What pollutes our water, and why is eutrophication such a damaging process?
The main water pollutants and their sources, the process and consequences of eutrophication, the use of indicator species and biological oxygen demand to monitor water quality, and methods of controlling water pollution.
A focused answer to AQA A-Level Environmental Science 3.4.3, covering the main water pollutants and their sources, eutrophication and its consequences, monitoring with indicator species and biological oxygen demand, and control methods.
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What this dot point is asking
AQA wants you to identify the main water pollutants and their sources, explain the process and consequences of eutrophication, describe how water quality is monitored using indicator species and biological oxygen demand, and describe methods of controlling water pollution. Command words are Describe and Explain, and the eutrophication chain is the single most examined sequence in this topic.
The main water pollutants
Eutrophication
The key chain to reproduce in full is: nutrients in, algal bloom, light blocked, submerged plants die, bacterial decomposition, dissolved oxygen depleted, aquatic life dies. Note that the bloom briefly raises oxygen by day through photosynthesis, but the later decomposition stage causes the lethal oxygen crash, which is why the timing matters.
Monitoring water quality
Indicator species give a picture integrated over time (they reflect conditions over weeks), whereas a BOD test gives a precise snapshot of the sample, so the two methods complement each other.
Controlling water pollution
- Treating sewage in works that remove organic matter (reducing BOD) and increasingly strip out nutrients before discharge.
- Reducing fertiliser runoff by applying only what crops need, avoiding application before rain, and leaving vegetated buffer strips beside rivers to trap runoff.
- Regulating industrial discharges with consent limits, and containing and cleaning up oil spills.
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 20185 marksDescribe the process of eutrophication and explain why it leads to the death of fish in a lake.Show worked answer →
Markers reward the ordered chain, with the final oxygen step essential for the fish-death link.
Nutrient enrichment: nitrates and phosphates from fertiliser runoff or sewage enter the lake. This causes a rapid growth of algae and surface plants, an algal bloom. The bloom covers the surface and blocks light, so submerged plants cannot photosynthesise and die.
Decomposition and oxygen: the dead algae and plants are decomposed by aerobic bacteria, whose populations grow rapidly and consume the dissolved oxygen in the water. Oxygen levels fall sharply, so fish and other aerobic organisms suffocate and die.
Full marks need the explicit step that fish die from oxygen depletion caused by decomposition, not from the nutrients themselves.
AQA 20215 marksExplain how indicator species and biological oxygen demand can each be used to assess the level of organic pollution in a river.Show worked answer →
Markers split this between the two methods, rewarding a clear link from measurement to pollution level.
Indicator species: certain organisms tolerate different oxygen levels, so their presence indicates water quality. Stonefly and mayfly nymphs need clean, well-oxygenated water, so their presence indicates low pollution; bloodworms and sludge worms tolerate low oxygen, so their dominance indicates heavy organic pollution.
Biological oxygen demand: a water sample is sealed and incubated, and the fall in dissolved oxygen over a set period (typically five days) is measured. A high biological oxygen demand means microorganisms are using a lot of oxygen to break down organic matter, indicating heavy organic pollution. Award the inverse link for indicator species and the high-equals-polluted point for biological oxygen demand.
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