Hi Tara. Water is the biggest topic on the 0680 syllabus — six sub-topics where most topics have four — and it is also the one that turns up most often in Paper 2, because water gives examiners exactly what they want: a diagram to label, a table of figures to compare, and a management decision with two defensible sides.
Here is the shape of it. 3.1 is where water is and how people get hold of it: the cycle, the sources, treatment, desalination, dams. 3.2 is what we put into it and what that does. 3.3 is the two diseases the syllabus names — malaria and cholera — and how each is controlled. 3.4 is fishing and fish farming. 3.5 is oil. 3.6 is plastic. Notice that 3.2 to 3.6 are all the same shape: causes → impacts → strategies. Once you spot that, the topic gets much smaller than it looks.
Three things carry more marks here than anything else, and they are all precision rather than volume. First, the water cycle is examined as named stores and named transfers — you have to produce the actual words, and “water goes up” earns nothing where “evaporation” earns one. Second, ordered chains are marked in order: water treatment, desalination and malaria transmission each have a fixed sequence and you lose marks for scrambling it. Third, bioaccumulation and biomagnification are two different words for two different things, they appear as separate objectives, and the examiner is checking whether you know which is which.
One habit to build from the start. Every time the syllabus says “discuss the benefits and limitations” — and in this topic it says it four times — it is asking for a balanced answer with a judgement at the end. A page of benefits and one line of limitations is not a balanced answer. So for each of those I have given you a two-column table, and I have added two extra columns you should always be thinking about even when the table does not print them: who bears the cost, and over what timescale.
The command words in this topic, and what each one is asking for
Identify — take the answer straight off a source. It is the standard opener on a source-led Paper 2 question: name the ocean at letter C, name the largest bar, name the year of the peak. One or two words, plus the figure and its units if the source carries one. No explanation is wanted and none is credited.
State — give the fact from memory, in a phrase. Like identify, it wants no reasoning: “screening, sedimentation, filtration, chlorination” is a complete answer to “state the stages of water treatment”.
Describe — say what happens or what the data show, with figures where there are figures. For a graph that means direction, turning point and size of change, all three.
Calculate — produce a number, and write the working down. In this topic almost every calculation is a percentage change, a difference or a ratio, and the rule for percentage change never varies: divide the change by the starting value, then multiply by 100. Write the division out even when you can do it in your head, give the answer to a sensible number of figures, and carry the units through. If the arithmetic slips, visible working is the difference between nothing and nearly everything.
Explain — give the mechanism, not the description. Every sentence should contain a because or a so that.
Compare — put both halves in one sentence. “Region North has 41% while region East has 92%” is a comparison; two separate descriptions are not.
Suggest — this is the one to understand properly, because it appears eight times in this guide and it is the command word 0680 uses when it wants you to apply what you know to material you have never seen. There is no single remembered answer. You are given an unfamiliar context — a council’s proposal, a district’s figures, a coastline — and asked what would follow, or what could be done, or why something might be so. A good “suggest” answer names something specific, ties it to a detail in the source, and gives the reason: “extend sewage treatment in the North region, because at 12% it is the lowest figure in the table and sewage is the route by which cholera spreads”. That is AO3, and AO3 is 40% of Paper 2, so this command word is worth more marks than any other in the subject.
Discuss the benefits and limitations and evaluate — both want two sides and then a judgement. Name who gains, who pays and over what timescale, then say what the decision turns on.
An honest note about mark allocations. Where this guide says something like “aim to include this” or “spread your answer across the four stages”, that is my judgement about how a question of that shape is likely to be marked. I have not checked those statements against a published Cambridge mark scheme, and you should treat them as advice on how to write a full answer rather than as facts about marking.
3.1 Water Sources and Supply
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▶ Watch: Water sources, treatment, desalination and dams
Opens on YouTube in a new tab; nothing loads from Google on this page. These are supporting explainers, not 0680 lessons — the syllabus is the authority on what is in scope. Two are badged for 0680 but they follow the old chapter numbering, so “Chapter 4” is this topic. Be aware of three gaps: nothing here covers the five oceans (3.1.3) or the comparison of water availability between regions (3.1.6), and the dam entry is an engineering documentary that shows what a multipurpose dam is like rather than teaching the syllabus list of eight uses. Use Sources A, B and the dam questions below for those.
The Water Cycle: Five Stores and Nine Transfers
Start with the distinction the syllabus is built on, because every mark in 3.1.1 depends on it. A store is a place where water sits. A transfer is a journey water makes from one store to another. Stores are nouns you could point at on a map; transfers are processes with a beginning and an end. If a question gives you a diagram with boxes and arrows, the boxes are stores and the arrows are transfers, and mixing the two up is the fastest way to lose a whole question.
The syllabus lists the stores in two groups, and the grouping itself is worth a mark, so learn it as two groups rather than as one list of five.
| Group | Stores the syllabus names | What to say about it |
|---|---|---|
| Salt water | the oceans | By far the largest store of water on Earth, and unusable as it is — you cannot drink it, irrigate with it or run most industry on it without treating it first. |
| Fresh water | ice sheets and glaciers | The largest store of fresh water, but frozen, and mostly a long way from where people live. |
| ground water | Water held in the pore spaces and cracks of rock below the surface. Slow to fill, slow to move, and out of sight, which is why it is easy to over-use. | |
| the atmosphere | Water vapour and cloud droplets. A tiny store by volume, but the whole cycle passes through it, so it turns over very fast. | |
| lakes and rivers | The surface fresh water people actually use most. Small as a store, but easy to reach. |
The trap in the store list
The atmosphere and lakes and rivers are small stores that people depend on heavily; ice sheets and the oceans are enormous stores that people can barely touch. A question that asks “suggest why fresh water shortages occur even though most of the Earth is covered in water” is testing exactly this. The answer is not “there is not much water” — it is that most of the water is salt water, and most of the fresh water is frozen or underground, leaving only a small fraction that is liquid, fresh and reachable.
Now the nine transfers. Read them as a loop — water leaves the sea, travels through the air, falls on land, and finds three different routes back — because a loop is much easier to reproduce under pressure than nine separate definitions.
| Transfer | What is actually happening |
|---|---|
| evaporation | Liquid water at the surface of the sea, a lake or wet soil gains enough energy to become water vapour and enters the atmosphere. |
| transpiration | Water taken up by plant roots is lost as vapour from the leaves. It is the same journey as evaporation but it goes through a plant on the way. |
| condensation | Rising water vapour cools, changes back to tiny liquid droplets and forms cloud. |
| precipitation | Water leaves the atmosphere and falls to the surface as rain, snow, sleet or hail. |
| interception | Precipitation is caught by vegetation — leaves, branches, the canopy — before it ever reaches the ground. |
| surface run-off | Water flows over the top of the ground, downhill, into streams, rivers and lakes. |
| infiltration | Water soaks downwards from the surface into the soil. |
| through-flow | Water moves sideways through the soil, downslope, without reaching the deep rock. |
| ground water flow | Water moves slowly sideways through the saturated rock below the water table, eventually reaching a river or the sea. |
All five stores and all nine transfers on one picture. Trace the loop with your finger a few times — sea, air, cloud, land, and then three separate ways back.
Three ways down, three ways along
Once precipitation has landed, water has exactly three routes, and they differ only in how deep it is travelling. Over the top → surface run-off. Through the soil → through-flow. Through the rock below the water table → ground water flow. They also differ in speed, and that is the follow-up mark: surface run-off is fast (hours), through-flow is slower (days), ground water flow is slowest of all (years to centuries). So if a question asks why a river keeps flowing weeks after the rain stopped, the answer is through-flow and ground water flow still arriving.
Two of these are worth an extra sentence because they get confused. Interception is not precipitation. Precipitation is the fall from cloud to surface; interception is what happens when that fall is stopped by a leaf on the way. And transpiration is not evaporation, even though the water ends up in the same place — aim to include “from the leaves of plants”. If you are asked what happens to the water cycle when a forest is cleared, both of those give you marks: less interception and less transpiration, therefore more water reaching the ground and more surface run-off.
Worked example
The diagram below shows the water cycle with the stores numbered 1 to 5 and the transfers lettered A to I. Name store 3 and store 5, and name transfers C, D, G and I. [6]
Cover the labelled version above before you try this.
The two stores
Store 3 is ground water — it is the band below the dashed water table, inside the rock. Store 5 is lakes and rivers. Both are fresh water stores.
The four transfers
C is precipitation (from cloud down to the surface). D is interception (arriving on the tree canopy, not on the ground). G is infiltration (straight down from the surface into the soil). I is ground water flow (sideways, below the water table, towards the sea).
Where the marks go missing
Writing “rain” for C instead of precipitation is risky — precipitation is the word the syllabus uses, and snow is precipitation too. Writing “run-off” for G is the common error: G points downwards into the ground, and run-off travels across the top of it. And writing “through-flow” for I loses the mark, because I is below the water table in rock, while through-flow (H) is above it in soil.
3 = ground water; 5 = lakes and rivers; C = precipitation; D = interception; G = infiltration; I = ground water flow.
Where People Get Their Fresh Water
Objective 3.1.2 is a short list and an easy four marks, but it is graded — each source is harder and more expensive to use than the one before it. Learn it in that order and the list doubles as an argument.
| Source | Named examples | What using it involves |
|---|---|---|
| atmosphere | rain and snow | Collect it directly — roof catchment, tanks, snowmelt. Free, but only as reliable as the weather, and there is nowhere to draw from in a dry spell. |
| surface water | rivers, lakes and reservoirs | Easy to reach and easy to move in quantity, which is why most large cities use it. Also the easiest to pollute, since anything upstream ends up in it. |
| ground water | aquifers and wells | Available in dry regions and naturally filtered by the rock, but it refills slowly, so it can be pumped out faster than it is replaced. |
| oceans | desalination plants | Effectively unlimited, and the only source that does not depend on rainfall — but it has to be manufactured, which costs energy and money. |
The Five Oceans
Objective 3.1.3 asks you to identify the world’s oceans, usually on a blank map. There are five and the syllabus names them: Pacific, Atlantic, Indian, Southern and Arctic.
PAISA
Pacific, Atlantic, Indian, Southern, Arctic. Five letters, five oceans. The two that get forgotten are the last two: the Southern Ocean rings Antarctica, and the Arctic is the small one at the top around the North Pole. If you can place the Pacific (biggest, between Asia and the Americas), the Atlantic (between the Americas and Europe/Africa) and the Indian (south of Asia, east of Africa), the other two are the poles.
Potable Water and How It Is Made
Learn this one word for word
Potable water is water that is safe to drink. That is the whole definition and it is exactly what the syllabus says. Note what it does not say: it does not say pure, and it does not say clean. Potable water still contains dissolved substances — that is fine. “Pure water” is a chemistry answer and it will not score here.
Making water potable is a four-stage chain, and the syllabus lists the stages in order. That order is not arbitrary: you take out the biggest solids first, then the medium ones, then the fine ones, and only kill the pathogens at the end — because chlorine works far better in water that is already clear, and dosing dirty water simply wastes it.
Screening, sedimentation, filtration, chlorination. Biggest first, pathogens last.
| Stage | What it removes | How |
|---|---|---|
| 1 screening | large floating objects | The water passes through a mesh or grid that holds back sticks, leaves, litter and anything else of that size. |
| 2 sedimentation | heavier suspended solids | The water is held still in a tank so that the denser particles sink to the bottom and are drawn off as sludge. |
| 3 filtration | fine suspended solids | The water passes down through beds of sand and gravel, which trap the small particles that would not settle. |
| 4 chlorination | bacteria and other pathogens | Chlorine is added in a controlled dose to kill the micro-organisms that cause disease. |
Command word check
“State the stages of water treatment [4]” wants four words in the right order. “Describe the stages of water treatment [4]” wants the four names plus what each one does — and with four stages against four marks, aim to say something about each — a beautiful paragraph on chlorination and nothing on the other three leaves three quarters of the question unanswered. Spread your effort evenly across the marks available.
Who Has Potable Water and Who Does Not
Objective 3.1.6 asks you to compare availability, and it names three comparisons. “Compare” means you must put both halves in the same sentence — “X is higher than Y” — not describe one and then the other.
| Comparison | What tends to be true | Why |
|---|---|---|
| water-rich and water-poor regions | Regions with high, reliable rainfall or large rivers and aquifers have far more water available per person than arid regions and those with a long dry season. | Supply follows climate and geology. A region can also be water-poor because its rain arrives in a few weeks and there is nowhere to store it. |
| urban and rural regions | Within one country, a much higher proportion of urban households usually have a piped, treated supply than rural households, who may rely on wells, standpipes or surface water. | Pipes, treatment works and sewers are cheaper per person where people are packed together. Serving scattered villages costs more per household and is usually built last. |
| global inequalities in availability and sewage treatment | The gap in sewage treatment is wider than the gap in water supply. Many places have improved drinking water long before they have treated sewage. | A tap is visible and popular; a sewage works is expensive, invisible and its benefit is shared. So it is politically the easier thing to postpone — and postponing it feeds straight back into water-borne disease. |
This is the link question
3.1.6 exists so that examiners can join 3.1 to 3.2 and 3.3. The chain to have ready is: no treated sewage → sewage enters rivers and wells → the water people drink carries pathogens → cholera and other water-borne disease. Whenever you are given data on sanitation coverage and asked to “suggest” a consequence, that chain is the answer.
Desalination
Desalination is removing the salt from sea water to make it potable. The syllabus gives you two methods and it gives the steps of each almost word for word, so these are recall marks. Give them as steps.
distillation
1. water is heated until it boils2. the salt remains behind in the liquid3. the steam given off is pure water4. the steam is cooled and condensed to form potable water
reverse osmosis
1. water is put under high pressure2. it is passed through a membrane3. the membrane allows water molecules to pass through but stops most ions and other molecules
The difference in one line: distillation separates by boiling point and needs heat; reverse osmosis separates by molecule size and needs pressure. That is also why their costs differ — heating a large volume of water is energy-hungry, whereas pressurising it is less so, which is why most modern large plants use reverse osmosis.
| Benefits of desalination | Limitations of desalination |
|---|---|
| Provides a supply of potable water in regions with very little rainfall, ground water or surface water. | Building the plant and running it is expensive, so it is out of reach for many of the countries that most need water. |
| The supply does not depend on rainfall, so it is reliable during drought and does not vary from season to season. | It uses a large amount of energy. Where that energy comes from fossil fuels, it adds to greenhouse gas emissions and air pollution. |
| The oceans are effectively an unlimited source, so the supply cannot be used up in the way an aquifer can. | The waste left over is brine — very salty water, often warm. Returning it to the sea raises salinity locally and can harm organisms there. |
| It takes pressure off rivers and aquifers, letting over-used fresh water sources recover. | Plants must be on the coast, so inland regions need long pipelines, adding cost and further energy for pumping. |
Who bears the cost, and over what timescale
Use this on every “discuss” question in this topic. For desalination: the benefit goes to the water users, immediately. The financial cost falls on the government or the customer, immediately and then permanently as running costs. The environmental cost of brine and emissions falls on the coastal ecosystem and on everybody, and it accumulates over decades. Stating who gains and who pays is what turns a list into a discussion, and it is where the top band starts.
Multipurpose Dams
“Multipurpose” is the key word: one structure doing several jobs at once. The syllabus names eight uses and you should be able to give them all.
| Use | How the dam does it |
|---|---|
| flood control | The reservoir holds back peak river flow after heavy rain and releases it slowly, so the flood does not reach settlements downstream. |
| hydro-electric power | Water released under pressure turns turbines, generating electricity without burning fuel. |
| irrigation | Stored water is released to farmland through canals, allowing cultivation in the dry season. |
| storage of water | The reservoir holds a supply for domestic and industrial use through periods of low rainfall. |
| transport | The deep, slow water of the reservoir can be navigated by boats where the original river was too shallow or too fast. |
| recreation | Sailing, fishing and swimming on the reservoir for people living nearby. |
| tourism | The dam and reservoir attract visitors, bringing income and jobs to the area. |
| fish farming | Cages or pens in the still water of the reservoir produce a supply of fish. |
Getting all eight out
Group them: two about controlling water (flood control, storage), two about producing something (hydro-electric power, fish farming), two about the land around (irrigation, transport), two about people visiting (recreation, tourism). Four pairs is much easier to hold than eight separate items, and if you can only recall three pairs you have still given six.
Now the discussion, and this is one of the genuinely contested trade-offs on the syllabus. Give both sides properly. A dam is not simply good or bad; it moves benefits and costs between different groups of people and between different points in time.
| Benefits of a multipurpose dam | Limitations of a multipurpose dam |
|---|---|
| Generates electricity continuously without burning fossil fuels, so it reduces emissions from the power sector. | Construction is very expensive and takes many years, and the money is often borrowed. |
| Protects settlements and farmland downstream from flooding, which saves lives and property. | Flooding the valley displaces the people who lived there, who lose homes, land and often a way of life. |
| Stores water through the dry season, making a reliable supply for homes, industry and irrigation. | Habitats in the flooded valley are lost, and the reservoir replaces a river ecosystem with a lake one. |
| Irrigation allows more than one harvest a year, raising food production and farm incomes. | Silt that used to be carried downstream settles in the reservoir instead, so fields below the dam become less fertile and the reservoir slowly fills up. |
| Creates jobs during construction and afterwards in tourism, fishing and maintenance. | The dam blocks the movement of migratory fish, and reduced flow downstream can affect water quality and the delta. |
| Recreation and tourism bring income to a region that may have had few other options. | Where a river crosses a border, one country storing the water reduces what reaches the next, which can cause disputes. |
How to finish a dam question
The judgement sentence is where the last marks are, and it should name the groups. Something like: “The benefits are national and long-term — electricity, flood protection and irrigation for millions — while the heaviest costs fall on a small number of people in the flooded valley, immediately and permanently. Whether the dam is justified depends on how the displaced communities are compensated and resettled, and on whether the sediment loss downstream is managed.” That names who gains, who pays, over what timescale, and what the decision turns on. That is the shape of a strong ending.
Data response
Reading a water availability table
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A country reports the following. Region P: rainfall 1850 mm per year, 96% of households with piped treated water, 71% with treated sewage. Region Q: rainfall 410 mm per year, 44% of households with piped treated water, 9% with treated sewage. Region Q is largely rural; region P contains the two largest cities.
(a) Calculate how many times greater the proportion of households with treated sewage is in P than in Q. [1]
(b) Suggest two reasons for the difference in piped water coverage. [2]
(c) A minister argues that region Q should be given a desalination plant. Evaluate that proposal. [4]
(a) Calculate how many times greater the proportion of households with treated sewage is in P than in Q. [1]
(b) Suggest two reasons for the difference in piped water coverage. [2]
(c) A minister argues that region Q should be given a desalination plant. Evaluate that proposal. [4]
(a)
71 ÷ 9 = 7.9 times (accept about 8). Show the division — the working carries the mark if you slip on the arithmetic.
(b)
Any two of: Q is rural, so households are scattered and the cost of pipes per household is much higher; Q has far less rainfall, so there is less surface water to treat and supply; P contains the largest cities, where infrastructure is built first and where more people can be served by one treatment works; P is likely to have more investment because more of the country’s economic activity is there.
(c) — the two sides
For: Q is dry (410 mm) so rainfall and surface water are unreliable, and desalinated supply does not depend on rainfall at all; it would relieve pressure on any aquifer Q is using; and it would raise the 44% coverage figure. Against: desalination is expensive to build and to run, and Q is rural, so the water would still have to be piped to scattered households — the pipes may cost more than the plant; a desalination plant must be on the coast, and if Q is inland it is not even possible; and Q’s worst figure is sewage treatment at 9%, which desalination does nothing about.
(c) — the judgement
A conclusion that scores: “Desalination would address supply but not the larger problem shown in the data, which is sanitation. Spending the same money on sewage treatment and distribution in Q would reduce water-borne disease more, and more cheaply. Desalination is worth considering only if Q is coastal and its water shortage is severe enough that no cheaper source exists.”
The move to copy
In part (c) the strongest answers used the numbers in the stem. “9% sewage treatment” is the piece of evidence that turns a general opinion into an evaluation. Always look for the figure that the proposal fails to address.
Source A
Water and sanitation in the five regions of one country
A table is the commonest source on Paper 2 and it is the one this guide had none of. Work through every part on paper before you press Show Answer. Reading a model answer you have not attempted feels like learning and is not.
| Region | Mean annual rainfall / mm | People with a piped treated water supply / % | People with treated sewage / % | Population / millions |
|---|---|---|---|---|
| North | 380 | 41 | 12 | 6.2 |
| East | 1750 | 92 | 68 | 14.5 |
| South | 2240 | 88 | 55 | 9.8 |
| West | 620 | 57 | 23 | 7.4 |
| Central | 1120 | 74 | 42 | 11.1 |
Figures for one year. The regions are labelled by compass direction only; nothing here depends on which country it is.
identify(a) Name the region with the lowest percentage of people who have a piped treated water supply, and give that percentage. [1]
describe(b) Describe the relationship between mean annual rainfall and the percentage of people with a piped treated water supply. Quote figures from the table. [3]
calculate(c) Calculate the number of people in the North region who have no piped treated water supply. Show your working. [2]
calculate(d) Calculate how many times greater the percentage with treated sewage is in the East than in the North. [1]
explain(e) In every region the percentage with treated sewage is lower than the percentage with piped treated water. Explain why. [3]
suggest(f) The government can improve one service in one region. Suggest which, and justify your choice using the table. [4]
Source A — question 1
Look at Source A. Which region has the largest gap between the percentage of people with a piped treated water supply and the percentage with treated sewage?
A. You have to work out five differences before you can answer, which is exactly what a 'largest gap' question is for: West 57 − 23 = 34, South 88 − 55 = 33, Central 74 − 42 = 32, North 41 − 12 = 29, East 92 − 68 = 24. B is the near miss at 32, and it is the answer you get if you eyeball the columns rather than subtract. C has the second largest gap and both of its numbers are big, which makes it look worse than it is. D has the lowest figures in the table but one of the smallest gaps — low is not the same as unequal, and that distinction is worth a mark on its own.
Source A — question 2
Using Source A, the number of people in the East region who have no treated sewage is closest to:
C. 68% have treated sewage, so 100 − 68 = 32% do not, and 14.5 × 0.32 = 4.64 million. Two steps, and the first one is the one people skip. A is 68% of 14.5 — the people who do have it, which is the commonest slip in this kind of question. B is 32% of the North region's population instead of the East's. D is the whole population of the region. Always ask yourself whether the percentage you have been given is the group the question wants.
(a)
North, at 41%. Give the number as well as the name — 'identify' on a source-led paper means take the value off the source, and a name on its own often does not earn the mark.
(b)
The general relationship is positive: the regions with more rainfall have a higher percentage supplied. North has the least rain at 380 mm and the lowest coverage at 41%, while East has 1750 mm and 92%. But it is not perfect, and saying so is worth a mark. South is the wettest region at 2240 mm and yet its coverage is 88%, below East at 1750 mm. So rainfall is not the only thing that decides supply.
(c)
100 − 41 = 59% have no supply. 6.2 × 59 ÷ 100 = 3.66 million people (about 3.7 million). Write both steps down; the working carries the mark if the arithmetic slips.
(d)
68 ÷ 12 = 5.7 times (accept about 6).
(e)
Three ideas, and any two or three of them will do. A tap is visible and personal — the household that pays for it gets the benefit — while a sewer is buried and its benefit falls on everyone downstream, so it is the easier thing for a government to postpone. Sewers and treatment works are also more expensive per household than a supply pipe, because the waste has to be collected as well as delivered and then treated before release. And a water supply can be built one standpipe at a time, while a sewerage system only works once a whole network exists.
(f) — the two sides
The strongest case is treated sewage in the North region, at 12%, the lowest figure anywhere in the table. Against it: the North region has the lowest water coverage at 41% and the least rain at 380 mm, so it has the largest absolute number of people without a supply — 3.7 million by part (c) — and water supply is the more immediate need. The honest answer sets one against the other.
(f) — the judgement
A conclusion that scores names the choice, the evidence and the reason: “I would extend treated sewage in the North region. At 12% it is the worst figure in the table, sewage is the service that carries the risk of cholera, and the largest gap between water and sewage coverage anywhere is 34 percentage points in the West region — the pattern across every region is that sanitation is the neglected half. Water supply matters more for the number of people affected, so a government with more money should do both, in that order.”
(a) North, 41%. (c) 3.66 million. (d) 5.7 times.
Source B
A world map to label, and a shaded key to read
3.1.3 asks you to identify the five oceans and that is normally done on a blank map, so here is one. The same map carries the water-availability shading for 3.1.6. Work through every part on paper before you press Show Answer. Reading a model answer you have not attempted feels like learning and is not.
Schematic and not to scale. Cover the answer, name the five letters, then read the key before you attempt part (b).
identify(a) Name the ocean marked by each of the letters A to E. [5]
state(b) State the class shown on the map for Australia, and give the range of values that class covers. [2]
describe(c) Describe the pattern of water availability shown on the map. [3]
explain(d) A region can have plenty of rain and still be short of potable water. Explain how. [3]
suggest(e) Suggest two reasons why availability can differ sharply between two parts of the same country. [2]
Source B — question 1
On Source B, which ocean is marked by the letter D?
B. D lies in the band of water that runs right around the bottom of the map, below every continent and above the ice — that is the Southern Ocean, and it is the one candidates most often leave out of the list of five. A is C, between Africa, southern Asia and Australia. C is E, in the band across the top. D is B, between the Americas and Africa. Learn the five as a shape on a map rather than as a list, because 3.1.3 says 'identify', and identify usually means on a diagram.
Source B — question 2
Source B shades Europe in the middle class. According to the key, that means each person there has:
D. Read the key, not the colour you expected. The middle band on Source B is 1700 to 5000 cubic metres of renewable fresh water per person per year. A is the top band, used here for northern North America, South America and northern Asia. B is the bottom band, used for north Africa, southern and western Asia and Australia. C treats a class boundary as a single value, which a shaded map never means. On a source-led paper the key is part of the source, and quoting its wording back is what turns a description into evidence.
(a)
A = the Pacific Ocean (the widest, on the far side of the Americas), B = the Atlantic Ocean (between the Americas and Europe and Africa), C = the Indian Ocean (between Africa, southern Asia and Australia), D = the Southern Ocean (the band that runs right round the bottom), E = the Arctic Ocean (the band across the top). The Southern and the Arctic are the two most often missed, and the syllabus names all five.
(b)
Australia is shown in the lowest class, which the key gives as under 1700 cubic metres of renewable fresh water per person per year. Quote the key's own wording; a colour is not an answer.
(c)
Availability is very uneven. The lowest class covers north Africa, southern and western Asia and Australia; the highest covers northern North America, South America and northern Asia; Europe, southern North America and central and southern Africa sit in the middle. Notice that the low band runs roughly across one belt of the world rather than being scattered at random, and that neighbouring parts of the same continent can be in different classes — northern Asia is in the highest class and southern and western Asia in the lowest.
(d)
Availability is not the same as usable supply. Rain that arrives in a few weeks and runs off is not available for the rest of the year unless there is storage. Water that is present may be polluted, so it is not potable until it has been treated. And water may be there but not reach people, because there is no pipe network to distribute it — which is why the table in Source A separates rainfall from the percentage of people supplied.
(e)
Any two of: one part is urban and one rural, and pipes cost far more per household where houses are scattered; rainfall differs across a country, as Source A shows; the water may sit in an aquifer or a river at one end of the country and the people at the other; investment follows economic activity, so the wealthier region is served first.
A = Pacific, B = Atlantic, C = Indian, D = Southern, E = Arctic. Australia is in the lowest class, under 1700 cubic metres per person per year.
Checkpoint 3.1
Answer, then read the explanation even when you were right.
Your Score
0 / 23
Question 1
Which of these is a store of fresh water in the water cycle?
B. A store is a place water sits; ice sheets and glaciers are the largest store of fresh water on Earth. A, C and D are all named in the syllabus, but every one of them is a transfer — a journey between two stores. That is the whole point of the first objective: the same list splits into two kinds of thing, and questions are written to see whether you can split it.
Question 2
A river is still flowing three weeks after the last rain fell anywhere in its catchment. Which transfer best explains this?
A. The three routes water takes after landing differ mainly in speed: surface run-off arrives within hours, through-flow within days, ground water flow over years. Only the slowest one is still delivering water three weeks later. B is tempting because run-off does feed rivers — but it would have finished long ago. C restates the stem: the question says no rain has fallen. D takes water out of the route to the river rather than adding to it.
Question 3
Rain lands on the leaves of a forest canopy and never reaches the soil below. Which transfer is this?
C. Interception is precipitation caught by vegetation before it reaches the ground. A is the commonest wrong answer: precipitation is the fall from the cloud, and it has already happened by the time the drop hits the leaf. B is right that a plant is involved but wrong about the direction — transpiration is water leaving the leaf as vapour, not arriving on it. D is the wrong surface: infiltration is water soaking into soil.
Question 4
Which statement matches the syllabus definition of potable water?
A. The definition is four words: water that is safe to drink. B is the chemistry answer and it is wrong here — tap water contains dissolved calcium, chloride and much else, and is still potable. C describes water that has been screened, settled and filtered but not yet chlorinated: clear, and still capable of causing cholera. D is close but it invents a condition the syllabus does not state; safety, not a list of what is absent, is the test.
Question 5
Why is chlorination placed at the end of water treatment rather than at the start?
B. Suspended solids shield micro-organisms and use up the chlorine, so dosing dirty water wastes most of it. Take the solids out first, then disinfect. A confuses a dissolved gas with a suspended solid — chlorine dissolves, it does not sink. C imagines a mesh that stops a dissolved substance; a screen only holds back sticks and litter. D is invented chemistry. The real reason is about efficiency, and the order of the four stages is the mark.
Question 6
In desalination by reverse osmosis, the sea water is:
D. The membrane lets water molecules through and stops most ions and other molecules; the pressure is what pushes the water across it. A is distillation — a real method, but the other one, so it is the answer to a different question. B is not on the syllabus at all. C is the water treatment chain from 3.1.5, which makes fresh water potable but removes no salt whatever.
Question 7
Which is a limitation of desalination rather than a benefit?
C. Brine is warm, very salty waste; discharging it raises the salinity around the outfall and can harm the organisms living there. The other three are all genuine benefits, and they are the three the syllabus itself points to, so learn them: independence from rainfall, relief for over-used fresh water sources, and an unlimited source. Being able to sort a mixed list into benefits and limitations is exactly what a “discuss” question needs you to do first.
Question 8
A farmer whose land lies downstream of a new dam finds his fields less fertile than before. The most likely reason is that:
D. Silt settles in the still water of the reservoir instead of being deposited on the floodplain below, so downstream fields stop being renewed. The same process slowly fills the reservoir, which is a second mark. A is true of many dams but it is about land upstream, not his. B is invented — turbines transfer energy, they do not filter. C describes salinisation, which is a real problem of irrigation but takes years and would not follow simply from building a dam.
Question 9
Which list names all five of the world’s oceans?
B. The five the syllabus names are Pacific, Atlantic, Indian, Southern and Arctic — PAISA. A is the near-miss to watch for: the ocean around Antarctica is called the Southern Ocean, and “Antarctic Ocean” is not one of the five, so this list has the Southern Ocean twice and no Arctic. C and D each swap in a sea rather than an ocean; the Mediterranean and the Caribbean are both seas, and neither is on the list.
Question 10
In many countries a smaller share of households has treated sewage than has treated drinking water. Which is the best explanation?
A. A tap is visible and each household gains from its own; a sewage works is expensive, buried and its benefit falls on everyone downstream, so it is the easier thing for a government to postpone. That is why the sanitation gap is usually wider than the water supply gap. B confuses sewage works with desalination plants, which do have to be coastal. C invents a shortage. D is the wrong reason for a real pattern — rural coverage is lower because households are scattered and pipes cost more per home, not because there is too little sewage.
Question 11
Which of the four sources of fresh water named in the syllabus is reached by drilling a well?
C. The syllabus lists four sources and pairs each with how it is reached: atmosphere as rain and snow, surface water as rivers, lakes and reservoirs, ground water as aquifers and wells, and the oceans through desalination plants. A well is a hole sunk to below the water table, so it takes water out of the rock. A, B and D are all genuine sources on the same list, which is the point of the question — you are being asked to match the source to its route, not to recognise a word.
Question 12
A city has almost no rainfall and no usable river, so it builds a plant that supplies drinking water from the sea. Which source of fresh water is it using?
B. The oceans are a source of fresh water for people, but only once the salt has been taken out, which is why the syllabus writes the source as 'oceans: desalination plants' rather than just 'oceans'. A is what the city would do if it had an aquifer, and it may well be doing that too — but the stem says the water comes from the sea. C needs a river to fill it. D needs rain, and the stem says there is almost none. Match the source to what the stem actually supplies.
Question 13
What does screening remove at the first stage of water treatment?
D. A screen is a mesh, and a mesh holds back things big enough to be caught by it. That is why it goes first: taking the branches out protects everything downstream of it. A is sedimentation, B is filtration and C is chlorination, so all four options are real stages of the same process in the right order — the question is whether you can attach the right job to the right name. If you can say what each stage removes, you can answer any version of this.
Question 14
In sedimentation the water is:
B. Sedimentation does nothing to the water except stop it moving. Once the flow stops, anything denser than water settles out as sludge and is drawn off the bottom. That is also why it needs large, quiet tanks and time. A is filtration, C is chlorination and D is screening. Notice the pattern in the four stages: the first three take out solids in order of size, biggest first, and only the fourth attacks living things.
Question 15
Filtration is placed after sedimentation because:
D. The order of the four stages is itself examinable, and the reason is practical every time: each stage is made easier by the one before it. Sending unsettled water straight into a sand bed would block it almost at once. A puts chlorination in the wrong place — it is last. B invents a step that is not in the syllabus at all. C is a real distinction but it belongs to desalination: an ordinary sand filter removes suspended solids, not dissolved ions.
Question 16
In desalination by distillation, the potable water is produced when:
A. Distillation separates by boiling point. The water boils, the salt does not, so the salt stays behind in the liquid and the steam that leaves is pure water — cool it and you have your product. B has it exactly backwards, and it is the commonest error in the whole sub-topic: the salt is what stays. C is reverse osmosis, which separates by molecule size using pressure instead of heat. D is not on the syllabus. Learn the pair as heat-and-boiling-point against pressure-and-size.
Question 17
A dam turns a shallow, fast-flowing river into a deep, still lake. Which two uses come from the water now being deep and still, rather than from storing water behind the dam or from the drop in height at the dam?
Deep water can be navigated where a shallow river could not, and still open water is what sailing, swimming and fishing for pleasure need. Flood control, irrigation and drinking water all come from storing water behind the dam, and electricity comes from the drop in height at the dam — real uses of a multipurpose dam, but not ones that follow from the water being deep and still. That is the difference between recalling a list and using it.
Question 18
How does a multipurpose dam provide flood control?
C. A reservoir is storage, and storage turns a short, high flow into a long, low one. The flood water still arrives; the dam decides when it leaves. A is wrong upstream, where the level rises a great deal. B is not something a dam can do — the rain falls whatever is built downstream of it. D is a real effect of a large reservoir but it is far too small and slow to control a flood, and it is a cost of dams rather than a use of them.
Question 19
Which of these is not one of the eight uses of a multipurpose dam given in the syllabus?
A. The eight are flood control, hydro-electric power, irrigation, storage of water, transport, recreation, tourism and fish farming. Desalination is not among them, and it would make no sense there anyway — a river reservoir already holds fresh water, so there is nothing to remove. B, C and D are three of the eight, and they are the three most often forgotten, because people stop at power and water. Recreation and tourism are separate entries: recreation is what people do on the lake, tourism is the money visitors bring.
3.2 Water Pollution
▼
▶ Watch: Water pollution, eutrophication and bioaccumulation
Opens on YouTube in a new tab. Supporting explainers, not 0680 lessons. The first is a walk-through of past-paper questions rather than a taught lesson. The two acid-rain videos go further into chemistry than 3.2.2(e) needs — the syllabus asks only for the effect on organisms in rivers and lakes.
Where the Pollution Comes From
Five sources, and the syllabus is closed — that is the whole list. It is worth noticing that they fall into two kinds: things people throw away (domestic waste, sewage, plastic waste) and things that leak out of production (industrial processes, agricultural practices). Sorting them that way makes them easier to recall and gives you a sentence of structure at the start of a long answer.
| Source | What actually reaches the water | How it gets there |
|---|---|---|
| domestic waste | Detergents, cleaning products, cooking oil and fat, household chemicals, food waste, litter. | Down the drain from kitchens and bathrooms, or from household rubbish that is dumped or blown into rivers and drains. |
| sewage | Human faeces and urine, which carry pathogens and a heavy load of organic matter and nutrients. | Untreated or partly treated discharge from drains and outfalls, overflowing latrines, and pipes that leak into ground water. |
| plastic waste | Bottles, bags, packaging, fishing gear and fragments broken off larger items. | Dropped or dumped on land, carried by drains and rivers to the sea, or lost directly from boats and coastal sites. |
| industrial processes | Toxic substances such as heavy metals, oils, acids and warm water from cooling. | Discharged from pipes into rivers and estuaries, spilled at sites, or leached out of waste tips into ground water. |
| agricultural practices | Fertilisers (nitrates and phosphates), pesticides, and slurry and manure from livestock. | Washed off fields by surface run-off, or leached down through the soil into ground water after rain or irrigation. |
Two words worth using precisely
Run-off is pollution washed over the surface into a river. Leaching is pollution carried downwards through the soil by water, usually ending up in ground water. Naming the route, not just the substance, is worth doing: “nitrate fertiliser is leached from the field into the ground water” scores more than “fertiliser gets into the water”.
Impact 1: Water-borne Disease
Where sewage reaches drinking water, the bacterium that causes cholera can be swallowed. Cholera produces severe watery diarrhoea and vomiting; the danger is dehydration, and it can develop within hours. Because it spreads through contaminated water and food, one polluted well can infect a whole settlement, and an outbreak feeds itself: infected people produce more infected waste, which returns to the same water. The control strategies are in 3.3.3, and the link to 3.1.6 is direct — where sewage treatment coverage is low, the risk of infectious bacterial disease is high.
Impact 2: Toxic Substances — and the Two Words That Get Confused
Industrial processes release toxic substances such as heavy metals into rivers, lakes and the sea. Many of them are persistent: they are not broken down quickly, and they are not easily excreted, so once an organism takes one in it tends to keep it. That single property is what drives everything that follows.
Now the two words. They are separate objectives, they are examined separately, and students lose marks by treating them as synonyms. Read the syllabus wording carefully — the difference is stated in it.
| bioaccumulation | biomagnification | |
|---|---|---|
| Where it happens | Inside one single organism. | Along a food chain, from one trophic level to the next. |
| What increases | The amount of the toxic substance in that organism, as time passes. | The concentration of the toxic substance, as you move up the levels. |
| The axis to picture | Time. An old individual holds more than a young one of the same kind. | Trophic level. A top predator holds more than the animals it eats. |
| Why it happens | The substance is taken in faster than it can be broken down or excreted, so it builds up. | Each predator eats many prey, and takes in all of the toxin each prey had accumulated. |
| One-line definition | The build-up of a toxic substance in a single organism over time. | The increase in concentration of a toxic substance as it is passed up a food chain. |
Accumulate = one. Magnify = many.
Bioaccumulation is what happens in one body over its lifetime — think of a bank account that only ever receives deposits. Biomagnification is what happens between bodies, up the chain — the predator collects every prey animal’s account into its own. The two are not rivals: biomagnification only works because bioaccumulation happened first in every organism at every level. If a question gives you 6 marks for “explain how a top predator ends up with a high concentration of a pesticide”, you need both words and that sentence linking them.
One pollutant, four trophic levels. Check the axis before you read any value off it.
Worked example
Using the figure above: (a) calculate how many times greater the concentration is in the tertiary consumers than in the producers; (b) explain why the concentration rises as it does. [4]
(a) the arithmetic [1]
48 ÷ 0.04 = 1200 times. Read the values off the bar labels rather than trying to measure the bars, because the axis is logarithmic — the bar for 48 ppm is nowhere near 1200 times the length of the bar for 0.04 ppm, and anyone who measures will get this badly wrong.
(b) the mechanism [3]
The pollutant is persistent: it is not broken down and it is not excreted, so it stays in the body. Each organism therefore bioaccumulates it over its lifetime. Each consumer has to eat many organisms from the level below, and takes in all of the pollutant each one had stored, so the concentration in its tissue is higher than in any single item of its food. Repeating that at every level is biomagnification, and it is why the top predator carries the highest concentration.
The mark you would probably lose
Most answers say “it builds up as it goes up the food chain” and stop. That is one mark. The two that are left need the words persistent / not excreted and each predator eats many prey. Mechanism, not description — that is what “explain” means.
(a) 1200 times. (b) Persistent, so it accumulates in each organism; each predator eats many prey and takes in all of their stored pollutant; repeated at each level, so concentration magnifies up the chain.
Impact 3: Acid Rain in Rivers and Lakes
How acid rain forms belongs to the atmosphere topic; what you need here is its effect on organisms in rivers and lakes. Acid rain falls on the catchment and drains in, and the pH of the water falls. Three things follow.
- Organisms are killed directly. Many freshwater species can only tolerate a narrow pH range. As the water becomes more acidic, fish eggs and young fish fail first, then adults; insect larvae, molluscs and amphibians are also lost.
- Aluminium ions are released from the soil and rock by the acid and washed into the water. These damage fish gills, causing a build-up of mucus so that the fish cannot take in enough oxygen. (Background, not required. 3.2.2(e) is capped at “the effect of acid rain on organisms in rivers and lakes”, and this mechanism is GCSE Biology depth. It is here because it is a good extra point, not because you have to have it.)
- Food chains break down. Losing the small invertebrates at the bottom removes the food supply of the fish above them, and losing the fish removes the food supply of birds and mammals that feed on them. A lake can end up looking clear and healthy while being almost empty of life.
The word to use
Say pH falls or the water becomes more acidic. “The water gets acid” is vague and “the acid burns the fish” is wrong. The aluminium point is a bonus rather than a requirement — the syllabus asks only for the effect on organisms — but few candidates make it, so add it if you have room.
Impact 4: Nutrient Enrichment and Eutrophication
This is the longest chain in the topic and it is marked step by step, so learn it as a sequence of arrows rather than as a paragraph. It starts with either inorganic ions (nitrate and phosphate from fertiliser) or organic content (sewage, slurry, manure), reaching the water by run-off or by leaching.
eutrophication
1. nutrients leach or run off into the water2. nutrient enrichment of the river or lake3. rapid growth of algae at the surface — an algal bloom4. the bloom blocks the light5. the plants beneath cannot photosynthesise and die6. bacteria decompose the dead plants and algae7. the bacteria multiply and respire aerobically8. dissolved oxygen in the water falls9. fish and other aerobic organisms die
Where the marks actually sit
Almost everyone writes “fertiliser gets in, algae grow, fish die”. That is the outline and not much more. The three steps in the middle are the ones that separate answers: the bloom blocks light, the plants below die and are decomposed, and the decomposing bacteria use up the dissolved oxygen. The fish are not poisoned by the fertiliser and they are not smothered by the algae — they suffocate, because bacteria took the oxygen. If you write nothing else, write the oxygen step.
Strategies for Improving Water Quality
Three strategies, and they attack the problem at three different points: before it reaches the water, on the way, and at the source of the discharge.
| Strategy | What it involves | What it fixes, and what it does not |
|---|---|---|
| improved sanitation | Toilets and latrines that contain waste safely, sewers that carry it away, and hand-washing facilities — so that human waste does not reach drinking water in the first place. | Directly reduces water-borne disease and organic pollution. Does nothing about industrial or agricultural pollution, and it needs households to be able to reach and afford the facilities. |
| treatment of sewage | Screening and settling out solids, then breaking down the organic matter biologically, before the effluent is discharged. | Removes pathogens and the organic load, so it reduces both disease and eutrophication. It is expensive to build and to run, and needs trained staff and a reliable power supply. |
| pollution control and legislation | Laws setting limits on what may be discharged and on which substances may be used, permits for outfalls, monitoring of water quality, and fines or prosecution for breaches. | The only strategy that reaches industry and agriculture. It works only where it is enforced — a law with no inspection and no penalty changes nothing, and this is the point examiners want you to make. |
Always say “and enforced”
Legislation appears again in 3.5 and 3.6, and the same evaluation point applies every time: a law is only as good as the monitoring and the penalty behind it. Writing “pass a law banning it” is one mark; writing “pass a law setting discharge limits, and fund inspection and fines so that it is enforced” is two, and it is the sentence that makes an evaluation look like an evaluation.
Data response
A lake downstream of farmland
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A lake is monitored through one year. In March, nitrate concentration is 4 mg per litre and dissolved oxygen is 9.1 mg per litre. Fertiliser is applied to the surrounding fields in April, followed by heavy rain. By July, nitrate has risen to 21 mg per litre, the surface of the lake is covered by green algae, and dissolved oxygen has fallen to 2.4 mg per litre. Dead fish are reported in August.
(a) Calculate the percentage decrease in dissolved oxygen between March and July. [2]
(b) Explain the sequence of events that links the fertiliser to the dead fish. [5]
(c) Suggest two measures the farmers could take. [2]
(a) Calculate the percentage decrease in dissolved oxygen between March and July. [2]
(b) Explain the sequence of events that links the fertiliser to the dead fish. [5]
(c) Suggest two measures the farmers could take. [2]
(a)
Decrease = 9.1 − 2.4 = 6.7. Percentage = 6.7 ÷ 9.1 × 100 = 73.6% (accept 74%). Divide by the starting value, not the final one — that is the commonest slip in this kind of question.
(b) — five linked steps
Heavy rain washed the fertiliser off the fields as run-off, and leached nitrate through the soil, so the lake was enriched with nutrients (nitrate rose from 4 to 21 mg per litre). The nutrients allowed rapid growth of algae at the surface, forming a bloom. The bloom blocked light from reaching the plants below, so they could not photosynthesise and died. Bacteria decomposed the dead plants and algae, multiplied, and respired aerobically, using up the dissolved oxygen — which is why it fell to 2.4 mg per litre. The fish then died because there was not enough dissolved oxygen for them to respire.
(c)
Any two of: apply less fertiliser, or apply it at the rate the crop actually needs; do not apply it just before heavy rain is expected, and avoid applying it to bare or frozen ground; leave an uncultivated buffer strip of vegetation between the field and the lake to intercept run-off; use organic manure worked into the soil rather than surface application; keep livestock and their slurry away from the water’s edge.
The move to copy
Part (b) is worth five marks and there are five arrows in the chain. When the marks and the steps match like that, it is a signal: write one sentence per mark, in order, and use the figures from the stem as evidence for the steps you can support.
Source C
Nitrate and dissolved oxygen in one river across a year
A chart with two vertical axes. The first thing to do with one is to decide which line belongs to which axis, out loud. Work through every part on paper before you press Show Answer. Reading a model answer you have not attempted feels like learning and is not.
One river, one sampling point, twelve monthly readings. Both quantities are in milligrams per cubic decimetre but the scales are different.
identify(a) State the month in which the nitrate concentration is highest, and give that concentration. [1]
describe(b) Describe how the dissolved oxygen concentration changes over the year, quoting figures. [3]
calculate(c) Calculate the percentage increase in nitrate concentration between January and May. Show your working. [2]
explain(d) Explain the shape of the dissolved oxygen line, and why its lowest point comes after the nitrate peak. [5]
suggest(e) Suggest two measures that would reduce the nitrate peak, and state a limitation of each. [4]
Source C — question 1
Read Source C. What is the dissolved oxygen concentration in August?
B. Find August on the horizontal axis, follow it up to the blue line, then across to the right axis: 7.4. A is August on the other line — it is the nitrate value, and reading the wrong series is the single commonest error on a twin-axis chart. C is September's oxygen, one month out. D is the June minimum. Before you read anything off a chart with two axes, say out loud which line belongs to which side.
Source C — question 2
On Source C the oxygen minimum comes about a month after the nitrate peak. The best explanation is that:
D. The chain has steps and each takes time: nitrate arrives, algae grow into a bloom, the bloom shades and kills the plants below, and only then do decomposing bacteria use up the dissolved oxygen. The lag is evidence that the bacteria, not the nitrate, are doing the taking. A invents chemistry — nitrate is very soluble. B invents an instrument fault, which a data question never wants. C is a real effect but it points the wrong way here: June is warmer, not cooler, and the recovery from July onwards happens while the water is still warm.
(a)
May, 14.2 mg per dm3.
(b)
Dissolved oxygen falls from 11.4 mg per dm3 in Jan to a minimum of 5.2 mg per dm3 in Jun, then rises steadily to 11.2 mg per dm3 by Dec. Describing a curve means naming the direction, the turning point and the size of the change, with a figure attached to each.
(c)
Change = 14.2 − 4.2 = 10.0. Percentage increase = 10.0 ÷ 4.2 × 100 = 238.1% (about 240%). Always divide by the starting value.
(d) — the chain
Nitrate is washed off farmland into the river in spring, when fertiliser has been applied and rain moves it. The nitrate is a nutrient, so algae grow rapidly and form a bloom. The bloom blocks the light reaching the plants below, which die. Decomposing bacteria break down the dead material, and it is those bacteria that use up the dissolved oxygen. Fish and other organisms then suffocate. The fish are not poisoned by the fertiliser; that step is the one worth writing above all the others.
(d) — the lag
Each step in that chain takes time, so the oxygen minimum comes about a month after the nitrate peak: nitrate peaks in May and oxygen bottoms out in Jun. The lag is evidence for the mechanism — if nitrate itself removed the oxygen the two would move together. That single observation turns a description of the graph into an explanation of it.
(e)
Any two, each with its limitation. Apply less fertiliser, or apply it at times when rain is unlikely to wash it straight off — but yields may fall, and the farmer bears that cost while the benefit is shared downstream. Leave an untreated strip of vegetation along the river bank to take up nutrients before they reach the water — but it takes land out of production and does nothing about nitrate that has already leached into ground water. Legislation setting limits on application, with inspection — but it needs a body able to monitor many farms, and nitrate already in the ground water will keep arriving for years.
(a) May, 14.2 mg per dm3. (c) 238.1% increase. Oxygen minimum 5.2 mg per dm3 in Jun.
Source D
The same pollutant measured along a linear food chain
The biomagnification chart earlier in this section has a logarithmic axis and tells you to read the bar labels. Here are numbers on a plain linear table instead, so that you meet both. Work through every part on paper before you press Show Answer. Reading a model answer you have not attempted feels like learning and is not.
| Step in the food chain | Mean concentration of the pollutant in body tissue / mg per kg |
|---|---|
| river water | 0.002 |
| producers | 0.04 |
| primary consumers | 0.5 |
| secondary consumers | 6 |
| tertiary consumers | 48 |
Illustrative figures for one persistent, fat-soluble pollutant in one river system. No species is named, and none is needed.
state(a) State the concentration of the pollutant in the primary consumers. [1]
describe(b) Describe what happens to the concentration as the pollutant passes along the food chain. [2]
calculate(c) Calculate how many times greater the concentration in the tertiary consumers is than in the producers. [2]
explain(d) Explain, using both of the syllabus terms, why the concentration rises at every step. [6]
suggest(e) The same data are drawn as a bar chart on a logarithmic axis earlier in this guide. Suggest one advantage and one disadvantage of drawing them that way. [2]
Source D — question 1
Using Source D, the concentration in tertiary consumers is how many times the concentration in the river water?
A. 48.0 ÷ 0.002 = 24 000. Dividing by a number smaller than one is where this goes wrong, so write it as 48 ÷ 0.002 and move the decimal point deliberately. B is 48 ÷ 0.04, which compares the top of the chain with the producers rather than with the water — a real figure, but the answer to a different question. C and D are what you get by losing a factor of ten on the way. The point of the number is the scale: a concentration far too low to measure usefully in the water is lethal four steps later.
Source D — question 2
Which step in Source D shows the largest increase in concentration, measured as a multiple of the step below it?
C. Work out each multiple rather than trusting the picture: 0.04 ÷ 0.002 = 20, then 0.5 ÷ 0.04 = 12.5, then 6.0 ÷ 0.5 = 12, then 48 ÷ 6.0 = 8. The biggest jump is the first one, into the producers. D is the tempting answer because it is the biggest jump in raw milligrams (42 mg per kg), and that is precisely the trap: the question says as a multiple. When a question specifies how to measure a change, the raw difference and the ratio can point at different steps.
(a)
0.5 mg per kg. Quote the units; a bare number often does not earn the mark.
(b)
It increases at every step, and it increases by a large multiple rather than by a small amount: from 0.002 mg per kg in the water to 48 mg per kg in the tertiary consumers. Saying increases is one mark; supporting it with the two end values is the second.
(c)
48 ÷ 0.04 = 1200 times.
(d)
Both words are needed and they mean different things. Bioaccumulation happens inside one organism over time: the pollutant is persistent and fat-soluble, so it is not broken down or excreted, and each organism takes in more than it loses, so its own concentration climbs as it gets older. Biomagnification happens between organisms, up the chain: each predator has to eat many prey to survive, and it takes in all of the pollutant in every one of them, so the concentration is higher at each level than at the one below. The link sentence is the one that earns the top marks: biomagnification only works because bioaccumulation happened first, in every organism at every level.
(e)
Advantage: a logarithmic axis lets values 24000 times apart be drawn on one page at a readable size — on a linear axis the producers' bar would be invisible next to the top one. Disadvantage: the steps look small and even when they are enormous, so the chart understates the effect unless you read the axis carefully — and reading a value off a log axis is much harder than reading one out of this table.
(a) 0.5 mg per kg. (c) 1200 times. Tertiary against the river water: 24000 times. Largest single step: river water to producers, ×20.0.
Checkpoint 3.2
Answer, then read the explanation even when you were right.
Your Score
0 / 15
Question 1
A single fish is caught at age 2 and another of the same kind at age 11. The older one contains far more mercury. This is an example of:
D. Only one organism is involved and the variable is time — that is bioaccumulation exactly as the syllabus defines it. A is the answer to a different comparison: biomagnification would need two different trophic levels, not two ages of the same animal. B is nutrient enrichment, which is about nitrate and phosphate rather than a toxic metal. C names a route into water, not what happens inside a body.
Question 2
Why does a top predator end up with a higher concentration of a persistent pesticide than its prey?
B. Two properties do the work: the pesticide is persistent, so it is not broken down or excreted, and each predator eats many prey, collecting all of their stored pesticide into one body. A is not the route — the pesticide comes in with food. C is a real misconception: the substance does not change, only its concentration does. D invents a distribution the question does not give you.
Question 3
In eutrophication, what causes the fish to die?
D. The fish suffocate. Decomposing bacteria multiply on the dead plants and algae and respire aerobically, and dissolved oxygen falls below what the fish need. A is the intuitive answer and it is wrong — nitrate at these concentrations is not the poison. B sounds plausible but the bloom is at the surface, not on the gills; that mechanism belongs to oil pollution. C gets the light right but the victim wrong — the blocked light kills the submerged plants, and that is a step in the chain, not the end of it.
Question 4
Nitrate from a fertilised field is carried downwards through the soil into an aquifer. This route is called:
C. Leaching is the downward movement of dissolved substances through the soil, and it is the word the syllabus uses in 3.2.2(f). A is the other route by which fertiliser reaches water, but it travels over the ground into a river, not down into an aquifer. B is a water treatment stage. D is a water cycle transfer involving vegetation. Naming the route correctly is often worth a mark on its own.
Question 5
Which effect of acid rain on a lake is most often missed in answers?
A. All four statements are true, which is what makes this question a good check — the acid releases aluminium ions from soil and rock, they wash into the lake, and they cause a build-up of mucus on fish gills so the fish cannot take in enough oxygen. Most candidates give the direct effect of low pH (B and C) and the food chain effect (D), and stop there. The aluminium step is the one that lifts an answer, so make it the one you always include.
Question 6
Which strategy for improving water quality is the only one that reaches industry and agriculture?
D. Sanitation and sewage treatment both deal with human waste; only legislation can set limits on a factory outfall or on how fertiliser is used. A and B are genuine strategies from 3.2.3 but they address a different source. C is a water treatment stage from 3.1.5, which makes water safe to drink after the pollution has already happened — it does not improve the quality of the river. And remember the second half of D: a law without monitoring and penalties changes nothing.
Question 7
Which pairing of source and pollutant is correct?
C. Livestock slurry and manure, fertilisers and pesticides are all agricultural. A attaches an industrial pollutant to a domestic source. B attaches an agricultural pollutant to sewage — sewage does carry nutrients, but nitrate applied to fields is fertiliser, and the phrase gives it away. D swaps the two the other way round: detergents and cooking fat go down a household drain. Questions like this are testing whether you learned the list as five separate sources or as one blur.
Question 8
A river has a high concentration of dissolved oxygen upstream of a town and a very low one just below its sewage outfall. The best explanation is that:
C. Sewage is a heavy load of organic matter. Bacteria multiply on it and respire aerobically, and their respiration removes the dissolved oxygen — the same mechanism as the last step of eutrophication, arriving by a different route. A is a real effect (warm water holds less oxygen) but far too small to explain the fall. B invents a reaction. D reverses the cause: the oxygen is being consumed, not blocked from dissolving.
Question 9
A town's untreated household waste water and the contents of its street drains both discharge into the same river. Which two sources of water pollution are these?
D. The syllabus lists five sources and the stem names two of them: waste water from homes carries detergents, food waste and household chemicals, which is domestic waste, and human waste is sewage. A adds a factory that the stem does not mention. B adds farming, which is not in the stem either. C names two things a street drain certainly carries some of, but they are not what the stem describes. Answer from the stem, not from what a town generally contains.
Question 10
Why does water polluted with sewage carry a risk of cholera?
B. Cholera is an infectious bacterial disease. Sewage is dangerous because it carries the bacterium from an infected person's gut into water that other people drink, which is why the controls in 3.3.3 are about keeping the two apart. A confuses the oxygen story with the disease story — falling oxygen is eutrophication, and it kills fish, not people. C invents a pH effect. D is the misconception the objective exists to correct: dirty water is the route, the bacterium is the cause.
Question 11
Improved sanitation raises water quality mainly because it:
D. Sanitation is toilets, latrines and the pipes that take what leaves them somewhere contained. It works at the start of the chain: the waste never reaches the river, so nothing has to be taken out of the river later. A is a job for treatment and for controlling what farms apply, not for sanitation. B is water treatment, which happens on the way in to a house rather than on the way out. C is sewage treatment, the next strategy on the same list — prevention first, then treatment, then legislation.
3.3 Water-related Diseases
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▶ Watch: Malaria and cholera
Opens on YouTube in a new tab. No 0680-badged video exists for this sub-topic, so these are general science and documentary. The malaria animations go well past 0680 into the parasite's life cycle; watch them for the two-way transmission between human and mosquito and ignore the rest, because that is all 3.3.1 asks for.
Two Diseases, Two Completely Different Routes
The syllabus names two diseases, and the single most useful thing to fix in your head is that water does a different job in each. In cholera, the water is what you swallow — the bacterium is in it, and drinking it makes you ill. In malaria, you never drink anything: standing water is simply where the mosquito breeds, and the disease arrives through a bite. That is why the control strategies for the two look nothing like each other, and why a question that mixes them up is easy to spot.
How Mosquitoes Spread Malaria
Three sentences from the syllabus, and they are marked as three separate points, so give all three.
- Female Anopheles mosquitoes are the vector. A vector is an organism that carries a pathogen from one host to another without being made ill by it. Two details matter: it is only the female that bites (she needs a blood meal to develop her eggs), and it is only the Anopheles type that carries malaria.
- The malaria parasite, Plasmodium, is transmitted to a human when that person is bitten by an infected mosquito. The mosquito is not the disease; it is the delivery system. The disease is the parasite.
- The parasite is transmitted to a non-infected mosquito when it feeds on the blood of an infected human. This is the sentence most people leave out, and it is the one that makes malaria a loop rather than a one-way street.
The parasite travels in both directions. That is why treating infected people also reduces transmission.
Vector, parasite, host
The mosquito is the vector. Plasmodium is the parasite. The human is the host. Say “the mosquito causes malaria” and you have lost the mark, because it does not — it carries the thing that does. The same three-word structure works for any vector-borne disease, so it is worth the ten seconds it takes to learn.
Controlling Malaria
The syllabus splits the strategies into two groups, and the split is the structure of your answer. Personal protection defends one person against the bite or against the parasite once it is in. Vector control attacks the mosquito population itself, so it protects everyone in the area at once.
| Personal protection | How it works |
|---|---|
| nets | A net over the bed is a physical barrier between the sleeping person and the mosquito, which bites mainly at night. Nets treated with insecticide also kill mosquitoes that land on them. |
| insect repellent | Applied to skin or clothing, it discourages the mosquito from landing and biting. |
| vaccination | Prepares the immune system so that the parasite is attacked if it does enter the body, reducing the chance of serious illness. |
| antimalarial drugs | Taken to prevent the parasite establishing, or to treat someone already infected. Treating infected people also lowers the chance of a mosquito picking the parasite up. |
| Vector control | How it works |
|---|---|
| cover or drain breeding areas | Mosquitoes lay eggs in standing water, and the larvae develop there. Draining ponds and ditches, or covering water tanks and containers, removes the places the next generation would come from. |
| spraying large areas with insecticides | Kills adult mosquitoes over a wide area, cutting the number able to bite. Spraying inside houses targets the mosquitoes that rest on walls after feeding. |
| sterilise male mosquitoes | Large numbers of sterilised males are released. Females that mate with them lay eggs that do not hatch, so the population falls over successive generations. |
| biological control | Introducing an organism that eats mosquito larvae — for example fish stocked in ponds and rice fields — so that fewer larvae survive to become adults. |
Controlling Cholera
Cholera is caused by a bacterium that is swallowed in contaminated water or food. Break that route and you break the disease, and the four strategies do it at four points along the route.
| Strategy | Where in the route it acts |
|---|---|
| handwashing | At the last step, between hands and mouth or hands and food. Washing with soap after using a latrine and before preparing food removes bacteria before they can be swallowed. |
| adequate sanitation and sewage treatment | At the first step. Toilets, latrines and sewers contain human waste, and treatment destroys the bacteria in it, so infected waste never reaches the water supply. |
| potable water supply: boiling and chlorination | In the middle. Boiling the water kills the bacteria in the home; chlorination does the same at the treatment works for a whole town at once. |
| vaccination | At the person. Prepares the immune system so that swallowing the bacterium is less likely to cause serious illness. |
Sanitation is the one that ends it
Boiling, chlorination and vaccination all protect people from water that is already contaminated. Only sanitation and sewage treatment stop the contamination happening, which is why it is the permanent answer and the others are, in effect, defences. If a question asks which strategy is most effective in the long term and why, that is the argument — and the counter-argument is that sanitation is the slowest and most expensive to build, so the others are what protect people meanwhile.
Discussing the Strategies: Benefits and Limitations
Objective 3.3.4 is a “discuss”, so it needs both columns and a judgement. Notice as you read that the pattern repeats: the cheap, fast strategies protect individuals and have to be repeated for ever, while the expensive, slow ones protect everybody permanently.
| Strategy | Benefits | Limitations |
|---|---|---|
| nets | Cheap, simple, need no power or training, and protect the people most at risk at the time they are most at risk. | Only work while someone is under them, so they do not help against biting in the evening outdoors. They tear, and they have to be replaced and re-treated. |
| insect repellent | Protects a person anywhere, indoors or out, and can be used immediately. | Has to be bought and re-applied constantly, which is a recurring cost, and it protects only the individual using it. |
| antimalarial drugs and vaccination | Reduce serious illness and death, and treating infected people lowers transmission because there is less parasite for mosquitoes to pick up. | Cost money and need a health service to deliver them. Parasites can become resistant to drugs, so the drugs used have to change over time. |
| drain or cover breeding areas | Removes the next generation of mosquitoes rather than the current one, so the effect lasts, and it protects a whole community. | Labour-intensive, and impossible to complete — mosquitoes breed in very small volumes of water, including containers around houses, so every site missed refills the population. Draining wetland also destroys habitat. |
| spraying insecticides | Reduces the adult mosquito population quickly over a wide area, which is useful when cases are rising. | Mosquitoes develop resistance, so it becomes less effective; the insecticide also kills harmless and useful insects; and spraying must be repeated, so it is a permanent cost. |
| sterilising males and biological control | Target the mosquito without spreading chemicals, so other organisms are not poisoned, and the effect builds over generations. | Sterilising needs a facility to rear and release very large numbers, which is technically demanding. An introduced predator may eat other species too, so it needs care. |
| handwashing | Extremely cheap, and it reduces several diseases at once, not only cholera. | Depends on having water and soap available at the right place, and on habit — behaviour change is slow and needs continued education. |
| boiling and chlorination | Boiling works in any home with fuel; chlorination protects a whole town from one point and is cheap per person. | Boiling uses fuel, which costs money and time and produces smoke indoors. Chlorination needs a piped network, so it does not reach households without one. |
| sanitation and sewage treatment | The only strategy that stops contamination at the source, so it protects everyone permanently and reduces eutrophication as well. | The most expensive and slowest to build, needs trained staff, power and maintenance, and in a settlement that already exists the pipes are disruptive to install. |
The judgement sentence
For a 6-mark “discuss the benefits and limitations of strategies to control water-related disease”, a strong ending names the trade-off rather than picking a favourite: “The individual measures — nets, repellent, boiling, handwashing — are cheap and work immediately, but they protect one person at a time and must be repeated for ever, so the cost never ends. The community measures — draining breeding sites, sewage treatment, a piped chlorinated supply — cost far more and take years, but they protect everyone and the effect is permanent. The practical answer is usually to use the individual measures to protect people now while the community measures are being built.”
Data response
A district trials two measures
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A district records 4200 cases of malaria in the year before any action is taken. Insecticide-treated nets are then distributed to every household. Cases fall to 1470 in the following year. In the third year the number rises again to 2600, although net ownership is unchanged. A survey finds that 38% of nets are torn and that a large area of ditches nearby has filled with standing water after unusually heavy rain.
(a) Calculate the percentage fall in cases in the first year of the nets. [2]
(b) Suggest two reasons for the rise in the third year. [2]
(c) Discuss whether the district should now spend its budget on insecticide spraying or on draining the ditches. [4]
(a) Calculate the percentage fall in cases in the first year of the nets. [2]
(b) Suggest two reasons for the rise in the third year. [2]
(c) Discuss whether the district should now spend its budget on insecticide spraying or on draining the ditches. [4]
(a)
Fall = 4200 − 1470 = 2730. Percentage = 2730 ÷ 4200 × 100 = 65%. Again, divide by the starting figure.
(b)
Any two: over a third of the nets are torn, so they are no longer a barrier — nets wear out and need replacing; the flooded ditches are standing water where mosquitoes breed, so the mosquito population has risen; nets only protect people who are under them, so biting outdoors in the evening is unaffected; mosquitoes may be developing resistance to the insecticide on the treated nets.
(c) — both sides
Spraying: kills adult mosquitoes quickly over a wide area, so it would bring cases down within weeks, which matters if people are ill now. But it must be repeated, so it is a permanent cost; resistance can develop; and it kills harmless insects too. Draining: removes the breeding sites, so it reduces the next generation rather than this one — slower to show results, but the effect lasts, it costs nothing to run once done, and the survey has identified the ditches as the specific cause. Against it: draining is labour-intensive, mosquitoes breed in very small volumes of water elsewhere, and wetland habitat may be lost.
(c) — the judgement
“The evidence points to the ditches, because cases rose in the year they flooded while net ownership did not change. Draining therefore addresses the cause and its benefit is lasting, so it is the better use of a limited budget — with the qualification that it will take a season to show, and that replacing the torn nets is cheaper than either and should come first.” Notice that the strongest point is drawn from the data, not from general knowledge.
The move to copy
When a “discuss” question comes with data, the judgement must rest on the data. “Net ownership was unchanged but cases rose” is the sentence that rules out one explanation and points at another. Look for the variable that did not change — it is usually the key to the question.
Source E
Cholera and sanitation coverage in four districts
Bars on one axis, points and a line on the other. Name the axis before you read any value. Work through every part on paper before you press Show Answer. Reading a model answer you have not attempted feels like learning and is not.
Four districts of one region, same year. Sanitation coverage means the percentage of people with a contained, treated waste system.
identify(a) Name the district with the highest number of cholera cases and give that number. [1]
describe(b) Describe the relationship between sanitation coverage and the number of cholera cases. Use figures. [3]
calculate(c) Calculate the percentage decrease in cases between district P and district S. [2]
explain(d) Explain how adequate sanitation reduces cholera. [4]
suggest(e) District S still records 4 cases per 10 000 people even at 93% coverage. Suggest two reasons, and name one further strategy the region could use. [3]
Source E — question 1
Read Source E. How many cholera cases per 10 000 people per year are recorded in district Q?
C. The bars use the left axis, and Q's bar reaches just above 30. A is Q's sanitation figure, which is on the right axis and is a percentage, not a case rate — reading the point instead of the bar. B is district R's case rate. D is district P's. Whenever a chart carries bars and a line, name the axis in your head before you read the value, and quote the units when you write the answer down: '31 cases per 10 000 people per year', not '31'.
Source E — question 2
Source E shows cases falling as sanitation coverage rises. What is the safest conclusion to draw from the data alone?
A. Four districts show a clear association, and that is exactly as far as the data go. B claims cause from a pattern in four places; the mechanism makes it very likely, but the source does not establish it, and 'the data show an association, and the mechanism explains why' scores better than an overclaim. C is contradicted by the source itself — district S still records 4 cases per 10 000 at 93% coverage. D adds a variable the source never measured. Reading only what is in front of you is a skill the examiner is testing directly.
(a)
District P, 48 cases per 10 000 people per year.
(b)
There is a clear negative relationship: as sanitation coverage rises, cases fall. District P has 22% coverage and 48 cases per 10 000, while district S has 93% and 4 cases. Every district in between fits the same order. Say as one rises the other falls in a single sentence — that is what 'describe the relationship' asks for, rather than four separate descriptions.
(c)
Decrease = 48 − 4 = 44. Percentage decrease = 44 ÷ 48 × 100 = 91.7% (about 92%).
(d)
Cholera is a bacterial disease spread when water or food is contaminated with the waste of an infected person. Adequate sanitation puts a barrier in that route: human waste is collected and contained rather than reaching the ground, the drains and the water people drink. Fewer bacteria reach the supply, so fewer people are infected, and each infected person is then less likely to pass it on. It works at the start of the chain, which is why it is more effective than treating people afterwards.
(e)
Any two reasons: coverage of 93% still leaves people without it, and one contaminated source can serve many households; water can be contaminated after it leaves a safe source, in the container or in the home; people travel, so cases can arrive from a district with lower coverage. One further strategy, with its mechanism: handwashing with soap, which breaks the route from hand to mouth; boiling or chlorinating drinking water, which kills the bacterium at the point of use; or vaccination, which protects the individual even where the water is unsafe.
(f) — the caution worth adding
Four districts showing a pattern is an association, not proof of cause. The mechanism in part (d) is what makes the causal claim reasonable, and the honest sentence is: “the data show a strong association, and the known route of infection explains it.” Writing that is worth more than asserting cause from a graph.
(a) District P, 48 per 10 000. (c) 91.7% decrease.
Checkpoint 3.3
Answer, then read the explanation even when you were right.
Your Score
0 / 10
Question 1
Which statement about malaria is correct?
D. The mosquito carries the parasite from person to person without being made ill by it — that is what a vector is — and the disease is caused by Plasmodium. A is the classic lost mark. B describes cholera, not malaria: standing water is where mosquitoes breed, not what you drink. C is wrong on a detail the syllabus states: only the female bites, because she needs a blood meal for her eggs.
Question 2
How does a mosquito that has never carried the parasite become infected?
D. This is the third sentence of 3.3.1 and it is the one most often left out. The loop runs both ways: mosquito to human when she bites, human to mosquito when she takes blood from someone infected. A muddles breeding with infection — the larvae are in water, but the parasite is in blood. B and C are routes the syllabus does not give; if it is not in the objective, it is not the answer.
Question 3
Which of these is vector control rather than personal protection?
C. Vector control acts on the mosquito population, so it protects a whole area; sterilising males reduces the number of mosquitoes in the next generation. A, B and D all protect the individual using them and leave the mosquito population exactly as it was. D is the one worth thinking about: a treated net does kill mosquitoes that land on it, but the syllabus lists nets under personal protection, and its purpose is to shield the sleeper.
Question 4
Why does draining ditches reduce malaria?
C. Standing water is the breeding site: eggs are laid on it and the larvae develop in it, so removing it removes the next generation of adults. A puts the parasite in the water, which is the cholera model rather than the malaria one. B invents a mechanism. D is again cholera thinking — with malaria nobody is drinking anything. The give-away in all three wrong options is that they treat the water as the thing that infects you.
Question 5
Which cholera control strategy stops the contamination happening in the first place?
D. Sanitation and sewage treatment contain and destroy the bacteria in human waste, so it never reaches the water. Everything else on the list is a defence against water that has already been contaminated: A kills the bacteria after they are in the water, C does the same on a larger scale, and B protects the person rather than the water. All four are correct strategies from 3.3.3 — the question is about where in the route each one acts.
Question 6
A limitation shared by insecticide spraying and by antimalarial drugs is that:
D. Mosquitoes can become resistant to insecticides and parasites can become resistant to drugs, so both lose effectiveness over time and have to be changed — a strong evaluation point. A is wrong for spraying, whose whole purpose is to kill adults. B is invented. C describes nets, not either of these. Watch for questions that ask what two strategies have in common; the answer is usually a limitation rather than a benefit.
Question 7
Handwashing with soap is recommended against cholera mainly because it:
C. Cholera is swallowed, so the last step of the route is hand to mouth or hand to food. Washing breaks that step, which is why it is both the cheapest strategy and one of the most effective. A reverses the direction — washing does not treat water. B describes vaccination. D describes sanitation. The strategies are easy to confuse because they all reduce the same disease; sort them by where in the chain they act.
Question 8
Which pair of statements correctly contrasts individual and community measures?
A. Nets, repellent, boiling and handwashing are cheap and act at once, but protect one person and must be repeated indefinitely. Draining breeding sites, sewage treatment and a piped chlorinated supply cost far more and take years, but they protect everybody and the effect lasts. B and C reverse the two halves of that. D reverses the coverage: a community measure is defined by reaching everyone in the area at once.
3.4 Marine Aquaculture
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▶ Watch: Overfishing, aquaculture and fisheries management
Opens on YouTube in a new tab. Three of these are badged for 0680 and the rest are documentary. The documentaries name farmed species; the syllabus states that knowledge of specific species is not required, so read every species name as “a farmed fish” or “the target species”. Watch them for escape, disease, waste and feed, which are the four impacts that are hardest to picture from a table.
Before you start: you do not need any species names
The syllabus states plainly that knowledge of specific species is not required for this sub-topic. So do not spend a minute learning which fish are farmed where. Write “the target species”, “a farmed fish”, “a crustacean”, “a wild predator”. The marks are for the processes, and a named example earns nothing extra here.
What Exploiting Marine Species Does
Overfishing means catching fish faster than the population can replace itself by breeding. Overharvesting is the same idea applied to anything else taken from the sea. The definition is worth getting right because it explains why a fishery can collapse while boats are still catching plenty: the catch is being taken from the breeding stock, so each year there are fewer adults left to produce the next generation, and the decline accelerates.
| Impact | What happens, and why |
|---|---|
| on the target species | The population falls, and the fish caught become smaller and younger because few survive long enough to grow large. Fewer mature adults means less breeding, so the population recovers more slowly than it is being reduced. If fishing continues, the stock can fall to a level from which it does not recover for many years. |
| on bycatch species | Bycatch is everything caught that was not the target — other fish, young of the target species, marine mammals, seabirds. Most of it is discarded, usually dead. So a fishery reduces populations that nobody is deliberately harvesting, and it does so invisibly, because discarded catch is not recorded in landings. |
| on food chains | Removing one species changes every level connected to it. Take out a predator and the animals it fed on increase, which reduces their food supply in turn. Take out a species that many things eat and its predators lose their food source and decline. Because the effects run both up and down the chain, a fishery aimed at one species can reshape a whole community. |
Answering “explain the effect on food chains”
Do not just write “the food chain is affected”. Pick a direction and follow it: “If the target species is a predator, the organisms it ate are no longer controlled and their numbers rise, so the food supply of those organisms is reduced.” One clear knock-on, stated with a because, is worth more than three vague sentences. This is the same skill as tracing a food web in Biology.
Marine Aquaculture: The Definition
Learn this one word for word
Marine aquaculture is the farming of marine species in captivity. The syllabus limits the species to three groups: fish, crustaceans and seaweeds. Two words in that definition are doing work. Marine — in the sea or in salt water, so a freshwater pond is not marine aquaculture. In captivity — the organisms are contained, in cages, pens, tanks or on ropes and frames, which is what separates farming from fishing.
The Impacts of Marine Aquaculture
Eight impacts, and they are deliberately mixed — some are advantages, some are problems, and one of them is the fact that people usually forget. Read the list as three groups.
Group 1: what it does for people
| Impact | Explanation |
|---|---|
| reduced exploitation of natural fisheries | Every tonne produced on a farm is a tonne that does not have to be caught from the wild, so pressure on wild stocks falls and populations have a chance to recover. |
| increased food supply for humans | Production is predictable and can be raised to meet demand, rather than depending on what the sea provides in a given year. It also supplies protein close to where people live, and creates jobs. |
Group 2: what escapes from the farm
| Impact | Explanation |
|---|---|
| risk of escape | Cages tear and pens fail, and farmed organisms get out. They compete with wild ones for food and space, and where they breed with the wild population they change it — farmed stock is bred for growth rather than for survival in the wild. |
| risk of disease | Animals held at high density are easy for parasites and disease to pass between, so infections spread quickly inside a farm and can then spread outwards to wild populations nearby. Treating them means using medicines, which enter the surrounding water. |
| local food webs | A farm concentrates organisms in one place, which attracts wild predators and scavengers to feed around it. Escaped or introduced organisms may eat, or be eaten by, wild species. Either way the balance of the local web shifts. |
| nutrient enrichment from waste | Uneaten food and faeces sink below the cages. This is organic matter and it enriches the water and the sediment, so the eutrophication chain from 3.2 can run underneath a fish farm: nutrient enrichment, algal growth, decomposition, falling dissolved oxygen. |
Group 3: what it costs to run
| Impact | Explanation |
|---|---|
| energy usage | Pumping and aerating water, running boats and equipment, refrigerating and transporting the harvest all use energy, so farmed production is not free of emissions. |
| source of food for farmed species | This is the one people miss. Farmed carnivorous fish are fed on fishmeal made from wild-caught fish. So aquaculture can increase pressure on wild stocks rather than relieve it — the fishing has simply moved to a different species. Farming seaweeds or species that do not need fishmeal avoids this completely. |
The point that lifts an aquaculture answer
Almost every candidate writes “fish farming reduces overfishing”. The examiner is waiting for the qualification: it reduces exploitation of the farmed species, but if the farmed animals are fed on fishmeal from wild-caught fish, pressure on wild stocks continues. Whether aquaculture relieves the sea depends on what the farmed organisms eat. That single sentence turns a description into an evaluation.
Managing the Harvesting of Marine Species
Nine strategies. They look like a long list until you see that they answer four different questions: how much gear, what the gear catches, how much and when, and who agrees to it.
| Question | Strategy | How it works |
|---|---|---|
| How much gear? | limits on size of boat and overall net size | A smaller boat with a smaller net simply cannot take as much, so the total catch is capped by the equipment itself rather than by trusting the crew. |
| limited number of fishing days | Restricting how many days a boat may fish reduces effort directly, and is easy to check from records of when boats left and returned. | |
| What does the gear catch? | increased mesh size of nets | Larger holes let small and young fish swim through, so they survive to breed at least once. It also reduces bycatch of smaller species. |
| sustainable methods: pole and line | Catching one fish at a time with a pole and line takes only the target species and almost no bycatch, and cannot strip an area the way a large net can. | |
| How much, and when? | quotas | A legal maximum mass of a species that a boat, fleet or country may land in a period, set so that enough adults remain to breed. |
| closed seasons | Fishing is banned during the breeding season, so adults can spawn undisturbed and the young have time to develop. | |
| protected areas | Areas where fishing is restricted or banned entirely. Populations inside can recover and breed, and the young then spread into the areas around, which is why protected areas can raise catches nearby. | |
| Who agrees to it? | conservation laws | National law making the above enforceable — licences, landing rules, protection for particular species, and penalties for breaking them. |
| international agreements: implementation and monitoring | Fish do not stop at borders and much fishing happens in international waters, so countries agree shared rules. The syllabus names the two hard parts: implementation (turning the agreement into national law and practice) and monitoring (inspections, observers on boats, satellite tracking and checks on what is landed). |
Why quotas produce discards
This is a favourite examiner point. If a boat is over its quota for a species and catches more of it, the extra is thrown back — usually dead — because landing it is illegal. So a quota can reduce what is landed without reducing what is killed. It is the cleanest example on the syllabus of a management strategy with an unintended effect, and it is exactly the kind of point a “discuss” question is looking for.
Discussing the Management Strategies
| Benefits of managing marine harvesting | Limitations of managing marine harvesting |
|---|---|
| Stocks are allowed to recover, so catches can be larger and more reliable in future than they would be without any limits. | Catches and incomes fall in the short term, which is hard for fishing communities that have few other sources of work. |
| Increasing mesh size and using pole and line reduce bycatch, so species nobody is targeting are protected as well. | Selective gear is usually slower and costs more per tonne landed, so it is not adopted unless there is a price or a rule that rewards it. |
| Closed seasons and protected areas let breeding happen undisturbed, and young fish from protected areas spread out and raise catches nearby. | Setting a quota or a closed season needs good data on the stock, which is expensive to collect and often uncertain, so the limit may be set too high or too low. |
| Rules on boat and net size are simple to check at the quayside, so they are cheap to enforce compared with counting fish. | Quotas can lead to discarding: fish over quota are thrown back dead, so the stock is not spared even though the landings figure falls. |
| International agreements can cover stocks that move between countries and waters that belong to nobody, which national law cannot reach. | Agreements are only as strong as their implementation and monitoring. Enforcing rules far out at sea is difficult and costly, and a country that does not sign is not bound at all. |
Who bears the cost, over what timescale
For fisheries management the split is sharp and it is worth naming: the cost is immediate and falls on the fishing communities — smaller catches and smaller incomes, this season. The benefit is delayed and shared — a recovered stock several years later, for everyone who fishes it and everyone who eats. That mismatch is precisely why the rules are resisted and why enforcement matters, and saying so is what a top-band conclusion sounds like.
Data response
A fishery under two different rules
▼
A coastal fishery lands 12 000 tonnes of one target species in year 1. A quota of 8000 tonnes is introduced in year 2 and landings meet it exactly. Observers on boats estimate that a further 2100 tonnes of the same species were caught and discarded in year 2. In year 3 the quota is kept and the minimum mesh size is increased; landings are 8000 tonnes and estimated discards fall to 400 tonnes.
(a) Calculate the total mass of the species removed from the sea in year 2. [1]
(b) Explain why landings alone are a poor measure of the pressure on a stock. [3]
(c) Suggest why increasing mesh size reduced discards. [2]
(a) Calculate the total mass of the species removed from the sea in year 2. [1]
(b) Explain why landings alone are a poor measure of the pressure on a stock. [3]
(c) Suggest why increasing mesh size reduced discards. [2]
(a)
8000 + 2100 = 10 100 tonnes. The quota reduced landings by a third but total removal by only about a sixth.
(b)
Landings count only what is brought ashore. Fish caught over quota are discarded at sea, usually dead, so they are removed from the stock but never appear in the figures. Bycatch of other species is discarded in the same way. So a fall in landings can look like a fall in pressure when the actual mortality has barely changed — here 8000 landed but 10 100 killed.
(c)
Larger holes let small and young fish escape from the net instead of being hauled up, so they are never caught and never need to be discarded — and they survive to breed. Preventing the catch is better than sorting it afterwards, because fish that have been hauled up and returned mostly die.
The move to copy
The insight in this question is that a strategy can improve the measurement without improving the situation. Whenever data show a target being met exactly, ask what is happening to the part that is not measured. Examiners like that question because it is the difference between describing a policy and evaluating it.
Source F
Landings and discards from one fishery over ten years
The natural line graph of this whole topic, and the one figure that shows why landings and removal are not the same number. Work through every part on paper before you press Show Answer. Reading a model answer you have not attempted feels like learning and is not.
One fishery, one target species, ten years. Discards are estimated by observers carried on the boats.
identify(a) State the year in which landings were highest, and give the mass landed. [1]
describe(b) Describe the trend in landings across the ten years. Quote figures. [3]
calculate(c) Calculate the percentage decrease in landings between year 3 and year 7. Show your working. [2]
calculate(d) Calculate the total mass of the species removed from the sea in year 7. [1]
explain(e) Explain why landings on their own are a poor measure of the pressure on a stock. [3]
discuss(f) Discuss whether the two management measures shown on the chart worked. [6]
Source F — question 1
Read Source F. What is the estimated mass of discards in year 5?
D. Both lines share one axis here, so the reading is direct: the red dashed line at year 5 sits just below 3. A is year 6 and B is year 4 — one place either side, which is what happens when you count the year gridlines from the wrong end. C is year 5 on the landings line. Put a finger on the year first and move it straight up; do not try to follow a line across from the axis.
Source F — question 2
Using Source F, in which year was the greatest total mass of the species removed from the sea?
B. Total removed is landings plus discards, so add the two lines: year 1 gives 16.6, year 2 gives 18.6, year 3 gives 20.2, year 4 gives 18.3. Year 3 is also the peak of the landings line, which is why it is worth checking whether the two peaks coincide — here they do, but they need not. A, C and D are all near neighbours, and each of them is the answer if you add only one pair or misread a single point. The idea underneath this question is the one 3.4 keeps returning to: landings are not the same as removal.
Source F — question 3
On Source F discards fall sharply between years 7 and 8. Which change marked on the chart best explains that fall?
D. The dashed marker between years 7 and 8 is the mesh change, and it is the only event in the right place. Larger holes let small fish escape from the net, so they are never hauled up and never have to be thrown back. A came a year earlier and, if anything, pushed discards up rather than down — a boat over quota must discard the excess. B happens after the fall, so it cannot have caused it. C describes the landings line, not the discards line. When a chart carries dated policy markers, check which side of the marker the change actually falls on.
(a)
Year 3, 16.8 thousand tonnes.
(b)
Landings rise from 14.0 thousand tonnes in year 1 to a peak of 16.8 in year 3, then fall steadily to 8.0 in years 7 and 8, and finally recover slightly to 9.2 by year 10. Three phases, each with a figure. A trend answer that names only the direction is a one-mark answer.
(c)
Decrease = 16.8 − 8.0 = 8.8. Percentage decrease = 8.8 ÷ 16.8 × 100 = 52.4% (about 52%).
(d)
8.0 + 2.1 = 10.1 thousand tonnes — landings plus discards, because a discarded fish has still been taken out of the stock.
(e)
Landings count only what is brought ashore. Fish caught over quota are thrown back at sea and mostly die, and bycatch is discarded in the same way, so both are removed from the stock and never appear in the figures. That is why a falling landings line can look like falling pressure when it is not: in year 7 the chart shows 8.0 thousand tonnes landed but 10.1 thousand tonnes removed. Total mortality, not landings, is what the stock experiences.
(f) — the quota
It worked on the measured figure and only partly on the stock. Landings fell to 8.0 thousand tonnes and stayed there, which is what the quota was for. But discards in year 7 were still 2.1 thousand tonnes, so total removal was 10.1 — a fall from the year 3 peak of 20.2, but a smaller fall than the landings line suggests. A quota caps what may be landed, not what may be caught, and the excess goes over the side.
(f) — the mesh size
This is the measure that changed the outcome. Discards fall from 2.1 to 0.9 thousand tonnes between years 7 and 8, a decrease of 57.1%, because larger holes let small and young fish escape from the net instead of being hauled up and thrown back. Preventing the catch beats sorting it afterwards, since fish that have been hauled up and returned mostly die anyway.
(f) — the judgement
“The quota alone reduced landings without reducing removal by nearly as much, so on its own it improved the measurement more than the stock. The mesh increase reduced what was caught in the first place, and only after both were in place did total removal fall to about 8.9 thousand tonnes and landings begin to recover, reaching 9.2 by year 10. The evidence here is that a rule about how much may be landed needs a rule about what the gear catches alongside it.” Naming the evidence for each half is what turns this into a discussion rather than an opinion.
(a) Year 3, 16.8. (c) 52.4%. (d) 10.1 thousand tonnes. Greatest total removal: year 3 at 20.2.
Checkpoint 3.4
Answer, then read the explanation even when you were right.
Your Score
0 / 18
Question 1
Which statement best defines marine aquaculture?
D. The syllabus wording is “the farming of marine species in captivity”, limited to fish, crustaceans and seaweeds. A is fishing, however careful the gear. B is restocking, which is a different activity — the organisms are released rather than kept. C fails on one word: marine. Freshwater farming is aquaculture but it is not marine aquaculture, and the objective is specific.
Question 2
What is bycatch?
B. Bycatch is everything caught that was not the target, including other fish, marine mammals and seabirds, and most of it is discarded. A is about price, which is irrelevant. C is over-quota catch — related, because it is also discarded, but a different idea. D reverses it: escaping is what an increased mesh size is designed to allow, and it is the solution rather than the problem.
Question 3
Why does farming carnivorous fish sometimes fail to reduce pressure on wild stocks?
B. If wild fish have to be caught to feed the farmed ones, fishing pressure has moved to another species rather than disappeared. This is the qualification that separates a good answer from an average one. A, C and D are all real impacts of aquaculture listed in 3.4.3, but none of them is about how much fishing takes place — read exactly what the question asks.
Question 4
Increasing the minimum mesh size of nets helps a stock recover mainly because:
A. Bigger holes release the small and young fish, so they survive to spawn and the stock replaces itself. It reduces bycatch of small species too. C gets it backwards — a larger mesh lets the small ones out, not the large ones. B is a side effect at best and has nothing to do with the stock. D would reduce fishing effort, but by accident, and it is not why the rule exists.
Question 5
A quota can reduce recorded landings without sparing the stock because:
A. Landing over-quota fish is illegal, so it is discarded — and the fish is dead either way. The landings figure falls; the mortality does not. B is a real problem but it is about where boats go, not about the gap between landings and mortality. C may sometimes be true but it is a claim about how the number is chosen, and the question asks about the mechanism. D is true of every fishery and explains nothing.
Question 6
Waste food and faeces settling under a marine fish farm most directly cause:
C. Uneaten food and faeces are organic matter, so they enrich the water and the sediment and the chain from 3.2 follows: enrichment, algal growth, decomposition, falling oxygen. A needs a source of heavy metals, which waste food is not. B confuses this with acid rain. D is exactly backwards, and it is the tempting one — decomposition consumes dissolved oxygen, it does not release it.
Question 7
Which is the strongest argument for a marine protected area?
A. Populations inside recover and breed, and their young move outwards, so fishing just beyond the boundary can improve — which is also the argument that persuades fishing communities to accept one. B is doubtful: enforcing a boundary far offshore is difficult and costly. C overstates it wildly; a protected area covers one area. D is the trap of treating strategies as alternatives — they are used together, and the syllabus lists nine for that reason.
Question 8
Which limitation applies to international fishing agreements in particular?
A. An agreement binds only those who join, and monitoring compliance far out at sea is expensive — which is why the syllabus names implementation and monitoring rather than just “agreements”. B states the opposite of their purpose: shared stocks are exactly what they exist for. C reverses their scope. D invents a conflict; national law is how an agreement is usually put into effect.
Question 9
Overfishing is best defined as catching fish:
B. The definition is about rate, and that is what makes it useful: it explains how a fishery can collapse while boats are still landing plenty, because the catch is being taken out of the breeding stock faster than the breeding stock can rebuild. A, C and D all describe breaking a rule. Rules are broken to overfish, but a fishery with no rules at all can overfish perfectly legally, and a boat inside its quota can still be part of overfishing if the quota was set too high.
Question 10
A fishery removes almost all of one predatory species from an area. What happens next in the food chain?
C. Take a predator out and the thing it ate is no longer controlled, so that population rises and eats more of whatever it feeds on. One clear knock-on, stated with a because, is what 'explain the effect on food chains' is asking for. A describes a general decline that nothing in the stem causes. B is the misconception the objective exists to remove — the whole point is that removing one species reaches every level connected to it. D reverses the direction: the pressure moves down the chain from the predator, and the producers are hit last, not first.
Question 11
Why does the escape of farmed organisms matter to wild populations?
A. Two separate harms and the answer needs both. Escaped organisms take food and space that wild ones would have used, and where they interbreed they change the wild population, because farmed stock is selected for fast growth in a cage rather than for survival in the sea. B is a real concern but it belongs to disease treatment, not to escape. C describes the waste route under the cages, which happens whether anything escapes or not. D contradicts the whole objective — if escapees simply died there would be nothing to describe.
Question 12
Disease spreads quickly on a marine fish farm mainly because:
A. Density is the mechanism. Animals packed together pass parasites and infections between one another far more readily than scattered wild ones can, which is why an outbreak runs through a farm quickly and can then reach the wild population outside the cages. B overstates the waste effect: oxygen falls near a farm, but it does not reach zero. C is not true of any animal. D has the direction of travel backwards — the usual worry is disease moving from the farm outwards.
Question 13
Pole and line fishing is called a sustainable method because it:
C. The syllabus names pole and line as its example of a sustainable method, and the reason is selectivity: a line catches what takes the hook, one at a time, so almost nothing unwanted is hauled up and an area cannot be stripped the way a large net strips it. A confuses a method with a closed season, which is a separate strategy. B invents a rule. D is the opposite of the truth, and it is also why the method needs a price or a rule behind it — it lands less per boat, so it is not adopted on its own.
Question 14
What is the difference between a closed season and a limit on the number of fishing days?
B. Both reduce fishing, but they are aimed at different things and a question will reward you for saying so. A closed season is timed to the breeding season, so adults spawn undisturbed; a limit on days can fall anywhere in the year and simply reduces how much fishing happens. A invents a distinction about where the rule bites. C invents one about who sets it — either can be national. D describes quotas and mesh size, which are two other strategies on the same list of nine.
Question 15
Conservation laws matter to fisheries management because they:
D. A quota, a closed season or a mesh rule is only a suggestion until a law makes it binding and attaches a penalty for breaking it. That is why the syllabus lists conservation laws alongside the measures rather than instead of them. A has it backwards — the law is what makes those measures work. B confuses management with markets. C describes international agreements, the next entry on the list: national law stops at the national boundary, which is exactly why agreements exist.
3.5 Oil Pollution
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▶ Watch: Oil pollution and clean-up
Opens on YouTube in a new tab. These are documentaries about particular spills, and 0680 never asks you to name one. They are here to show what oil does to birds, mammals and beaches, which is 3.5.2. Two gaps you must fill elsewhere: there is nothing here on prevention — no MARPOL, no double-hulled tankers, no risk assessments and no maintenance (3.5.3) — and only one of the five response methods, booms, is covered. Source G below is where you practise the rest.
Again: no species names needed
As in 3.4, the syllabus states that knowledge of specific species is not required. The seven groups it names — birds, marine mammals, fish, crustaceans, seaweeds, coral reefs and beaches — are the answer. Learn what oil does to each group, and do not spend time on named examples.
Where Oil Pollution Comes From
Five causes, and it is worth noticing before you learn them that the dramatic one is not the biggest one. Large tanker accidents make the news, but a great deal of oil enters the sea in small, routine, continuous amounts — from tank washing, from pipeline leaks, from refinery discharges. That contrast is a useful evaluation point, because it explains why prevention focuses on maintenance and rules rather than only on emergency response.
| Cause | How the oil reaches the sea or the coast |
|---|---|
| off-shore and on-shore oil extraction | Blow-outs and leaks at the wellhead of a rig at sea release oil directly into the water. On-shore, oil spilled at a well or during storage runs off the land or seeps into ground water and reaches rivers and then the coast. |
| pipelines | Pipelines corrode, are damaged by ground movement or machinery, and are sometimes deliberately tapped. Because a pipeline can run for hundreds of kilometres, a leak may go unnoticed for a long time. |
| shipping | Collisions and groundings can tear a tanker open and release a large volume at once. Ships also lose smaller quantities routinely from fuel handling and from bilge water. |
| cleaning of tanks at sea | Washing out cargo tanks and discharging the oily water overboard. Each release is small, but it happens constantly and far from land, which is what makes it hard to police — and it is exactly what MARPOL was written to stop. |
| refineries | Refineries are usually on the coast, and oily effluent and spills from their storage and loading operations enter coastal water directly. |
What Oil Does — Seven Groups
Two properties do most of the damage. Oil floats and spreads into a thin film across the surface, and it coats whatever it touches. Almost every impact below follows from one of those two, so if you can remember the two properties you can reconstruct the list.
| Group | What the oil does, and why it matters |
|---|---|
| birds | Oil coats the feathers so they no longer trap a layer of air. The bird loses its insulation and its buoyancy, so it becomes cold and may drown. Birds also swallow oil while preening, which poisons them. Coated wings cannot be used to fly to feeding grounds. |
| marine mammals | Oil coats fur, removing its insulating property in the same way. Animals that surface to breathe inhale oil and vapour, damaging lungs and eyes, and they swallow oil when feeding and grooming. |
| fish | Oil clogs the gills, so less oxygen is absorbed. Toxic components are absorbed into the body and reduce growth and reproduction. Eggs and young stages, which develop near the surface where the oil is, are affected most. |
| crustaceans | Many live on the sea bed or in shallow water where oil settles and persists. Oil coats their gills and body surfaces, toxic components accumulate in their tissue, and populations recover slowly because the oil stays in the sediment. |
| seaweeds | A surface film blocks light, so photosynthesis is reduced. Oil coating the fronds does the same and damages the tissue. Losing seaweeds removes shelter and a food source, so the effect passes into the food web. |
| coral reefs | Oil smothers the coral surface and blocks light, and toxic components damage the tissue. Reefs in shallow water are exposed at low tide. Recovery takes years to decades because corals grow very slowly. |
| beaches | Oil is washed ashore and coats sand, rock and the organisms living between the tides. Beaches become unusable for recreation and tourism, and cleaning is slow and expensive. Oil that soaks into sand or between rocks can persist for years and be released slowly. |
Two properties, seven groups
It coats, and it floats. Coating explains birds, mammals, fish gills, crustaceans and beaches. Floating explains the loss of light for seaweeds and corals, and why eggs and young at the surface suffer most. Add toxic when swallowed or absorbed and you can build an answer for any group the question happens to name.
Do not stop at the organism
Questions on 3.5.2 carry several marks, and a good share of them are for consequences beyond the individual animal: the food web (losing seaweeds or crustaceans removes food and shelter for everything above them), fisheries and income (catches fall and shellfish may be unsafe to sell), and tourism (a coated beach has no visitors). Environmental Management rewards the human dimension — it is in the name of the subject.
Preventing Oil Spills
| Strategy | What it is | What it cannot do |
|---|---|---|
| MARPOL | The International Convention for the Prevention of Pollution from Ships. An international agreement setting rules on what ships may discharge, including limits on oily water, and requirements on how tanks are washed and where waste is delivered. | It binds only the countries that have signed and ratified it, and checking what a ship discharged in the middle of an ocean is difficult, so it depends on port inspections and record keeping. |
| double-hulled oil tankers | Two layers of steel with a space between them, so that a collision or grounding that tears the outer hull may not reach the cargo tanks inside. | Costs more to build, and a severe impact can still breach both hulls. It does nothing about routine operational discharges. |
| risk assessments | Identifying in advance what could go wrong at a terminal, a rig or on a route — and how likely and how serious each event would be — so that controls are put in place before an operation begins. | Only as good as the honesty and the imagination behind it, and it must be acted on. An assessment that is filed and forgotten changes nothing. |
| regular maintenance | Inspecting and repairing pipelines, valves, tanks and hulls on a schedule, so that corrosion and wear are found before they become leaks. | Costs money continuously and shows no visible benefit when it works, which is precisely why it is the thing that gets cut. |
Prevention has two halves
Rules and hardware. MARPOL and risk assessments are rules — they change what people are allowed and required to do. Double hulls and maintenance are hardware — they change the equipment itself. A full answer names at least one of each, and adds that rules need monitoring and enforcement to mean anything.
Minimising the Impact Once Oil Is in the Water
The syllabus gives five, and they are not alternatives — a real response uses several at once, chosen according to how thick the oil is, how rough the sea is and how close the coast is. Notice that the first one is really prevention that has been listed here because it reduces the chance of a spill happening at all.
| Method | How it works | Its limitation |
|---|---|---|
| improved navigation systems | Better charts, positioning and traffic control reduce the collisions and groundings that cause large spills in the first place. | Does nothing about oil already spilled, and cannot prevent equipment failure or a pipeline leak. |
| booms | Floating barriers placed around the slick or across a harbour entrance to contain the oil and stop it spreading, so it can be collected and sensitive coasts protected. | Ineffective in rough water and strong currents, where oil is carried over or under them. They need boats and crews to deploy quickly. |
| sorbents | Materials that soak up oil and are then lifted out, taking the oil with them. Useful for thin films and for the last stage of a clean-up. | Only practical for small quantities, and they create a large volume of oily solid waste that has to be disposed of. |
| detergent sprays | Chemicals sprayed onto the slick break the oil into small droplets that mix into the water, so less oil reaches birds at the surface and less comes ashore. | The oil is dispersed, not removed — it is moved into the water column, where fish and organisms on the sea bed meet it. The detergents themselves can be toxic. |
| skimmers | Devices on boats that lift floating oil off the surface and pump it into tanks, so the oil is physically recovered rather than moved elsewhere. | Slow, and effective only where the oil is thick and the sea is calm. They collect a lot of water with the oil, which must then be separated. |
| controlled burning | Igniting oil that has been concentrated inside fire-resistant booms, removing a large volume from the water surface quickly. | Produces smoke and air pollution, leaves a residue that can sink, and needs fresh, thick oil and calm conditions, so the window is short. |
The evaluation that examiners want
Sort the five methods by what happens to the oil. Booms, sorbents and skimmers remove or contain it — the oil ends up somewhere it can be dealt with. Detergent sprays and controlled burning transfer it — into the water column, or into the air. That is not an argument that dispersing and burning are wrong; they are used because they are fast and can work in conditions where a skimmer cannot. It is an argument that a response is a choice between imperfect options, and saying so, with a reason, is what “evaluate” means.
Data response
Choosing a response to a spill
▼
A tanker grounds 30 km offshore and releases about 9000 tonnes of oil. The slick is thick and the sea is calm for the first two days, then the forecast is for strong winds. The coast 30 km downwind has a shallow reef, an area of seaweed beds that supports a small fishery, and a tourist beach. Of the oil, an estimated 2700 tonnes is recovered by skimmers in the first two days.
(a) Calculate the percentage of the spilled oil recovered by skimmers. [2]
(b) Explain why the response team may switch from skimmers to detergent spray on day three. [3]
(c) Suggest one impact on the seaweed beds and one on the fishery that follows from it. [2]
(a) Calculate the percentage of the spilled oil recovered by skimmers. [2]
(b) Explain why the response team may switch from skimmers to detergent spray on day three. [3]
(c) Suggest one impact on the seaweed beds and one on the fishery that follows from it. [2]
(a)
2700 ÷ 9000 × 100 = 30%. Worth noting how ordinary that figure is: recovering a third of a spill by skimming in ideal conditions is a good result, not a poor one.
(b)
Skimmers work only where the oil is thick and the sea is calm, and the forecast is for strong winds, so from day three they would recover very little. Detergent spray can be applied from aircraft in rougher conditions and works quickly, breaking the oil into droplets so that less of it reaches the surface-feeding birds and less comes ashore on the reef and the beach. The trade-off, which should be stated, is that the oil is dispersed into the water column rather than removed, where fish and sea bed organisms are exposed to it, and the detergent itself can be toxic.
(c)
Seaweeds: the surface film and the coating of the fronds block light and reduce photosynthesis, and the tissue is damaged, so the beds die back. Fishery: the seaweed beds provided shelter and food, so the fish that depend on them decline and catches fall — and shellfish and fish from the area may be unsafe or unsaleable, so incomes drop even where stocks survive.
The move to copy
Part (b) is a three-mark “explain” and the third mark is the trade-off. Whenever you recommend a method in this topic, add the sentence beginning “the cost of doing this is…”. It is almost always worth a mark, and it is the habit that carries into every “discuss” and “evaluate” question on the paper.
Source G
A diagram to complete: five causes and five responses
The second unlabelled diagram in this guide, built the same way as the water cycle exercise in 3.1 — nothing on it is named. Cover the answer and write out all ten before you look. Work through every part on paper before you press Show Answer. Reading a model answer you have not attempted feels like learning and is not.
Top half: how the oil reached the sea. Bottom half: what is being done about it. Schematic; no species and no real place is shown.
identify(a) Name the cause of oil pollution shown at each of A to E. [5]
identify(b) Name the response shown at each of 1 to 5. [5]
state(c) State which of the five responses removes or contains the oil, and which transfers it somewhere else. [2]
explain(d) Explain why the cause at D is difficult to prevent, and name the agreement written to stop it. [3]
suggest(e) A tanker runs aground close to a coast in calm weather. Suggest which two responses should be used first, and justify your choice. [4]
Source G — question 1
On Source G, D shows a vessel far from any coast leaving a thin, continuous trail behind it. Which cause of oil pollution is this?
B. Three details in the drawing decide it: the vessel is a ship, it is far out, and the trail is thin and continuous rather than a single dark mass. That is washing out cargo tanks and putting the oily water over the side — small each time, constant, hard to police, and the practice MARPOL was written to stop. A is E, drawn on the coast with an outfall. C is A, drawn as a fixed platform with a plume from the sea bed. D is B, drawn on land. Read what the picture actually shows before you name it.
Source G — question 2
Responses 3 and 5 on Source G differ from responses 2 and 4 because they:
C. 3 sprays the slick so the oil breaks into droplets that mix down into the water column, and 5 burns it into the air. In both the oil leaves the surface without ever being collected. 2 contains and soaks it up and 4 lifts it off into a tank, so in both the oil ends up somewhere it can be dealt with. A reverses the conditions — both need reasonably calm water. B describes response 1 only. D is untrue of every one of them. Sorting the five into remove and transfer is the classification that makes an evaluation possible.
(a)
A = off-shore oil extraction (a fixed platform, with oil escaping at the sea bed). B = a pipeline (running across land, leaking to a watercourse). C = shipping (a vessel aground on rocks near the coast). D = cleaning of tanks at sea (a ship far out, leaving a thin continuous trail). E = a refinery (a coastal plant with an outfall). Those are the five causes the syllabus names, and the picture gives you a detail for each: what the object is, where it is, and whether the oil arrives all at once or continuously.
(b)
1 = improved navigation systems (a vessel held inside a marked traffic lane). 2 = booms and sorbents (a floating barrier ringing the slick, with absorbent material inside it). 3 = detergent sprays (sprayed from the air onto the slick). 4 = skimmers (a vessel lifting floating oil off the surface into a tank). 5 = controlled burning (oil concentrated inside a fire-resistant ring and set alight).
(c)
Remove or contain: 2 and 4. The oil is collected and can then be dealt with on land. Transfer: 3 and 5. Detergent moves the oil into the water column, where fish and sea-bed organisms meet it; burning moves it into the air and leaves a residue that can sink. 1 is neither — it works before there is any oil in the water at all.
(d)
It happens far from land, out of sight, and in small amounts each time, so nobody sees it and no single release looks serious. It is also routine rather than accidental, which means it can be planned around a rule instead of prevented by better equipment. The agreement is MARPOL — the International Convention for the Prevention of Pollution from Ships, and it depends on port inspections and record keeping rather than on catching a ship in the act.
(e)
2 and 4. The oil is close to a coast, so containment comes first: booms around the vessel and across any harbour entrance stop the slick spreading and protect the shore, and sorbents deal with what gets past. Then skimmers, because the sea is calm and the oil will still be thick, which is exactly the condition a skimmer needs, and it recovers the oil rather than moving it. Detergent would push the oil down into shallow coastal water where it reaches the sea bed; burning needs the oil concentrated and produces smoke over a populated coast. Say what the conditions in the stem allow — 'calm' and 'close to a coast' are both doing work in that answer.
A extraction, B pipeline, C shipping, D tank cleaning at sea, E refinery. 1 navigation, 2 booms and sorbents, 3 detergent sprays, 4 skimmers, 5 controlled burning.
Checkpoint 3.5
Answer, then read the explanation even when you were right.
Your Score
0 / 14
Question 1
What does MARPOL stand for?
B. In full it is the International Convention for the Prevention of Pollution from Ships, and the two words to hold on to are ships and prevention — it controls what vessels are allowed to discharge, including the washing of tanks at sea. The other three are invented, and each is invented in a plausible direction: ports, liability and a general ocean treaty. If you can write out the full name you have secured a straightforward mark that many candidates guess at.
Question 2
Why does oil coating the feathers of a bird often kill it?
C. Feathers work by trapping a layer of air, which insulates the bird and keeps it afloat. Oil destroys that structure, so the bird becomes cold and can drown; it is also poisoned by swallowing oil while preening. A invents an anatomy birds do not have. B has the right idea about weight but the wrong consequence — the problem is staying warm and afloat, not diving. D is invented chemistry.
Question 3
Which cause of oil pollution releases small amounts continuously rather than one large volume?
D. Tank washing releases a little oil at a time, but it happens constantly and far from land where nobody is watching, which is why the total is large and why MARPOL exists. A, B and C are all genuine causes, but each is a single event releasing a large volume at once. The distinction matters because it decides which strategy is appropriate: rules and inspection for the routine releases, response equipment for the accidents.
Question 4
How does a surface film of oil harm seaweeds?
B. Oil floats, so it shades whatever is beneath it; less light means less photosynthesis and the beds die back, taking food and shelter for other organisms with them. Oil coating the fronds directly does the same and damages the tissue. A belongs to acid rain, C to nutrient limitation and D to thermal pollution. All three are real processes in other parts of the syllabus, which is exactly why they are tempting here.
Question 5
A double-hulled tanker reduces the risk of a spill because:
B. The space between the two layers of steel means a tear in the outer hull from a collision or grounding may not open the cargo tanks. A confuses a hull with a filter. C is the opposite of true — a deeper ship is more likely to touch the bottom, not less. D is invented. Remember the limitation as well: a severe impact can breach both hulls, and a double hull does nothing about routine discharges.
Question 6
What is the main drawback of using detergent sprays on a slick?
D. Dispersing is not removing: the oil is broken into droplets that mix into the water column, where fish and sea bed organisms meet it, and the detergents can be toxic themselves. A describes skimmers and controlled burning, which do need calm water; sprays are used precisely because they work in rougher conditions. B describes controlled burning. C describes sorbents. Each wrong option is a true limitation of a different method.
Question 7
Which two methods physically remove oil from the water rather than transferring it elsewhere?
A. Skimmers pump floating oil into tanks and sorbents are lifted out with the oil held in them, so in both cases the oil ends up somewhere it can be handled. B and C include methods that move the oil into the water column or into the air. D mixes a response method with a prevention method — a double hull stops a spill happening and does nothing once oil is in the water. Sorting the five methods into “remove” and “transfer” is the fastest way to answer any question in 3.5.4.
Question 8
Why do coral reefs and crustacean populations recover slowly after a spill?
A. Two separate reasons in one answer: corals build their structure very slowly, so rebuilding takes years to decades, and oil that settles into sediment stays there and keeps exposing the animals living in and on it. B overstates it. C is wrong on the facts — reefs are in shallow water, which is part of why they are exposed. D invents a predator response. When a question names two groups, look for the property they share; here it is slow recovery from long-lasting exposure.
Question 9
Why can a leak from an oil pipeline release a large quantity before it is found?
B. Length is the whole problem. Most of a pipeline is nowhere near anybody, so corrosion, ground movement or damage from machinery can open a leak that runs unnoticed until oil appears in a river or on a shore. That is also why regular maintenance is listed as a prevention strategy. A overstates it — pipelines run on land, under water and along the sea bed. C is untrue, and pressure is part of why a breach empties quickly. D is invented.
Question 10
How does a refinery cause oil pollution of coastal water?
D. Refineries sit on the coast because that is where the tankers arrive, and that closeness is the point: oily process water and spills during storage and loading reach coastal water directly, in small amounts and continuously rather than as one event. A describes air pollution, which is a different topic. B confuses a refinery with an extraction platform. C invents a process. Notice that three of the five causes in 3.5.1 — pipelines, tank cleaning and refineries — are routine rather than accidental.
Question 11
Improved navigation systems reduce oil pollution because they:
A. Better charts, positioning and traffic control keep ships apart and off rocks, so the groundings and collisions that tear a hull open happen less often. It is listed under minimising impacts, but it works by stopping the spill rather than by cleaning one up, and saying that is worth a sentence. B is detergent spraying and C is a skimmer, both of which act after the oil is already in the water. D would if anything raise the amount at risk in a single accident.
Question 12
What is the main drawback of controlled burning of an oil slick?
C. Burning is fast and clears a large volume from the surface, but the oil is not recovered — most of it goes into the air as smoke, and what is left is a heavy residue that can sink and reach organisms on the sea bed. That is why it is grouped with detergent spraying as a method that transfers the oil. A reverses the condition: burning needs thick, fresh oil, so it is used before dispersal, not after. B reverses it too — it needs calm water and a short window. D names only the residue and misses the smoke.
3.6 Plastic Pollution
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▶ Watch: Plastics, bioplastics and microplastics
Opens on YouTube in a new tab. No 0680-badged video exists for this sub-topic. These are general science and documentary, and they are strongest on microplastics and on whether bioplastics really biodegrade, which is the definition 3.6.2 exists to test. None of them gives the five definitions in the syllabus wording, so take those from the table above, not from a video.
Five Words, Two Questions
This sub-topic opens with five definitions, and they are the sort of thing that looks easy and then collapses in the exam because the words sound similar. The way to keep them apart is to notice that they answer two different questions. One question is about what the plastic is made from. The other is about whether it decomposes. They are independent, and the syllabus is careful to say so.
Two axes, not one. The commonest error in this sub-topic is assuming that “bio” in the name means it rots.
| Term | The description to learn |
|---|---|
| conventional plastics | Plastics made from fossil fuels that are generally non-biodegradable. |
| bioplastics | Plastics that can be biodegradable or non-biodegradable and are fully or partly made from biological raw materials rather than from fossil fuels. |
| biodegradable plastics | Plastics that are designed to decompose in water or soil; that are decomposed by the action of bacteria and fungi into water, biomass and gases (carbon dioxide and methane); and that decompose at different rates depending on biotic and abiotic factors. |
| non-biodegradable plastics | Plastics that break down over a long period of time. |
| microplastics | Plastics that are less than 5 mm in length; that are formed when larger plastics break down; and that are used in commercial products. |
The three things people get wrong
One. “Bioplastic” describes the raw material, not the fate. A bioplastic can be non-biodegradable and last just as long in the sea as a conventional one. The syllabus states this in the definition itself, so a question on it is fair game.
Two. Non-biodegradable does not mean “lasts for ever”. The wording is that it breaks down over a long period of time — and that breaking down is precisely how microplastics are formed.
Three. Biodegradable does not mean “disappears wherever you leave it”. It decomposes at different rates depending on biotic and abiotic factors: temperature, moisture, oxygen, light and how many bacteria and fungi are present. Cold, dark, low-oxygen conditions — which is to say the deep sea — slow it right down. That single fact is the strongest limitation of switching to biodegradable plastic and it is worth a mark whenever the switch is proposed.
Getting the biodegradable products right
The syllabus names the products of decomposition and they are worth having exactly: water, biomass and gases — carbon dioxide and methane. Notice methane is on that list. If a question asks for a limitation of biodegradable plastic, “decomposition can release methane, which is a greenhouse gas” is a legitimate point drawn straight from the definition, and very few candidates make it.
Microplastics: two ways in
Less than 5 mm. They arrive by two routes and an answer that gives only one is half an answer. Route one: larger plastic items break down in the sea — waves, sunlight and abrasion tear a bottle or a net into ever smaller fragments. Route two: they are manufactured small on purpose and used in commercial products, and washed straight down the drain. Because they are already tiny, the second route bypasses the breaking-down stage entirely.
What Plastic Does in a Marine Ecosystem
Four impacts, and they climb from the obvious to the invisible. Answer them in that order and you have a structure.
| Impact | What happens |
|---|---|
| visual pollution | Plastic accumulates on beaches, in harbours and on the surface. It is unsightly, so tourism and recreation fall, and the income of coastal communities falls with them. It is the easiest impact to see and the cheapest to fix, which is why clean-ups get attention. |
| entanglement | Marine animals become caught in discarded netting, line, packaging bands and bags. They may be unable to swim or feed, may drown, or may be injured as the material cuts in as they grow. Lost fishing gear continues to catch animals long after it has been abandoned. |
| mistaken for food | Floating plastic resembles food to many marine animals. Swallowed plastic fills the gut without providing any nutrition, so the animal takes in less real food, and it may block or damage the digestive system. Animals also feed plastic to their young. |
| bioaccumulation and biomagnification | Microplastics are eaten by small organisms and pass along the food chain, and they carry toxic substances that stick to their surfaces. Because the particles and the toxins are not digested or excreted, they bioaccumulate in each organism over time and biomagnify in concentration as they are passed up the chain — exactly the process from 3.2.2, arriving by a plastic route. |
Why the last impact needs the most work
Visual pollution and entanglement are one-mark points that everyone writes. The fourth impact is the one that needs a mechanism, because it connects two sub-topics: microplastics are small enough to be eaten by organisms at the bottom of the chain → they are not digested or excreted → each organism accumulates them and the toxins they carry → each predator eats many prey → concentration increases up the chain. Write that and you have written 3.2 and 3.6 at once.
Managing Plastic Pollution
Five strategies. Sort them by where in the life of the plastic they act, because that is what makes an evaluation possible: two act before the plastic exists, two act after it is used, and one sits over the whole thing.
| Strategy | What it involves | Benefits | Limitations |
|---|---|---|---|
| alternative packaging | Replacing plastic packaging with paper, card, glass, metal or plant-based materials, or removing packaging altogether. | Cuts the amount of plastic entering the system at the start, and the alternatives are often easier to recycle or to compost. | Alternatives have their own costs — glass is heavy so transport uses more fuel; paper needs trees and water. Some products need plastic to keep food from spoiling, and food waste has an environmental cost too. |
| avoiding single-use plastics | Reusable bottles, bags and containers instead of items used once and thrown away; refill schemes; charges or bans on single-use items. | Attacks the largest category of plastic waste, and a reusable item replaces many disposable ones over its life. | A reusable item takes more material and energy to make, so it only pays back if it is genuinely reused many times. It also depends on people changing habits, which takes time and reliable access to alternatives. |
| safe disposal | Collecting waste properly and containing it in managed landfill or treating it, so that plastic does not reach drains, rivers and the sea. | Directly prevents plastic entering the marine environment, which is the whole problem, and it needs no change in what people buy. | Requires a collection service and managed sites, which many places do not have. Landfill occupies land and the plastic remains there, since it breaks down only over a long period. |
| recycling | Collecting, sorting and reprocessing plastic into new products, so the same material is used again. | Reduces the demand for new plastic made from fossil fuels, saves energy compared with making it from raw materials, and keeps waste out of the environment. | Plastics must be sorted by type and be reasonably clean, which is labour-intensive; mixed or contaminated plastic often cannot be recycled. Quality falls each time, so most plastic cannot be recycled indefinitely, and the process itself uses energy. |
| legislation and enforcement | Laws banning or taxing particular items, requiring recycled content or deposit-return schemes, setting producer responsibility — plus the inspection and penalties that make them real. | The only strategy that changes what everyone does at once rather than relying on individual choice, and it can shift the cost onto the producer rather than the public. | Needs political agreement and a body able to inspect and prosecute. Rules can be avoided where waste or production simply moves elsewhere, and a law that is not enforced changes nothing at all. |
Ranking them, if you are asked to
Prevention beats capture. Alternative packaging and avoiding single-use plastics stop the plastic existing; recycling and safe disposal deal with it once it does; legislation is what makes any of the other four happen at scale. A conclusion that says “reducing what is produced is more effective than managing what is thrown away, but it is also the slowest to achieve, so disposal and recycling are what limit the damage in the meantime, and legislation is what moves either of them beyond the people who volunteer” is the shape of a strong ending.
Data response
Would switching to bioplastic bags help?
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A coastal town surveys the plastic collected from its beaches. In year 1 it collects 1840 kg, of which 61% is single-use packaging. The council then proposes replacing all conventional plastic carrier bags in the town with bioplastic ones, and states that this will “remove plastic from our beaches because bioplastic is natural”.
(a) Calculate the mass of single-use packaging collected in year 1. [1]
(b) Explain two reasons why the council’s statement may be incorrect. [4]
(c) Suggest a measure likely to reduce beach plastic more than the bag switch, and justify it. [3]
(a) Calculate the mass of single-use packaging collected in year 1. [1]
(b) Explain two reasons why the council’s statement may be incorrect. [4]
(c) Suggest a measure likely to reduce beach plastic more than the bag switch, and justify it. [3]
(a)
1840 × 0.61 = 1122.4 kg (accept 1120 kg).
(b) — reason one
A bioplastic is not necessarily biodegradable. The term describes the raw material — fully or partly biological rather than fossil fuel — and the syllabus states that bioplastics can be biodegradable or non-biodegradable. A non-biodegradable bioplastic bag would persist on the beach exactly as the old one did.
(b) — reason two
Even if the bags are biodegradable, they decompose at different rates depending on biotic and abiotic factors. Sea water is cold, salty and often low in oxygen, and there are fewer of the bacteria and fungi that do the work than in warm soil, so decomposition may be very slow. A material that composts in weeks in a treatment facility can last for years in the sea.
(c)
The data point to the answer: 61% of the collected waste is single-use packaging, and carrier bags are only a part of that. So a measure aimed at single-use plastics as a whole — a charge or ban on single-use items, refill and deposit-return schemes, and improved waste collection so that packaging does not reach the drains — would act on a far larger share of the problem than changing one item. Justify it by the figure, and add that it needs legislation and enforcement to reach everyone rather than only those who choose to take part.
The move to copy
The council’s statement contains a word doing hidden work: “natural”. Definitions are where marks are won in 3.6 — when a claim uses a term loosely, quote the syllabus definition back at it. That is not pedantry; it is exactly what the examiner is testing.
Source H
A beach litter survey to read off a scale
Nothing is written on these bars. Every value has to be measured against the axis, which is what Paper 2 normally asks for. Work through every part on paper before you press Show Answer. Reading a model answer you have not attempted feels like learning and is not.
One survey, one 100 m stretch of beach, one day. Categories are counts of items, not masses.
identify(a) Name the largest category and read its value off the scale. [1]
describe(b) Describe what the survey shows about the composition of the litter. [3]
calculate(c) Calculate the percentage of all the items collected that were made of plastic. Show your working. [3]
explain(d) Explain why the fishing gear category is a concern out of proportion to its size. [3]
discuss(e) The council proposes a charge on single-use plastic items. Discuss the benefits and limitations of that proposal, using the data. [6]
Source H — question 1
Read Source H against the scale. The number of plastic bags collected is closest to:
A. The bag bar ends between the 50 and the 100 gridline and much nearer to 50, so a little over 60 is the honest reading. Nothing is printed on the bars, which is deliberate — on Paper 2 the value usually has to be measured, and 'about 60' with the units is worth the mark where a bare number is not. B is the glass and metal bar. C is the fishing gear bar. D is the cigarette filter bar. Read the axis first, decide what one gridline is worth, and then read the bar.
Source H — question 2
Using Source H, roughly what share of all the items collected was made of plastic?
A. Add the five shaded bars: 254 + 186 + 141 + 97 + 63 = 741, against a total of 820 for all seven, which is 90.4%. B is roughly what you get if you leave out the largest plastic category. C and D are the sort of figure a general impression produces, and a general impression is not evidence. In an answer, give the calculation and then the sentence it supports: 'about 90% of the items collected were plastic, so a strategy aimed at plastic reaches almost all of this litter'.
(a)
food packaging and wrappers, reading about 254 items per 100 m (accept anything from about 250 to 260 — the bar has to be measured against the scale, and an honest reading in that range earns the mark).
(b)
The litter is overwhelmingly plastic: the five shaded categories are the five largest bars. It is also concentrated in a few kinds of item — packaging at about 254 and bottles and caps at about 186 together make up more than half of everything collected — while the two non-plastic categories are the two smallest, at about 48 and about 31.
(c)
Plastic categories: 254 + 186 + 141 + 97 + 63 = 741. All categories: 820. Percentage = 741 ÷ 820 × 100 = 90.4% (about 90%). Set out the addition; the working carries the marks.
(d)
Lost or discarded netting, line and rope go on catching and entangling animals long after they were abandoned, so one item keeps doing harm for years without anybody using it. It is also strong, designed to be hard to break and to resist sea water, so it persists, and as it does break down it produces microplastics. A count of items therefore understates its effect: the number of pieces is not the same as the amount of harm, and saying so is the point of this part.
(e) — the case for
The data support it. Packaging, bottles and caps, cigarette filters and bags are all essentially single-use, and together they come to 644 of 820 items, about 79% of everything collected. A charge acts before the item exists rather than cleaning it up afterwards, it applies to everyone rather than only to people who volunteer, and it shifts some of the cost onto the producer and the buyer instead of the public purse.
(e) — the case against
A charge changes behaviour only if an affordable alternative exists, and some alternatives carry their own costs — glass is heavier to transport, paper needs trees and water, and packaging that keeps food from spoiling has a purpose. It needs enforcement: a rule with no inspection changes nothing. And it does not touch the categories a charge cannot reach: fishing gear, at about 97 items, is lost at sea rather than bought in a shop, and the 79 non-plastic items are outside it altogether.
(e) — the judgement
“About 90% of the items collected are plastic and about 79% are single-use, so a charge is aimed at the right target and would act on the largest share of this litter. It will not reach the fishing gear, which is the category that keeps doing harm after it is lost, so it should be paired with a scheme for collecting and returning gear, and with the enforcement without which neither works.”
(a) food packaging and wrappers, about 254 items. (c) 90.4% of items were plastic. Single-use categories: about 79%.
Checkpoint 3.6
Answer, then read the explanation even when you were right.
Your Score
0 / 13
Question 1
Which statement about bioplastics is correct?
B. The syllabus definition says exactly this: bioplastics can be biodegradable or non-biodegradable, and are fully or partly made from biological raw materials rather than fossil fuels. A is the assumption the definition was written to head off. C reverses the raw material. D drops half of the products of biodegradation — water, biomass and gases, meaning carbon dioxide and methane.
Question 2
Microplastics are defined as plastic pieces:
A. Less than 5 mm in length, arriving by two routes: larger plastics breaking down, and being manufactured small for use in commercial products. B has the unit wrong — centimetres, not millimetres — and misses the manufactured route. C invents a restriction on material; a microplastic is a size, not a substance. D is wrong twice: many microplastics are large enough to see, and the main source is conventional plastic wearing down.
Question 3
Why might a biodegradable bag still persist for years in the sea?
B. The definition says biodegradable plastics decompose at different rates depending on biotic and abiotic factors — temperature, moisture, oxygen and the decomposers present. Cold, salty, low-oxygen deep water is a poor environment for decomposition, so a material that composts in weeks on land can last years at sea. A is false. C invents a single requirement. D is invented chemistry. This is the strongest limitation to raise whenever a switch to biodegradable plastic is proposed.
Question 4
Which impact of plastic on marine ecosystems carries the most marks in an explain question?
C. All four are listed in 3.6.6 and all four earn marks, but the first three are each a single statement, while the fourth needs a chain of reasoning — small enough to be eaten low in the chain, not digested or excreted, accumulating in each organism, and increasing in concentration because each predator eats many prey. That chain is where several marks sit, and it is also the point at which 3.6 connects to 3.2.
Question 5
Non-biodegradable plastics are described by the syllabus as plastics that:
B. The wording is precise and it matters: they break down over a long period of time. A is the popular version and it is wrong — and getting it right explains microplastics, which are the fragments produced by exactly that slow breaking down. C gives the products of biodegradation, which is the other category. D describes bioplastics, and is a definition from a different axis altogether.
Question 6
Which is a genuine limitation of recycling plastic?
C. Sorting by type is labour-intensive, contaminated plastic is often rejected, and the material degrades with each cycle, so most plastic cannot be recycled indefinitely. A states the reverse of the main benefit. B is simply untrue. D is the interesting wrong answer — recycling does use energy, which is a fair limitation, but it generally uses less than manufacturing from raw materials, so the comparison as written is wrong. Read comparative claims carefully.
Question 7
A reusable bottle only reduces plastic pollution if:
A. A reusable bottle takes more material and energy to make than a disposable one, so the saving only exists once it has replaced enough single-use bottles. This is the standard evaluation point about avoiding single-use plastics, and it is the one that shows you are weighing rather than listing. B changes the raw material without changing how often it is used. C is desirable but does not decide the comparison. D describes the bottle rather than what determines the benefit.
Question 8
A country bans single-use plastic bags but does not inspect shops or issue penalties. The most likely outcome is that:
D. Legislation only works where it is enforced — that is why the syllabus writes “legislation and enforcement” as one item, and the same point applies to pollution control in 3.2 and to international agreements in 3.4. A assumes a law changes behaviour by itself. B repeats the bioplastic error: a bioplastic need not be biodegradable, and biodegradation at sea is slow anyway. C claims a link the situation does not support.
Question 9
A beach is covered with plastic, and nearby a marine animal is caught in a length of discarded netting. Which two impacts of plastic pollution are these?
C. The syllabus names four impacts and the stem shows the first two: plastic that is unsightly on a shore, and an animal caught in material it cannot free itself from. A, B and D each pair one right answer with one that needs something the stem does not show. Bioaccumulation and biomagnification need plastic to have been eaten, and 'mistaken for food' needs an animal to swallow it. Match each impact to the evidence in front of you rather than to the topic in general.
Question 10
Why does swallowed plastic harm a marine animal even when the plastic itself is not toxic?
A. The animal feels full, so it takes in less real food and slowly starves, and the plastic can also block or damage the gut. That is the mechanism the objective wants, and it works without any toxicity at all — which is precisely why 'mistaken for food' is a separate impact from bioaccumulation. B is not what plastic does. C borrows the decomposition products of a biodegradable plastic, which needs bacteria and fungi outside the animal. D invents an effect.
Question 11
Bacteria and fungi decompose a biodegradable plastic into:
D. The syllabus states the products and they are worth learning as a set: water, biomass and gases — carbon dioxide and methane. Methane is the one that matters most in an answer, because it is a greenhouse gas, so 'decomposition can release methane' is a legitimate limitation of switching to biodegradable plastic drawn straight from the definition. A, B and C each swap one product for oxygen or nitrogen. Decomposition is not photosynthesis; nothing here releases oxygen.