Notes · Environmental ScienceUK · A-Levels

Pollution

This chapter examines pollution across air, water and land: what makes a substance a pollutant, how the major pollutants behave and are controlled, and the principles of pollution management. It covers the properties that determine a pollutant's severity, air pollution and its control technologies, organic water pollution measured by biochemical oxygen demand and shown by the oxygen-sag curve, land pollution and the waste hierarchy, and the framework used to manage pollution.

5 sections~17 min reading time3 competenciesLevel Standard 3 · Advanced 2

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Exam profile

AO1 · Describe the properties of pollutants, the major pollutants of air, water and land, and their controlAO2 · Apply the ideas of BOD, dilution and bioaccumulation to data and interpret an oxygen-sag curveAO3 · Analyse pollution data and evaluate control strategies and technologies

Operators:describeexplaincalculateanalyseevaluateinterpret

basic level

AS-Level expects you to describe the main pollutants of air, water and land, their effects and the main control methods.

higher level

The full A-Level requires you to interpret pollution data such as the oxygen-sag curve, work with BOD and biomagnification, and evaluate control strategies.

Depth

Text

Contents · 5 sections

Pollution

The properties of pollutants and factors affecting severity#

StandardAQA 7447 3.4

Biomagnification along a food chain

Fig. 1Biomagnification: the concentration of a persistent pollutant rises at each trophic level (illustrative concentrations).

A pollutant is a substance (or form of energy, such as heat or noise) released into the environment in amounts that cause harm. Pollutants are classified in useful ways: a primary pollutant is harmful as released (such as sulfur dioxide), whereas a secondary pollutant forms in the environment from primary pollutants (such as the ozone in photochemical smog); and a point source releases pollution from a single identifiable place (a pipe or chimney), whereas a non-point or diffuse source releases it over a wide area (runoff from many fields), which is much harder to control.

The severity of a pollutant depends on several properties. Its toxicity is how poisonous it is and at what dose; its persistence is how long it lasts before breaking down, so a persistent, non-biodegradable pollutant accumulates and spreads far, while a biodegradable one is broken down; and its mobility is how readily it moves through air, water or living things, determining how widely it spreads. A pollutant that is toxic, persistent and mobile, such as some pesticides, is especially dangerous because it lasts, travels and harms.

Two linked processes make persistent pollutants particularly serious in food chains. Bioaccumulation is the build-up of a pollutant within a single organism over time, because it is taken in faster than it is excreted or broken down. Biomagnification is the increase in the concentration of the pollutant along a food chain: each consumer eats many contaminated organisms and retains the pollutant, so its concentration rises at each trophic level, reaching the highest levels in top predators. This is why persistent pollutants often harm top predators most, even where the pollutant is dilute in the environment.

The harm a pollutant does also depends on the amount released, how it disperses and dilutes, and the sensitivity of the receiving environment. The same amount of a pollutant may be harmless when well diluted in a large river but damaging in a small stream, and some organisms and habitats are far more sensitive than others. This is why pollution control considers not only the pollutant but the pathway it takes and the target it reaches, an idea developed in the management section.

Worked example

Reasoning about biomagnification

A persistent pollutant is present in a lake at 0.001 ppm in the water but at 25 ppm in a fish-eating bird. Calculate how many times more concentrated it is in the bird, and explain the increase.

Compare the concentrations
Factor = $\dfrac{25}{0.001}$ .
Evaluate
$\dfrac{25}{0.001} = 25\,000$ times more concentrated in the bird than in the water.
Explain
The pollutant is persistent, so it is not broken down; it accumulates in each organism and its concentration rises at each trophic level (biomagnification), reaching its highest level in the top predator.

Result: The pollutant is 25 000 times more concentrated in the bird, because biomagnification raises it at each trophic level.

Typical mistakes

Confusing bioaccumulation (build-up in one organism) with biomagnification (increase along a food chain).
Assuming a dilute pollutant is always harmless; biomagnification can concentrate it to harmful levels in top predators.

Active revision

Explain why a persistent pesticide sprayed at low concentration on farmland can nevertheless kill fish-eating birds.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Air pollution and its control#

StandardAQA 7447 3.4

A catalytic converter

Fig. 2A catalytic converter turns carbon monoxide, nitrogen oxides and hydrocarbons into carbon dioxide, nitrogen and water.

The major air pollutants come mainly from combustion. Particulates (fine soot and dust) damage the lungs and reduce visibility; sulfur dioxide, from burning sulfur-containing coal and oil, irritates the lungs and forms acid deposition; nitrogen oxides, formed at high combustion temperatures, irritate the lungs, form acid deposition and contribute to smog; carbon monoxide, from incomplete combustion, is a toxic gas; and volatile organic compounds, together with nitrogen oxides in sunlight, form the secondary pollutant tropospheric ozone. These give three linked problems: direct health effects, acid deposition, and photochemical smog.

Acid deposition forms when sulfur dioxide and nitrogen oxides react with water in the atmosphere to form acids that fall as acidic rain, snow or dry particles, sometimes far from the source. It acidifies lakes and soils, killing fish and damaging trees, releases toxic aluminium from soils, and corrodes buildings. Photochemical smog forms when sunlight drives reactions between nitrogen oxides and volatile organic compounds from traffic to produce ground-level ozone and other irritants, worst in warm, sunny cities, especially during a temperature inversion that traps the pollutants near the ground.

Air pollution is controlled in three broad ways: prevention, dispersion and removal. Prevention reduces the pollutant at source, by using cleaner or low-sulfur fuels, improving combustion, switching to low-emission energy, and reducing demand; this is the most effective approach. Dispersion, such as tall chimneys, does not remove the pollutant but spreads it to reduce local concentrations, often moving the problem elsewhere. Removal uses end-of-pipe technology to capture pollutants after they form but before release.

Several removal technologies are widely used. Catalytic converters on vehicles convert carbon monoxide, nitrogen oxides and unburnt hydrocarbons into less harmful carbon dioxide, nitrogen and water as the exhaust passes over a catalyst. Flue-gas desulfurisation scrubbers remove sulfur dioxide from power-station gases by reacting it with an alkali such as limestone. Electrostatic precipitators remove particulates by charging them and attracting them to oppositely charged plates. Evaluating which control to use means weighing effectiveness, cost and whether it prevents the pollutant or merely captures or disperses it.

Worked example

Explaining a control technology

Explain how a catalytic converter reduces the pollutants in a car's exhaust, and state one limitation of relying on it.

How it works
The exhaust passes over a catalyst that speeds reactions converting carbon monoxide and hydrocarbons to carbon dioxide and water and nitrogen oxides to nitrogen.
Effect
The gases leaving are far less harmful than those entering, reducing carbon monoxide, nitrogen oxides and hydrocarbon emissions.
Limitation
It is removal, not prevention: it still produces carbon dioxide, does nothing about the fuel used, and works poorly until warm, so reducing vehicle use would tackle the problem at source.

Result: A catalytic converter converts the harmful gases to less harmful ones, but it is end-of-pipe removal that still emits carbon dioxide.

Typical mistakes

Confusing the causes of acid deposition (sulfur dioxide and nitrogen oxides) with those of photochemical smog (nitrogen oxides and VOCs in sunlight).
Treating tall chimneys as removing pollution; they only disperse it, moving the problem elsewhere.

Active revision

Explain how acid deposition forms and why its effects are often felt far from the source of the pollution.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Water pollution, BOD and the oxygen-sag curve#

AdvancedAQA 7447 3.4

The oxygen-sag curve

Fig. 3Dissolved oxygen sags below an organic discharge as bacteria respire, then recovers as the pollution is used up.

Much water pollution is organic: sewage, farm slurry and food-processing waste add large amounts of biodegradable organic matter to water. This does its damage indirectly, by feeding aerobic bacteria that decompose it and, in doing so, consume the dissolved oxygen that fish and other aquatic life depend on. The amount of oxygen the microorganisms need to break down the organic matter in a sample is the biochemical oxygen demand (BOD), usually measured as the fall in dissolved oxygen over five days of incubation in the dark; a high BOD indicates heavy organic pollution.

The effect of a single organic discharge on a river is captured by the oxygen-sag curve. Upstream of the discharge the dissolved oxygen is high; immediately downstream it falls sharply as bacteria multiply and respire to decompose the pollution, reaching a minimum, the sag; then, further downstream, as the pollution is used up, decomposition slows, oxygen dissolves back from the air and is added by photosynthesis, and the oxygen level recovers. Reading such a curve, identifying the discharge point, the sag and the recovery, and relating them to the biology, is a classic data-response task.

The changing oxygen level changes which organisms can live at each point, so indicator species reveal the level of pollution. Just below the discharge, where oxygen is very low, only tolerant species such as sludge worms and rat-tailed maggots survive; as oxygen recovers downstream, less tolerant species reappear, until clean-water species such as stonefly and mayfly nymphs return. Surveying the species present is therefore a biological way to assess pollution that integrates conditions over time, complementing a chemical measurement taken at one instant.

The severity of the effect depends on the load of organic matter relative to the river's capacity to dilute and reoxygenate it, which is why the same discharge harms a small, slow stream more than a large, fast river, and why sewage treatment aims to reduce the BOD of effluent before discharge, as seen in the hydrosphere chapter. Understanding BOD and the oxygen-sag curve ties together water pollution, sewage treatment and the eutrophication met in the cycles chapter.

\text{BOD} = D_{\text{initial}} - D_{\text{after 5 days}}

Biochemical oxygen demand

The fall in dissolved oxygen (in mg/l) over five days of incubation; a higher BOD means more organic pollution and greater oxygen demand.

Worked example

Calculating and interpreting BOD

A water sample has a dissolved oxygen of 9.0 mg per litre; after five days incubation in the dark it has fallen to 2.5 mg per litre. Calculate the BOD and comment on the water quality.

Apply the definition
$\text{BOD} = D_{\text{initial}} - D_{\text{after 5 days}} = 9.0 - 2.5$ .
$\text{BOD} = 9.0 - 2.5 = 6.5\ \text{mg/l}$
Interpret
A BOD of 6.5 mg per litre is fairly high, showing a substantial amount of biodegradable organic matter and therefore appreciable organic pollution.
Consequence
Such a demand would lower the dissolved oxygen of a small river and could harm oxygen-sensitive species.

Result: The BOD is 6.5 mg per litre, indicating appreciable organic pollution that would deplete oxygen in a small river.

Typical mistakes

Saying the bacteria add oxygen; the bacteria consume oxygen as they decompose the organic matter, causing the sag.
Reading a high BOD as a high oxygen level; a high BOD means a high demand for oxygen and so heavy pollution.

Active revision

A river is sampled above and at intervals below a sewage outfall. Describe and explain the pattern of dissolved oxygen you would expect to find.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Land pollution, waste and the waste hierarchy#

StandardAQA 7447 3.4

The waste hierarchy

Fig. 4The waste hierarchy ranks options from the most preferred (reduce) to the least (dispose).

Land is polluted by the solid and hazardous waste that society produces in ever greater quantities. Much waste has traditionally gone to landfill, where it is buried; but landfill takes land, and as waste decomposes it produces methane, a potent greenhouse gas, and a polluting liquid called leachate that can contaminate groundwater if it is not contained. Hazardous wastes, including toxic chemicals and heavy metals, need special treatment because they can poison soil and water and enter the food chain, linking land pollution back to bioaccumulation.

The preferred framework for managing waste is the waste hierarchy, which ranks options from most to least desirable. Reducing the amount of waste made in the first place is best, because it avoids the resource use and pollution of both making and disposing of a product; reusing items avoids the energy of remaking them; recycling recovers materials to make new products; recovering energy, for example by burning waste to generate electricity, extracts some value from what remains; and disposal to landfill is the last resort. The hierarchy embodies the principle of preventing waste rather than dealing with it after the fact.

Each option has a role and a cost. Recycling saves materials and energy, as seen for metals in the minerals chapter, but needs collection, sorting and clean streams of material; energy recovery by incineration reduces the volume of waste and generates power but must control the air pollution it produces; and modern landfills are engineered with liners and gas collection to limit leachate and to capture methane for energy. Evaluating how a community should manage its waste means applying the hierarchy while weighing the practical costs and impacts of each option.

The strength of the hierarchy is that it directs effort towards prevention, where the environmental benefit is greatest, rather than towards ever more sophisticated disposal. This connects waste management to the wider ideas of resource efficiency and the circular economy developed in the sustainability chapter, where keeping materials in use and closing loops is shown to be the most sustainable long-term approach to both waste and resources.

Worked example

Applying the waste hierarchy

A council currently sends most of its plastic waste to landfill. Using the waste hierarchy, recommend a more sustainable strategy.

Start at the top
First reduce plastic waste, for example by cutting single-use packaging, because avoiding waste has the greatest benefit.
Then reuse and recycle
Encourage reusable containers, and collect and recycle the plastics that remain into new products.
Last resorts
Recover energy from any non-recyclable plastic by incineration with pollution control, and send only the final residue to landfill.

Result: The council should move up the hierarchy: reduce first, then reuse and recycle, with energy recovery and landfill only as last resorts.

Typical mistakes

Ranking recycling above reduce and reuse; the hierarchy prefers not making the waste, then reusing, before recycling.
Treating incineration as pollution-free; it reduces waste volume and recovers energy but must control the gases it emits.

Active revision

Using the waste hierarchy, suggest how a household could move up the hierarchy for its packaging waste, from disposal towards reduction.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Principles of pollution management#

AdvancedAQA 7447 3.4

The source-pathway-receptor framework

Fig. 5Pollution can be controlled at the source, along the pathway, or by protecting the receptor; prevention at source is best.

Pollution is managed most effectively by thinking about the whole pathway from source to receptor, rather than only the pollutant itself. Critical pathway analysis follows a pollutant from where it is released, through the routes it takes in the environment, to the target it eventually reaches, so that the most vulnerable route and the most exposed organisms can be identified and protected. This approach shows where control will do most good, for example by treating a discharge, blocking a pathway, or protecting a sensitive receptor.

The relationship between dose and effect is central to setting safe limits. Many pollutants show a threshold, a dose below which no harm is detected, so a standard can be set to keep exposure below it; others, such as some carcinogens and radiation, are often treated as having no safe threshold, so any exposure carries some risk and the aim is to keep it as low as reasonably achievable. Understanding whether a pollutant has a threshold shapes how it is regulated.

Control follows a clear order of preference. Preventing pollution at source, by using cleaner processes, substituting less harmful substances, or reducing the activity, is better than controlling it after it forms; capturing or treating a pollutant before release is next; and dispersing or diluting it is the weakest option because it does not remove the pollutant. Legislation sets standards and limits and requires monitoring and enforcement, and economic tools such as taxes, charges and the polluter-pays principle give an incentive to pollute less. In practice a mixture of regulation, technology and economics is used.

Evaluating a pollution-control strategy means weighing its effectiveness against its cost and practicality, and asking whether it prevents pollution or merely moves it. A tall chimney reduces local pollution but disperses it more widely; a treatment plant removes a pollutant but uses energy and produces its own waste; a tax reduces pollution only if it is high enough and enforced. The strongest answers judge a strategy against the source-pathway-receptor framework and the order of preference, and recognise that prevention is usually both the most effective and, over time, the cheapest approach.

Worked example

Choosing a control point

A factory discharges a toxic chemical that reaches a fishery downstream. Using the source-pathway-receptor framework, identify options and recommend the best.

At the source
Change the process or substitute a less harmful chemical so the toxin is not produced or released, preventing the pollution entirely.
Along the pathway
Treat the effluent to remove the chemical before it enters the river, or divert it away from the fishery.
At the receptor
Protect the fishery, for example by relocating it, which does not stop the pollution. The best option is prevention at source, because it removes the problem rather than shifting it.

Result: Options exist at source, pathway and receptor; prevention at source is preferable because it removes rather than relocates the pollution.

Typical mistakes

Assuming dispersion or dilution solves pollution; it lowers local concentration but does not remove the pollutant.
Ignoring whether a pollutant has a threshold when judging what level of control is needed.

Active revision

Explain, using the source-pathway-receptor framework, three different ways a factory's toxic discharge to a river could be managed, and which is preferable.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

References & sources

Sources

AQA

AQA AS and A-level Environmental Science (7447) specification

Department for Education

GCE AS and A level subject content

Pollution

5 sections~17 min reading time3 competenciesLevel Standard 3 · Advanced 2

Pollution

Key points

Reasoning about biomagnification

Key points

Explaining a control technology

Key points

Calculating and interpreting BOD

Key points

Applying the waste hierarchy

Key points

Choosing a control point

Sources

Pollution

Key points

Reasoning about biomagnification

Key points

Explaining a control technology

Key points

Calculating and interpreting BOD

Key points

Applying the waste hierarchy

Key points

Choosing a control point

Sources