What makes something a pollutant, and what properties decide how much harm it does?
The definition of pollution, the properties of pollutants that determine their impact (toxicity, persistence, bioaccumulation and biomagnification), and how pollutants are dispersed and degraded in the environment.
A focused answer to AQA A-Level Environmental Science 3.4.1, covering what pollution is, the properties of pollutants that determine their impact, and how pollutants are dispersed and broken down in the environment.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
AQA wants you to define pollution, explain the properties of pollutants that determine how much harm they do (toxicity, persistence, bioaccumulation and biomagnification), and explain how pollutants are dispersed and degraded in the environment. Command words are Explain and Discuss, so the marks lie in connecting a property to the harm it produces.
What is pollution
A crucial point for the definition mark is that a substance is only a pollutant when it causes harm; the same chemical may be a useful nutrient at low concentration and a pollutant at high concentration. Nitrate, for example, is an essential plant nutrient but becomes a pollutant when runoff causes eutrophication.
Properties that determine impact
Persistence and the tendency to dissolve in fat (rather than water) are what drive biomagnification, because fat-soluble substances are stored in body tissues rather than excreted. This is why organochlorine pesticides and mercury are so dangerous: they tick all three boxes of toxic, persistent and fat-soluble.
Bioaccumulation and biomagnification
A persistent toxic pollutant absorbed by a small organism is not excreted, so it accumulates in its body (bioaccumulation). When that organism is eaten, the predator takes in the pollutant from all of its prey, so the concentration increases up the food chain (biomagnification). Because a predator eats many prey items, each already carrying a load, the concentration can rise by orders of magnitude from the base of the food chain to the top. This is why top predators such as birds of prey and large fish were the worst affected by pesticides like DDT and by mercury pollution (as seen at Minamata).
Dispersal and degradation
Some pollutants are diluted and dispersed in air or water to harmless concentrations; gaseous emissions from a tall chimney, for instance, may be diluted before reaching the ground. Others are broken down (degraded) by sunlight (photodegradation), by bacteria (biodegradation) or by chemical reaction. Pollutants that are both persistent and toxic are the most damaging because they neither break down nor disperse quickly, so dilution and natural degradation cannot keep pace with their release.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksExplain the difference between bioaccumulation and biomagnification, using a named persistent pollutant.Show worked answer →
Markers award 2 marks for each correctly explained process tied to an example.
Bioaccumulation is the build-up of a persistent pollutant within a single organism over its lifetime, because the substance is absorbed or ingested faster than it can be excreted or broken down.
Biomagnification is the increase in concentration of that pollutant at each successive trophic level, so a predator that eats many contaminated prey accumulates the combined load, and top predators carry the highest concentrations.
Named example: DDT, a persistent organochlorine pesticide, bioaccumulated in fish and biomagnified up to birds of prey, thinning their eggshells. The discriminating point is within one organism (bioaccumulation) against up the food chain (biomagnification).
AQA 20226 marksDiscuss the factors that determine how much harm a pollutant causes once it is released into the environment.Show worked answer →
A 6-mark Discuss expects several factors developed with reasoning, not a bare list.
Toxicity: how poisonous the substance is per unit dose; highly toxic pollutants cause harm at very low concentrations.
Persistence: how long it remains before breaking down; persistent pollutants spread widely and accumulate, whereas biodegradable ones are removed quickly.
Bioaccumulation and biomagnification: substances that are fat-soluble and poorly excreted build up in tissues and concentrate up food chains, so even low environmental concentrations can become lethal in top predators.
Dispersal and degradation: pollutants that are diluted or broken down by sunlight or bacteria do less harm; those that resist both are most damaging. A strong answer concludes that the most dangerous pollutants combine high toxicity with high persistence and a tendency to biomagnify.
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