dissolved oxygen and speeds up respiration, so oxygen demand rises
Oil
Spills, bilge washing, leaks from rigs and pipelines
Forms a surface film that blocks gas exchange and ruins the insulation of fur and feathers
Radioactive material
Nuclear accidents, mining, badly stored waste
Long-term exposure risk, including mutation and cancer in wildlife and people
Pathogens
Untreated sewage and animal waste
Bacteria, viruses and parasites cause waterborne disease such as cholera and typhoid
Light
Street lighting, harbours, floodlit coastal developments
Breaks natural day-night cues used for feeding, breeding and navigation
Noise
Shipping, sonar, pile driving, offshore construction
Masks the sounds whales and dolphins use to communicate and find food
Invasive species
Ballast water, hull fouling, released aquarium stock
Outcompete native species and rewire the food web
Do not try to memorise fifteen rows word for word. Learn one clear example from each group and you will always have something to write.
Direct harm and indirect harm
Group the list a second way, because this is what turns a 2-mark answer into a 4-mark answer:
Directly toxic — mercury, pesticides, some industrial effluent. The chemical itself poisons the organism.
Oxygen removers — sewage, slurry, nutrients, hot water. Nothing here is poisonous. The oxygen loss does the killing.
Physical spoilers — suspended solids, oil, plastic. They smother, block light or trap, without any chemistry at all.
Sensory and biological disruptors — light, noise, invasive species. They change behaviour and community structure rather than chemistry.
Hold on to this diagram. The final page of this topic is built on exactly these four links.
🧩 How to attack a “sources of pollution” question
Name the source precisely. Not “farming” but “nitrate fertiliser applied before heavy rain”.
Say point or diffuse. One phrase, and it shows you understand the management problem.
Give the pathway. Surface run-off, groundwater, an outfall pipe, the atmosphere.
Give the effect on the water. Oxygen, light, toxicity or physical smothering — pick the right one.
Finish with an organism. Fish, invertebrates, aquatic plants, people. Marks live in the consequences.
Worked examples
WORKED EXAMPLE
A river runs past a dairy farm and then past a paper mill. Suggest one pollutant from each, and explain how each harms the river. [4]
Farm: pick the pollutant and the route in
Slurry and nitrate fertiliser, washed off the fields by rain — a diffuse source.
Effect: the organic matter feeds bacteria, whose respiration strips dissolved oxygen; the nitrate feeds algae, which shade the plants below.Mill: pick the pollutant and the route in
Warm effluent full of suspended fibre, leaving from one outfall pipe — a point source.
Effect: warm water holds less oxygen, and the suspended solids cloud the water and clog fish gills.Two sources, two mechanisms, two effectsThe mark scheme wants a mechanism, not just the word “pollution”.
WORKED EXAMPLE
Nitrate in a river measures 4 mg/L upstream of a village and 19 mg/L one kilometre downstream. Suggest why, and explain why it is hard to prove. [3]
Read the jump19 − 4 = 15 mg/L added along a single kilometre of river.
Suggest sources
A sewage discharge from the village (point), or fertiliser washing off nearby fields (diffuse).
Say why proving it is hardA point source can be sampled at the pipe. Diffuse run-off enters all along the bank, changes with rainfall, and lags days behind the day the fertiliser was spread.Rise of 15 mg/L; source likely diffuse
💡 Exam tip
Always label a source as point or diffuse. It is often a free mark and it shapes the rest of your answer.
Use real, named examples — nitrate from fertiliser, mercury from gold mining, ballast water from cargo ships. Vague answers score vaguely.
If asked for “impacts”, split them into ecological, human health and economic. Three headings, three easy marks.
Watch the command term. “Outline” wants a short chain; “explain” wants a mechanism; “evaluate” wants strengths and weaknesses.
Remember heat, light and noise are pollutants. Including one of them shows you understand the definition rather than a list.
⚠ Common mix-up
“Pollution” is not the same as “harm”. Something only counts as pollution when it arrives faster than the system can deal with it.
Confusing organic matter with inorganic nutrients. Sewage adds both, but the organic load feeds bacteria while the nutrients feed algae. Different first step, same oxygen loss.
Thinking suspended solids are toxic. They are not. They block light and clog gills — a physical effect.
Assuming a diffuse source is a small source. Diffuse run-off delivers far more nitrate to most rivers than any single pipe.
Forgetting groundwater. Pollutants can travel underground for years before surfacing, which is why a river can be polluted with no visible pipe anywhere.
Up next: Plastics in Aquatic Systems — where the plastic goes, why it gathers in the middle of oceans, and how it climbs the food chain.
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