1 · What the two papers actually pay for▶
Everything on the Topic 3 syllabus is already taught in the study guide — all six sub-topics, 3.1 water sources and supply through to 3.6 plastic pollution. This page is not more content. It is about the gap between an answer that knows the water cycle and an answer that scores, which in Environmental Management is almost always a gap in technique.
Why Topic 3 in particular
Topic 3 is the largest topic on the syllabus: 33 numbered objectives across six sub-topics. But the number that should change how you revise it is a different one. Topic 3 asks you to "discuss the benefits and limitations" four separate times:
| Objective | Discuss the benefits and limitations of… |
|---|---|
| 3.1.8 | desalination |
| 3.1.10 | a multipurpose dam |
| 3.3.4 | strategies to control water-related diseases |
| 3.4.5 | strategies for management of the harvesting of marine species |
Four questions, four different contexts, one shape of answer. Section 2 of this page teaches that one shape and then drills it on all four. It is the highest-value forty minutes you can spend on this topic.
Topic 3 also uses describe and explain six times — 3.2.2, 3.4.1, 3.4.3, 3.4.4, 3.6.6 and 3.6.7 — and that command word has its own rule, covered in section 3.
Where the marks actually are
0680 has two papers. Each is 1 hour 45 minutes, each is 80 marks, and each is worth 50% of the grade. They are not the same paper twice.
| Paper | AO1 knowledge | AO2 application | AO3 analysis & evaluation | What it feels like |
|---|---|---|---|---|
| Paper 1 Principles of Environmental Management | 55 | 30 | 15 | Mostly recall and short explanation, building to one longer discuss question. |
| Paper 2 Environmental Management in Context | 30 | 30 | 40 | Source-led. Nearly every question hangs off a figure, table, map or graph printed on the paper. |
40 marks in every 100 are AO3 on Paper 2
AO3 is not something you can learn by heart the night before. It is a set of habits: read the axes, quote the figures, say what the data cannot tell you, weigh both sides, land a judgement. Those five habits are the whole of section 4 of this page, and on Paper 2 they are worth more than any single sub-topic you could revise. Water is a topic examiners love to build sources around — river quality graphs, water-access tables, catch data, coastal maps — so Topic 3 and AO3 go together.
What each AO is asking for
| AO | The question is really asking | Topic 3 example |
|---|---|---|
| AO1 Knowledge and understanding | Give back what you were taught, accurately and in the right words. | State the four stages of water treatment. |
| AO2 Application | Take what you know and use it on a place, a plant or a situation you have never seen before. | Suggest why reverse osmosis rather than distillation was chosen for this island. |
| AO3 Analysis, interpretation and evaluation | Work from the source in front of you. Describe what it shows with numbers, work something out from it, say what it does not show, weigh two options and decide. | Using Fig. 2, evaluate the claim that the river has recovered. |
Command words, and what they pay
Read the command word before you read anything else. A very large share of lost marks in this topic are answers that describe when they were asked to explain, or list when they were asked to discuss.
| Command word | What earns the mark | What earns nothing |
|---|---|---|
| State / Name / Identify | The correct term. Nothing more is needed and nothing more is likely to be credited. | A paragraph. You are not paid for it and you have spent the time. |
| Define | The syllabus wording. Potable water is water that is safe to drink is the whole answer to that one. | An example instead of a definition. |
| Describe | What it is, or what the pattern is. For data: the direction, the figures and the units. | Reasons. A reason in a describe question is unlikely to be credited. |
| Explain | Why or how — the mechanism, in linked steps. | Restating the question. "The oxygen falls because there is less oxygen." |
| Describe and explain | Both. One mark names it; the rest are the chain. | Naming three impacts and explaining none. Very common, and it caps you low. |
| Suggest | An idea that fits the new situation. There is no single right answer; there is a right kind of answer. | Something true in general but not connected to the case on the page. |
| Discuss / Evaluate / To what extent | Two sides, then a judgement with a reason attached. | A list of benefits with no limitations and no conclusion. This is the Topic 3 killer. |
| Calculate | Working shown, then the answer with its unit. | A bare number that happens to be wrong. With working, a slip can still earn the method mark. |
Count the marks, then plan that many points
A [4] is four separate creditable points, not one point said four ways. Before you write, put four dots in the margin. If you can only fill three, you know to think again before you have used the space, not afterwards.
For a discuss question the dots are not all the same. On a [6] discuss, plan roughly two benefits, two limitations, and two sentences of judgement — not six benefits.
A standing health warning about the mark numbers on this page
Where this page writes (1) beside a sentence, that is my reading of what a question of that size is asking for. It is not copied from a published Cambridge mark scheme, and I have not checked it against one. Treat the allocations as a way of seeing the structure of a good answer, and aim to include the points shown — not as a promise about what an examiner will tick.
Checkpoint — how the papers pay
Three questions on what the examiner is buying.
Score: 0 / 3
Question 1
Topic 3 uses the words discuss the benefits and limitations four times. What does a discuss question pay for that a describe question does not?
Discuss, evaluate and to what extent all want the same three things: points in favour, points against, and a decision that says on what grounds. Detail, length and vocabulary are not what moves the mark, and 0680 does not require you to learn named case studies for these four objectives. If your discuss answer contains no sentence beginning "overall" or "on balance", you have almost certainly left the judgement marks behind.
Question 2
Which paper carries the larger proportion of AO3 marks, and roughly how large is that proportion?
Paper 2 is AO1 30 / AO2 30 / AO3 40, so AO3 is 40 marks in every 100 — about two fifths. Paper 1 is AO1 55 / AO2 30 / AO3 15, so its AO3 share is only about a seventh, which rules out both of the Paper 1 answers and the one that says the two papers are the same. Both papers are 80 marks and each is worth half the grade, so the AO3 habits are not optional.
Question 3
A question reads: Discuss the benefits and limitations of desalination. [6] A student writes six good benefits of desalination and nothing else. What is the most she is likely to score?
A discuss question splits its marks across the two sides and the judgement, so six benefits can only reach into the benefit half of the answer, however good they are. Six marks is therefore not available; nor is a deduction, because marks are awarded for what is there rather than taken away for what is missing. Nor is zero the right answer — good benefits are still creditable points. This is the commonest way to lose marks in Topic 3, and it is entirely avoidable: as soon as you see discuss, write LIMITATIONS in the margin before you start.
2 · One frame for all four "discuss" objectives▶
Four objectives, four contexts, one shape. Learn the shape once and you can walk into any of the four without knowing which one is coming.
The five-step discuss frame
Say it to yourself as a sentence: Good thing — bad thing — who pays — how soon — so what?
1
Benefit — with a mechanism. Not "it gives water" but what it does and how. One good benefit explained beats three named.
2
Limitation — with a mechanism. Same rule. "It is expensive" on its own is weak; why is it expensive, and what does the expense stop happening?
3
Who bears the cost, and who takes the benefit. In water management these are very often different people. Saying so is worth a mark on its own and almost nobody does it.
4
Over what timescale. Most water schemes give the benefit soon and the cost later, or the cost now and the benefit in ten years. Naming that mismatch is a genuine evaluation point.
5
Judgement — a position, the grounds, and the condition. "On balance X is worth doing where Y" is a judgement. "There are advantages and disadvantages" is not.
Steps 3 and 4 are where the grade 8 lives
Almost every candidate can produce step 1 and step 2 — a benefit and a limitation. What separates the top answers is that they notice the benefit and the cost land on different people, or at different times, and they say so. Those two sentences are cheap to write and they are the difference between a competent answer and a convincing one.
The frame filled in on all four Topic 3 discuss objectives
Read down one column at a time, not across. Then cover the table and rebuild one column from memory.
| Step | 3.1.8 Desalination | 3.1.10 Multipurpose dam |
|---|---|---|
| 1 Benefit | Sea water is effectively unlimited, so a coastal region with little rainfall, no large river and little ground water gains a supply that does not depend on the weather or on a neighbour upstream. The water produced is of very high quality. | One structure delivers many things at once: flood control, hydro-electric power, irrigation water, storage, transport, recreation, tourism and fish farming. The stored water evens out a seasonal river, so irrigation is possible in the dry season, and the electricity is generated without burning fuel. |
| 2 Limitation | Both routes need a large energy input — distillation must heat water to boiling, reverse osmosis must hold water at high pressure — so running costs are high, and where the electricity comes from fossil fuels there are emissions attached to every litre. The concentrated salt solution left behind has to go somewhere, usually back to the sea, where it raises the salinity near the outfall. | Land upstream is flooded to make the reservoir, so people are displaced and habitat and farmland are lost. The reservoir traps sediment, so the fertile silt that used to be spread on the floodplain no longer reaches it. The dam is a physical barrier in the river, so fish cannot move up and down it. In a hot climate a large amount of the stored water is lost by evaporation from the reservoir surface. |
| 3 Who pays | Coastal cities and industry gain, because the water has to be piped from the coast and piping is expensive; inland and rural users often gain nothing. The marine environment near the brine outfall bears the salinity. | The country gains power and irrigation. The people whose land is flooded upstream pay, and so do the farmers on the floodplain downstream who lose the silt. Where the river crosses a border, the downstream country pays for a benefit the upstream country takes. |
| 4 Timescale | Very high capital cost up front; supply reliable from the day it opens; the energy bill continues for as long as the plant runs. | Displacement is immediate and permanent. Power and irrigation last for decades. Sediment trapping and the loss of downstream fertility build up slowly over those same decades, so the cost grows while the benefit stays flat. |
| 5 Judgement | Defensible where there is genuinely no alternative source and energy is available — a coastal, water-poor region. It is not a general answer to water shortage, because it does not reach inland or rural users and does nothing at all about demand. | Usually justified where the displaced population is small and properly compensated and the downstream flow is negotiated in advance. The benefits do not cancel the limitations; they land on different people, which is why a dam can be a national success and a local disaster at the same time. |
| Step | 3.3.4 Controlling water-related disease | 3.4.5 Managing marine harvesting |
|---|---|---|
| 1 Benefit | Several of the methods are cheap and work immediately. Handwashing, boiling and chlorination break the route by which cholera bacteria reach the mouth; nets, repellent, antimalarial drugs and vaccination protect the individual from malaria; draining or covering standing water removes the places mosquito larvae develop, so the vector population falls. | Each measure attacks a different part of the problem. A larger mesh size lets young fish pass through the net so they survive to breed; a quota caps the total taken so the stock can replace itself; a closed season protects the breeding period; a protected area keeps a reservoir of breeding adults; pole and line takes one animal at a time so bycatch is very low; international agreements cover stocks that move across borders. |
| 2 Limitation | They must be sustained and used correctly: a net that is not used every night, a drug course not finished, a chlorination plant without a reliable power supply. Insecticides become less effective as resistant mosquitoes survive and breed, and they kill non-target insects. Sewage treatment and piped potable water need very large investment, trained staff and years of construction. Vaccination needs cold storage and repeated contact with every household. | Everything here has to be monitored and enforced at sea, which is expensive and difficult. Quotas can lead to fish that are over the limit being thrown back dead. A national rule does nothing to a fleet from another country fishing in international waters. Restrictions cut catch and income now, so compliance is hard to obtain, and the stock estimate the quota is built on is itself uncertain. |
| 3 Who pays | The individual gains the protection; the state or an aid agency pays for the infrastructure. The poorest and most rural households are the hardest and most expensive to reach and are usually the most exposed, so the people who need it most are the last to get it. | The long-term fishery, and the crews who are not yet working, gain. Today's crews and coastal communities pay first and pay most, which is the whole reason management is resisted. |
| 4 Timescale | Personal protection works from the first night. Sanitation and a treated water supply take years to build but then protect everyone continuously without anyone having to remember to do anything. Insecticide effectiveness can fall over a few years as resistance spreads. | The cost is immediate; the benefit arrives only after several breeding cycles. That mismatch is the single most quotable sentence in this whole objective. |
| 5 Judgement | No single method is enough, because each has a different failure mode. A defensible position is that a programme should pair one measure that works tonight (nets, boiling) with one that works permanently (sewage treatment, piped potable water), so that the failure of one does not leave people unprotected. | Measures that work by making the harmful act physically impossible — mesh size, a closed area — are more robust than measures that depend on honest reporting, such as a quota. So the defensible answer is a combination, with monitoring, rather than any one measure. |
One sentence you can carry into all four
"The benefit and the cost do not fall on the same people, or at the same time." That sentence is true of desalination, of dams, of disease control and of fisheries management. It is a step 3 and a step 4 in one line, and you can adapt it in the exam in about eight seconds.
Now fade it out — four questions, less help each time
Work them in order. Do not read the next one until you have written the one before.
Worked in fullDiscuss the benefits and limitations of desalination as a source of potable water for a coastal city. [6]
Read the model, then read the note underneath about which sentences did the earning. The five steps are labelled in the margin so you can see the frame inside the prose.
The answer
[1 Benefit] Desalination takes sea water, which for a coastal city is effectively unlimited, and removes the dissolved salt to leave water that is safe to drink (1). Because the source is the ocean rather than rainfall, the supply does not fail in a drought and does not depend on a river shared with anyone else (1).
[2 Limitation] However, both routes need a large and continuous energy input: distillation has to heat the water until it boils, and reverse osmosis has to hold it at high pressure against a membrane (1). That makes the water expensive to produce, and where the electricity is generated from fossil fuels the plant carries emissions with it. The concentrated salt solution left over is usually returned to the sea, where it raises the salinity around the outfall and can harm the organisms living there (1).
[3 Who pays] The city gains, but the plant must be on the coast and the water piped from there, so inland and rural users gain little, while the marine environment near the outfall bears the brine (1).
[4 Timescale + 5 Judgement] The capital cost falls at the start and the energy cost never stops. Overall, desalination is worth building for a coastal city that has genuinely run out of fresh water and has energy available, but it is a last resort rather than a first one, because reducing leakage and demand costs far less per litre (1).
[2 Limitation] However, both routes need a large and continuous energy input: distillation has to heat the water until it boils, and reverse osmosis has to hold it at high pressure against a membrane (1). That makes the water expensive to produce, and where the electricity is generated from fossil fuels the plant carries emissions with it. The concentrated salt solution left over is usually returned to the sea, where it raises the salinity around the outfall and can harm the organisms living there (1).
[3 Who pays] The city gains, but the plant must be on the coast and the water piped from there, so inland and rural users gain little, while the marine environment near the outfall bears the brine (1).
[4 Timescale + 5 Judgement] The capital cost falls at the start and the energy cost never stops. Overall, desalination is worth building for a coastal city that has genuinely run out of fresh water and has energy available, but it is a last resort rather than a first one, because reducing leakage and demand costs far less per litre (1).
Why it scores across the whole question, not half of it
Count the paragraphs: benefit, limitation, who pays, judgement. Nothing is repeated. Notice that the final sentence does three things at once — it takes a position (worth building), states the grounds (no fresh water left, energy available) and gives the condition under which the answer changes (cheaper options first). That is what a judgement looks like.
What a weak version looks like
"Desalination gives you fresh water from the sea which is good because there is a lot of sea. But it is expensive." That is one benefit and one bare limitation with no mechanism behind either — and no step 3, 4 or 5 at all.
Good thing → bad thing → who pays → how soon → so what.
Fading step 1 — the judgement is blankDiscuss the benefits and limitations of building a multipurpose dam on a large river. [6]
Steps 1 to 4 are written for you. You write step 5 — the judgement. It must take a position, give the grounds, and name the condition under which you would say the opposite.
Given: A multipurpose dam provides flood control, hydro-electric power, irrigation water, storage, transport, recreation, tourism and fish farming from a single structure, and the stored water lets farmers irrigate through the dry season. However, the reservoir floods land upstream, so people are displaced and habitat is lost; it traps sediment, so the silt that fertilised the floodplain downstream no longer arrives; and the dam blocks the movement of fish along the river. The country gains the power and the irrigation, but the displaced households upstream and the floodplain farmers downstream pay for it, and the displacement is immediate while the loss of downstream fertility builds up over decades.
You write: Overall, because , although this would change if .
Model judgement
Overall, a multipurpose dam is usually worth building where the population that has to be moved is small and is properly compensated, and where the flow released downstream is agreed in advance, because the same structure supplies power, dry-season irrigation and flood protection that no single alternative provides (1). This would change if the reservoir would displace a large population, or if the river crosses a national border and the downstream country has not agreed to the scheme — in that case the benefits are taken by one group and the costs paid by another, and the dam is not justified (1).
Marking your own
Three tests. Did you say what you think, not just that opinions differ? Did you attach a because that refers to something in the paragraph above? Did you name a condition that would flip your answer? If any of the three is missing, the sentence is a summary, not a judgement.
A judgement = position + grounds + the condition that would change it.
Fading step 2 — you write all five stepsDiscuss the benefits and limitations of strategies used to control water-related diseases. [6]
Nothing is given. Use the frame: benefit, limitation, who pays, how soon, judgement. Aim to name at least one malaria strategy and one cholera strategy, and to give a mechanism for each rather than just the name.
Model answer
Benefit. Several strategies are cheap and work at once. Sleeping under a net puts a physical barrier between a person and the female mosquitoes that carry the malaria parasite, so bites at night are prevented (1). Boiling or chlorinating drinking water kills the cholera bacteria in it, so the route from contaminated water to the mouth is broken (1).
Limitation. These depend entirely on being used correctly every single time: a net that is torn or not hung, water that is boiled today but not tomorrow. Spraying insecticide over large areas becomes less effective as resistant mosquitoes survive and breed, and it also kills insects that were not the target (1). The permanent solutions — adequate sanitation, sewage treatment and a piped potable supply — need very large investment, trained staff and a reliable power supply (1).
Who pays. The individual gets the protection, but the state or an aid agency pays for the treatment works and the spraying programme; the rural and poorest households are the most exposed and also the most expensive to reach (1).
Timescale and judgement. Personal protection works from the first night but only while it is kept up; sanitation takes years to build but then protects everyone without anyone having to remember anything. Overall the two types should be used together rather than chosen between, because they fail in different ways: a combination of an immediate measure and a permanent one is far more reliable than either alone (1).
Limitation. These depend entirely on being used correctly every single time: a net that is torn or not hung, water that is boiled today but not tomorrow. Spraying insecticide over large areas becomes less effective as resistant mosquitoes survive and breed, and it also kills insects that were not the target (1). The permanent solutions — adequate sanitation, sewage treatment and a piped potable supply — need very large investment, trained staff and a reliable power supply (1).
Who pays. The individual gets the protection, but the state or an aid agency pays for the treatment works and the spraying programme; the rural and poorest households are the most exposed and also the most expensive to reach (1).
Timescale and judgement. Personal protection works from the first night but only while it is kept up; sanitation takes years to build but then protects everyone without anyone having to remember anything. Overall the two types should be used together rather than chosen between, because they fail in different ways: a combination of an immediate measure and a permanent one is far more reliable than either alone (1).
The two things most answers drop
First, the mechanism: "nets stop malaria" is a name, "a net is a physical barrier so the mosquito cannot reach the skin to bite" is a mark. Second, vector control: many answers give only personal protection. 3.3.2(b) also lists covering or draining breeding areas, spraying, sterilising male mosquitoes and biological control, and an answer with nothing from that list has only used half the syllabus.
Two families of strategy — personal protection and vector control — and they fail differently.
Fading step 3 — timed, no help at allDiscuss the benefits and limitations of strategies for the management of the harvesting of marine species. [6]
Give yourself eight minutes and write it in one go, without looking anything up. Then open the model and mark yourself against the five steps.
Model answer
Benefit. Increasing the mesh size of nets lets young, small individuals pass straight through, so they survive long enough to breed and the stock can replace itself (1). Quotas cap the total mass that may be landed, closed seasons stop fishing during the breeding period, and protected areas keep a reservoir of breeding adults that restocks the surrounding water; pole and line takes one animal at a time, so very little bycatch is taken and other species and the wider food chain are protected (1).
Limitation. All of these have to be monitored and enforced far out at sea, which is expensive and hard to do (1). A quota can cause fish taken over the limit to be discarded dead rather than landed, so the stock is damaged anyway; and a national law has no effect on a fleet from another country in international waters, which is why international agreements with an agreed way of monitoring them are needed (1).
Who pays and how soon. The cost falls on today's crews and coastal communities in the form of a smaller catch, while the benefit — a stock that can still be fished in twenty years — goes to people who are not fishing yet and only arrives after several breeding cycles (1).
Judgement. On balance, measures that make overfishing physically impossible, such as mesh size limits, a limit on boat and net size or a closed area, are more dependable than measures that rely on accurate self-reporting such as quotas. The defensible position is a combination, backed by monitoring and by conservation law, rather than any single measure (1).
Limitation. All of these have to be monitored and enforced far out at sea, which is expensive and hard to do (1). A quota can cause fish taken over the limit to be discarded dead rather than landed, so the stock is damaged anyway; and a national law has no effect on a fleet from another country in international waters, which is why international agreements with an agreed way of monitoring them are needed (1).
Who pays and how soon. The cost falls on today's crews and coastal communities in the form of a smaller catch, while the benefit — a stock that can still be fished in twenty years — goes to people who are not fishing yet and only arrives after several breeding cycles (1).
Judgement. On balance, measures that make overfishing physically impossible, such as mesh size limits, a limit on boat and net size or a closed area, are more dependable than measures that rely on accurate self-reporting such as quotas. The defensible position is a combination, backed by monitoring and by conservation law, rather than any single measure (1).
Self-marking checklist
Tick each: (i) at least two named strategies with a mechanism, not just names; (ii) at least two limitations, one of which is about enforcement; (iii) a sentence saying who pays; (iv) a sentence about the delay between cost and benefit; (v) a judgement with grounds. Five ticks is a strong answer. If you have four names and no mechanisms, that is the classic list answer.
Physical limits beat paper limits, because a net with a big mesh cannot lie about what it caught.
Checkpoint — the discuss frame
Four questions on using the frame.
Score: 0 / 4
Question 1
Which of these is a limitation of desalination that is about the waste product rather than about the energy the plant uses?
Desalination has two separate limitations and most answers give only one of them. The energy limitation is that heating water to boiling, or holding it at high pressure, costs a great deal to run — which is what the pressure answer describes. The waste limitation is quite different: whatever salt you take out of the water still exists, and the concentrated solution has to be disposed of somewhere, usually back into the sea. Giving both, rather than the same point twice, is how you fill the limitation half of a [6]. Needing to be on the coast is a practical constraint rather than an environmental limitation, and high water quality is a benefit.
Question 2
In the multipurpose dam frame, which group is most often left out of student answers when they reach step 3 — who bears the cost?
Almost everyone remembers the displaced households upstream — it is the vivid image. The downstream loss is the one that gets dropped, and it is a genuinely separate point: the reservoir traps sediment, so the fertile silt that used to be deposited on the floodplain in each flood no longer arrives, and soil fertility there falls over the following decades. That one sentence gives you a second group who pay and a long timescale in a single move. The government does pay, but it also takes the benefit, so it is a weaker evaluation point; the builders are paid for their work.
Question 3
Why is the timescale step worth a mark of its own in the marine harvesting discuss?
The mismatch between when the cost falls and when the benefit arrives explains something the question is really about: why sensible management is so often resisted by the very people it is meant to help. Cost now, benefit after several breeding cycles, and largely to somebody else. The statements about quotas being annual and closed seasons lasting months are both true, but neither is an evaluation — they are description in disguise. Trip length is not a syllabus point at all.
Question 4
Which of these four sentences is a judgement rather than a summary?
A judgement takes a position, gives the grounds and, ideally, names the condition under which it would change — which is exactly what the sentence about a coastal city with no fresh water left does. The other three are all ways of avoiding making one. The "both advantages and disadvantages" sentence in particular feels like a conclusion and is not one; it is the single most common ending to a Topic 3 discuss answer, and it restates the command word instead of obeying it.
3 · Three answers, one question▶
The most useful thing anyone can show you is the same question answered three ways. Below are two Topic 3 questions, each with a weak answer, a middling answer and a strong answer — and, more importantly, exactly which words moved the mark.
First, the rule that governs the first question
Topic 3 uses describe and explain six times. The rule is short: one mark names it, the rest are the chain. Everything after the name comes from linking the steps between the thing and the outcome, using words like so that, because, which means and this causes. If your answer contains none of those words, you have written a list, and a list is capped at about one mark per item however many items it has.
Question A
Describe and explain two impacts of marine aquaculture on the
water and the wildlife around the farm. [6]
Six marks, two impacts. So the sensible plan is 1 for naming each impact and 2
for the chain behind each — two paragraphs of three points, not one long paragraph about fish
farms in general. Work that structure out from the number in the brackets before you write a word.
Weak answer1 of 6
Fish farms are bad for the environment. They pollute the water and the fish can escape into the sea which is bad for the wild fish. This damages the ecosystem.
What it earned: 1, for the idea of escape, if the examiner is generous.
Why the other five were lost: "They pollute the water" names no pollutant and no process, so it is not yet an impact. "Bad for the wild fish" and "damages the ecosystem" are the question repeated back — the question already assumes there is an impact, so saying there is one cannot be a reason. There is not one linking word in the whole answer, and "bad for the environment" is a verdict where a mechanism was asked for.
Why the other five were lost: "They pollute the water" names no pollutant and no process, so it is not yet an impact. "Bad for the wild fish" and "damages the ecosystem" are the question repeated back — the question already assumes there is an impact, so saying there is one cannot be a reason. There is not one linking word in the whole answer, and "bad for the environment" is a verdict where a mechanism was asked for.
Middling answer4 of 6
One impact is nutrient enrichment. The waste from the farm goes into the water and adds nutrients, so the water quality gets worse. Another impact is that farmed animals can escape from the cages into the wild population, which can affect the local wild animals and pass on disease.
What it earned: 1 for naming nutrient enrichment, 1 for the waste being the source, 1 for naming escape, 1 for the disease link.
Why the last two were lost: the nutrient chain stops one link too early. "Water quality gets worse" is not yet a mechanism — it needs to reach the oxygen: added nutrients cause rapid growth of algae, the algae block the light, the plants below die, bacteria decompose them and use up the dissolved oxygen. And "can affect the local wild animals" is vague where the syllabus is specific: 3.4.3 lists the effect on local food webs, so name that.
Why the last two were lost: the nutrient chain stops one link too early. "Water quality gets worse" is not yet a mechanism — it needs to reach the oxygen: added nutrients cause rapid growth of algae, the algae block the light, the plants below die, bacteria decompose them and use up the dissolved oxygen. And "can affect the local wild animals" is vague where the syllabus is specific: 3.4.3 lists the effect on local food webs, so name that.
Strong answer6 of 6
The first impact is nutrient enrichment of the surrounding water (1). Uneaten food and the animals' faeces fall through the cages and dissolve, adding nitrate and phosphate to the water below the farm (1). Those nutrients allow algae to grow very rapidly; the algae block the light reaching the plants beneath them, so those plants die, and the bacteria that decompose the dead material use up the dissolved oxygen, so the animals living in that water are left without enough oxygen and die (1). The second impact is the risk of escape (1). Farmed animals kept at very high density can get out through a damaged cage into the open sea, where they compete with the wild population for the same food and space (1). High density in the cages also allows disease and parasites to spread quickly, and escaped animals carry them into the wild population, so the local food web is affected well beyond the farm itself (1).
Why it is full marks: two impacts, each named and each followed by a chain that ends at a living thing rather than at the water. Notice the middle of the nutrient chain is about oxygen, not about "pollution" — the specific mechanism is what is being bought. Length is not what did it; the strong answer is barely longer than the middling one, it is just less vague.
The one-sentence diagnosis
Weak answers stop at the verdict ("it is bad"). Middling answers stop at the water ("quality gets worse"). Strong answers reach the organism ("so the animals living there do not have enough oxygen"). Before you put your pen down, ask: does my last sentence mention a living thing?
The same treatment on a discuss question
Question B
"Desalination is the answer to the world's water shortage." To what
extent do you agree? [4]
"To what extent" is a discuss command. Four marks means roughly:
a point for, a point against, a second point on one side, and a judgement that is not a shrug. And look
hard at the wording — the answer is an absolute, and absolutes are put in quotations on
purpose.
Weak answer1 of 4
I agree because there is a lot of sea water and we can turn it into drinking water so nobody will be thirsty. Countries should build more desalination plants.
One mark, for the idea that the ocean is an effectively unlimited source. There is no second side at all, "nobody will be thirsty" is not a mechanism, and "countries should build more" is an opinion with no evidence attached, which is not creditable. The word the in the quotation has not been noticed.
Middling answer3 of 4
Desalination removes salt from sea water to make it potable, either by distillation or by reverse osmosis, and the ocean is effectively unlimited so a coastal country with little rainfall gains a reliable supply. However, it uses a lot of energy because the water must be heated or held under high pressure, which makes it expensive, and the brine left over is returned to the sea. So there are advantages and disadvantages.
Three solid marks: the mechanism, the reliability benefit, and a genuine other side with a reason behind it. The fourth is lost on the last sentence — "there are advantages and disadvantages" is not a judgement, it is a refusal to make one. A to what extent question pays for the decision and the grounds.
Strong answer4 of 4
Desalination removes dissolved salt from sea water, by distillation or by reverse osmosis, to leave water that is safe to drink; because the ocean is effectively unlimited, a coastal region with little rainfall and no large river gains a supply that does not fail in a drought. Against that, both routes need a large continuous energy input — heat for distillation, high pressure for reverse osmosis — so the water is expensive per litre and carries emissions where the electricity is generated by burning fossil fuels, and the concentrated brine returned to the sea raises the salinity near the outfall. The word that makes the statement too strong is the: desalination can only serve places near a coast that can afford the energy, so it does nothing for inland and rural regions, which is where much of the world's shortage actually is, and it does nothing at all about demand or about leaking pipes. I agree that it is an answer for coastal, water-poor, energy-rich regions, but not that it is the answer.
The fourth mark is in the last two sentences. They take a position, say on what grounds, and do it by attacking the absolute in the quotation — which is nearly always the right move when a quoted statement contains one. Learn to look for the, always, never, only and best in a quotation; they are there on purpose and they are the easiest route into the judgement mark.
Checkpoint — reading your own answer
Three questions on self-marking.
Score: 0 / 3
Question 1
A student ends a six-mark aquaculture question with "and so it damages the ecosystem". Why does that sentence earn nothing?
The question is what are the impacts and why do they happen, so "there is an impact" is the given rather than the answer, and every mark has to sit on the path between the fish farm and that outcome. The claim about Topic 4 is false — 3.4.3 explicitly covers effects on local food webs. Habitat and ecosystem would both be acceptable words; the problem is not the noun, it is that there is no mechanism attached to it. This is the commonest wasted sentence in Topic 3 answers, and deleting it costs you nothing at all.
Question 2
The middling aquaculture answer said the waste "adds nutrients, so water quality gets worse". What was the cheapest available upgrade?
About twenty extra words, two extra marks. "Water quality gets worse" is a verdict; the oxygen chain is a mechanism, and the mechanism is what is being bought. None of the other three additions touches the missing link — they add material rather than depth, which is the whole problem with the middling answer. That same chain answers a eutrophication question at 3.2.2(f), a fertiliser run-off question and an aquaculture question, so it is one of the highest-value things in this topic to know word-perfect.
Question 3
In a to what extent question, why is it worth hunting for words like the, always, only and best in the quotation?
The absolute is the hinge of the question. You are not required to say the statement is false, and the answer claiming it is certainly false overstates the case badly; but you are required to say to what extent it holds, and the honest answer is nearly always "in these conditions yes, in those conditions no". That sentence is the judgement, and the absolute hands it to you. The counts have nothing to do with the marks, and both papers use quotations.
4 · The AO3 toolkit — working from a source▶
AO3 is 40 marks in every 100 on Paper 2. It is also the most trainable thing in the whole syllabus, because it is five repeatable moves rather than a body of knowledge. Here they are, demonstrated on one source, and then drilled on three more in section 6.
Demonstration source
Table 1 — a persistent toxic substance measured along a marine food chain
Samples were taken from one bay. The same substance, which is not broken down by the body and is not excreted, was measured in the water and in the tissue of organisms at four positions in the food chain.
| Where the substance was measured | Concentration of the substance / mg per kg |
|---|---|
| Water | 0.001 |
| Producers (algae and other plankton) | 0.05 |
| Primary consumers (small filter-feeding animals) | 0.4 |
| Secondary consumers (small predatory animals) | 3.2 |
| Top predators (large predatory animals) | 25.6 |
Table 1. Constructed for practice, not measurements from a named bay. The organisms are described by their position in the food chain rather than by name, which is how 0680 examines this: no knowledge of particular species is required.
Move 1 — read the headings and the units before you read the question
Thirty seconds, every time. In Table 1: the left column is position in the food chain, not a list of places; the right column is mg per kg, a concentration, not a total amount; and the first row is the water, which is not an organism at all. A student who misses that last point writes about "five organisms" and gets the whole analysis one step out.
Units are marks
"25.6" is not an answer; "25.6 mg per kg" is. On calculation questions the unit is frequently the mark most often dropped, and it costs nothing to write.
Move 2 — quote figures, and quote both ends
A describe-the-data answer without numbers in it is doing half the job. And one number is not enough: quote the smallest and the largest, so the examiner can see you have read the range and not just the first row.
Weak: "The concentration goes up along the food chain."
Strong: "The concentration rises at every step along the food chain, from
0.001 mg per kg in the water to 25.6 mg per kg in the
top predators — about 25,600 times greater."
The three calculations that actually come up
| Asked for | How to do it | On Table 1 |
|---|---|---|
| A difference | Larger − smaller. Keep the unit. | 25.6 − 0.4 = 25.2 mg per kg |
| How many times greater | Larger ÷ smaller. The answer has no unit — it is a ratio. | 25.6 ÷ 0.4 = 64 times greater |
| A percentage | Part ÷ whole × 100. Show the division before you divide. | See section 6, where two of the practice sets ask for one. |
Move 3 — say what the data does not show
This is the move almost nobody makes, and it is creditable in any evaluation. Ask four questions of every source:
- How long? Table 1 is a single set of samples. It shows a pattern along the food chain at one moment; it does not show anything changing over time, so on its own it is evidence of biomagnification, not of bioaccumulation.
- How many? One bay. Nothing here tells you whether another bay behaves the same way.
- What is missing? No information about where the substance came from, how long it has been entering the bay, or what effect these concentrations have on the animals.
- Does it show cause? The table shows that concentration rises with trophic level. It does not by itself prove the mechanism — though the mechanism is well established: a predator eats many prey animals and cannot break down or excrete the substance, so it keeps all of theirs.
Move 4 — weigh both sides
Every AO3 evaluation has a for and an against, and they both have to come from the source. On Table 1: for the claim that the substance is dangerous to top predators — the concentration in them is 64 times that in the primary consumers. Against — the table gives no information about what concentration is actually harmful, so a large number is not by itself evidence of harm.
Move 5 — land a judgement, and say on what grounds
Same rule as section 2. Position, grounds, condition. "The data support the conclusion that the substance biomagnifies, because it rises at every step from 50.0 times to 8.0 times, but they cannot show whether the top predators are being harmed, because no measurements of the animals' health were taken."
The five moves, in five words
Units. Figures. Gaps. Both sides. Verdict. Write those five words at the top of your Paper 2 answer booklet in the reading time. They apply to every source on the paper.
Checkpoint — working from a source
Four questions on Table 1. Read the table again before you start.
Score: 0 / 4
Question 1
Table 1 shows the concentration rising from 0.4 mg per kg in the primary consumers to 25.6 mg per kg in the top predators. Which process does that pattern demonstrate?
This is the distinction the syllabus draws at 3.2.2(c) and (d), and it is worth being exact about. Biomagnification is an increase in concentration up a food chain — between organisms — which is exactly what one set of samples taken at different trophic levels shows. Bioaccumulation is a build-up within one organism over time, and you would need repeated samples from the same individual to show it; a single snapshot cannot. Eutrophication and leaching are both real processes in this topic but neither is about concentration rising along a food chain.
Question 2
Calculate how many times greater the concentration in the top predators is than the concentration in the primary consumers.
25.6 ÷ 0.4 = 64. Three traps are sitting in these options. The answer of 25.2 is the difference (25.6 − 0.4), a correct calculation of the wrong thing, which is what happens when the command word is skimmed. The one with mg per kg attached has the right number but "times greater" is a ratio, so it carries no unit, and adding one can cost the mark. The 0.016 answer divides the two values the wrong way round.
Question 3
Which of these is a fair statement about the limitations of Table 1?
The statement about how long and how harmful is the honest limitation, and it is the sort of sentence that earns an evaluation mark. The complaint about naming is wrong twice over: knowledge of particular species is not required by this syllabus, and naming them would not make the measurements more reliable anyway. The complaint about the unit misunderstands it — mg per kg is the correct way to express a concentration in tissue. And dismissing the table entirely goes too far: one bay is a real limit on how widely you can generalise, but answers that throw a source away tend to score worse than answers that use it carefully and then state its limits.
Question 4
Which sentence would earn the judgement mark in an evaluation of Table 1?
The long sentence takes a position, gives the grounds from the data, and states exactly what the data cannot settle — position, grounds, limits. The sentence about concentration being higher further up is a description rather than a judgement. The "very worrying" sentence adds a feeling where a reason was needed. And "more research is needed" is the sentence students write when they have run out of ideas; on its own it says nothing about this particular source and is very unlikely to be credited.
5 · The ten Topic 3 traps▶
These are the ten confusions that cost Topic 3 marks most reliably. Most of them are a pair of words that sound similar and are not. Read one, close your eyes, and say the difference out loud before you move on.
Trap 1 — "bio" does not mean it rots
This is the single most confusing pair of definitions in the topic, because the syllabus uses the same three letters for two different ideas.
| Term | What it is about | The definition to learn |
|---|---|---|
| Bioplastic (3.6.2) | What it is made from | A plastic made fully or partly from biological raw materials rather than from fossil fuels. It can be biodegradable or non-biodegradable. |
| Biodegradable plastic (3.6.3) | What happens to it at the end | A plastic designed to decompose in water or soil, broken down by the action of bacteria and fungi into water, biomass and gases (carbon dioxide and methane). |
| Conventional plastic (3.6.1) | Both | Made from fossil fuels, and generally non-biodegradable. |
| Non-biodegradable plastic (3.6.4) | What happens to it | Breaks down only over a very long period of time. |
So a bioplastic can be non-biodegradable, and a shopping bag labelled "bio-based" may sit in the sea for exactly as long as an ordinary one. If a question asks whether switching to bioplastics solves plastic pollution, that sentence is the answer: it changes what the plastic is made from, not necessarily what happens to it afterwards.
Also worth noticing in 3.6.3(c): biodegradable plastics decompose at different rates depending on biotic and abiotic factors — so "biodegradable" is not a promise about speed either. Cold, dark, low-oxygen conditions on a sea bed are not the conditions such a plastic was designed for.
Trap 2 — bioaccumulation and biomagnification
| Term | Where | The distinguishing feature |
|---|---|---|
| Bioaccumulation (3.2.2c) | Within one organism | Over time. The individual takes the substance in faster than it can break it down or excrete it, so the amount in its body keeps rising as it gets older. |
| Biomagnification (3.2.2d) | Up a food chain | Between organisms. Each consumer eats many prey and keeps all of their load, so the concentration is higher at each trophic level. |
The give-away word is where. One organism, over time = accumulation. Along a chain of organisms = magnification. They usually happen together, and the second depends on the first, but a question that asks for one and gets the other scores nothing. A quick test on any data set: if it has a time axis it can show accumulation; if it has trophic levels it can show magnification.
Trap 3 — distillation and reverse osmosis separate by different things
| Distillation (3.1.7a) | Reverse osmosis (3.1.7b) | |
|---|---|---|
| Separates by | Boiling point | Molecule size |
| What you put in | Heat | Pressure |
| What happens | The water is heated until it boils. The salt stays behind in the liquid. The steam is pure water; it is then cooled and condensed back to liquid, and that liquid is potable. | The water is put under high pressure and forced through a membrane. The membrane lets water molecules through but stops most ions and other molecules. |
| No membrane / no boiling | There is no membrane anywhere in distillation. | Nothing is boiled in reverse osmosis. |
The commonest lost mark is writing "the water is boiled and passes through a filter", which mixes the two and describes neither. Learn the two verbs: distillation boils, reverse osmosis presses.
Trap 4 — the malaria return leg, the sentence almost everyone leaves out
3.3.1 has three parts, and nearly every student writes two of them:
- Female Anopheles mosquitoes are the vector.
- The malaria Plasmodium parasite is transmitted to a human when that human is bitten by an infected mosquito.
- The parasite is transmitted back to non-infected mosquitoes when they feed on the blood of an infected human.
Part 3 is the return leg, and it is the reason the disease keeps going in a population. Without it there is no cycle, only a one-way transfer, and an answer that stops at part 2 has described how one person catches malaria rather than how malaria spreads.
It also explains why the strategies at 3.3.2 work at two different points: personal protection (nets, repellent, drugs, vaccination) blocks the leg from mosquito to human, while vector control (covering or draining breeding areas, spraying, sterilising male mosquitoes, biological control) attacks the mosquito population itself and therefore both legs at once. If you can say which leg a strategy interrupts, you are answering at a level above the recall.
Trap 5 — interception is not precipitation
In the water cycle, 3.1.1 splits everything into stores and transfers. Interception is a transfer, and it is not rain.
- Precipitation is water falling from the atmosphere to the surface as rain, snow, hail or sleet.
- Interception is rain being caught by vegetation — leaves, branches — before it reaches the ground. Some of it later evaporates from the leaf, some drips down.
Two more pairs in the same objective that get mixed up:
- Infiltration = water soaking into the soil from the surface. Through-flow = water moving sideways through the soil towards a river. Ground water flow = the same sideways movement, but deeper, through rock.
- Transpiration = water lost from plants. Evaporation = water changing to vapour from any surface. A question asking for a transfer involving plants wants transpiration or interception; "evaporation" will not do.
And the store/transfer line matters: ice sheets and glaciers, ground water, the atmosphere, lakes and rivers, and the oceans are stores. Everything in the list above is a transfer. If a question says "name two stores" and you write "evaporation", no amount of correct detail after it will help.
Trap 6 — potable does not mean pure, and clear does not mean safe
3.1.4 defines potable water as water that is safe to drink. That is the whole definition, and it is not the same as pure. Potable water still contains dissolved substances; it is simply free of the things that would make you ill. Equally, water can be perfectly clear and completely unsafe, because bacteria are invisible. An answer that says treated water is "pure" or "clean" has drifted away from the syllabus wording, and the syllabus wording is what is being asked for in a define question.
Trap 7 — the four treatment stages do four different jobs, in order
| Stage | What it removes |
|---|---|
| 1 Screening | Large floating and suspended objects — branches, rubbish — caught on a screen or mesh. |
| 2 Sedimentation | Heavier suspended solids, which settle out under gravity when the water is held still. |
| 3 Filtration | The finer particles still suspended, as the water passes through a bed of sand or gravel. |
| 4 Chlorination | Micro-organisms. Chlorine is added to kill the pathogens. |
Two things to hold on to. The order matters — the big things first, so the later stages are not overwhelmed. And only chlorination deals with pathogens: the first three stages remove solids, and solids are not what gives you cholera. A student who leaves chlorination off the list has described how to make water clear, not how to make it safe.
Trap 8 — aquaculture is not simply the solution to overfishing
3.4.3 lists eight impacts of marine aquaculture, and two of them are benefits — reduced exploitation of natural fisheries, and increased food supply for humans. The other six are costs: escape, disease, effects on local food webs, nutrient enrichment from waste, energy usage, and the source of food for the farmed species.
That last one is the one to remember, because it closes a loop most answers miss: if the farmed animals are fed on fish caught from the wild, then farming them has not removed the pressure on wild stocks, it has moved it. An answer that says "fish farming solves overfishing" and stops there has taken one of the eight points and ignored the other seven.
Also keep bycatch and overfishing separate (3.4.1). Overfishing is taking the target species faster than it can reproduce. Bycatch is the non-target animals caught in the same nets and usually discarded. They are different problems with different solutions: a quota addresses the first, a larger mesh size and pole-and-line fishing address the second.
Trap 9 — microplastics have two origins, and a size limit
3.6.5 gives three facts and most answers give one. Microplastics are:
- less than 5 mm in length (written as < 5 mm)
- formed when larger plastics break down
- used in commercial products — that is, some are manufactured small on purpose, and were never a large piece of plastic at all
The second origin is the one that gets dropped. A question asking where microplastics come from wants both routes, and an answer with only the break-down route has given half of it. The size is worth stating too: 5 mm is a specific figure and specific figures tend to be credited where vague ones are not.
Trap 10 — preventing an oil spill and cleaning one up are two different objectives
| 3.5.3 — prevention: stopping the oil getting into the sea | 3.5.4 — minimising the impact: dealing with oil already there |
|---|---|
| MARPOL, the international convention for the prevention of pollution from ships Double-hulled tankers — a second hull inside the first, so a puncture of the outer hull need not release oil Risk assessments Regular maintenance |
Improved navigation systems for ships Booms — floating barriers that contain the slick Sorbents — materials that soak the oil up Detergent sprays — break the slick into droplets Skimmers — lift oil off the surface Controlled burning |
If a question says prevention and you write about booms and skimmers, you have answered the other objective. Booms do not prevent spills; they contain them once they have happened.
One genuine evaluation point that fits either question: a detergent spray breaks the oil into small droplets, which disperses the slick, but the detergent itself is a chemical added to the sea and can harm the organisms living there. So a clean-up method can carry its own environmental cost, and saying so is a step 2 in the discuss frame. The same goes for controlled burning, which removes the oil from the water surface but releases the products of combustion into the air.
Remember also that 3.5.1 lists five causes and only one of them is a shipping accident: the others are off-shore and on-shore extraction, pipelines, the cleaning of tanks at sea, and refineries. Tank cleaning is a routine, deliberate operation rather than an accident, which is precisely why an international convention was needed to control it.
Checkpoint — the ten traps
Five questions on the pairs that get confused.
Score: 0 / 5
Question 1
A carrier bag is made from plant starch rather than from crude oil, and is labelled a bioplastic. What can you conclude about how it will behave if it ends up in the sea?
"Bio" in bioplastic refers to where the raw material came from, not to what happens at the end of the object's life, and 3.6.2 states plainly that bioplastics can be biodegradable or non-biodegradable. The claim that no plastic decomposes is also wrong — biodegradable plastics exist and are a separate definition at 3.6.3. The water-and-biomass-only answer misquotes 3.6.3(b): a biodegradable plastic is broken down by bacteria and fungi into water, biomass and gases, namely carbon dioxide and methane. And even a genuinely biodegradable plastic decomposes at a rate that depends on biotic and abiotic factors, so cold deep water is not where it will perform best.
Question 2
A scientist measures the same substance in one individual animal every year for ten years and finds the concentration in its tissue rising each year. Which process has been demonstrated?
One organism, measured over time, with the amount rising because it is taken in faster than it can be broken down or excreted — that is bioaccumulation, 3.2.2(c). Biomagnification would need measurements at different trophic levels, because it is about concentration increasing as a substance is passed up a food chain, 3.2.2(d), and one animal is not a food chain. Eutrophication is about nutrient enrichment of water, and leaching is the movement of substances out of soil; neither describes what was measured here. The test is simple: a time axis shows accumulation, a food chain shows magnification.
Question 3
Which statement correctly describes reverse osmosis and not distillation?
Reverse osmosis separates by molecule size, using pressure and a membrane. Boiling the water so the salt is left behind, and condensing the steam afterwards, are both stages of distillation, which separates by boiling point using heat and has no membrane in it anywhere. The sand bed is filtration — a water treatment stage rather than a desalination method, and sand will not remove dissolved salt at all.
Question 4
A student describes malaria transmission: "Female Anopheles mosquitoes are the vector. When an infected mosquito bites a person, the Plasmodium parasite is passed into that person." What has been left out?
The return leg, 3.3.1(c), is the third of three parts and the one most often dropped. Without it there is no cycle: the answer has described how one person catches malaria, not how the disease keeps circulating in a population. The bacterium answer is wrong on the biology — Plasmodium is a parasite, and cholera is the bacterial disease in this topic. Breeding in standing water is true and useful, but it belongs to the control strategies at 3.3.2 rather than to the transmission objective.
Question 5
Which of these is a transfer in the water cycle that involves vegetation catching water before it reaches the ground?
Interception is rain caught by leaves and branches on its way down, and some of it later evaporates from the leaf while the rest drips through. Precipitation is the fall from the atmosphere to the surface — a different transfer, and the one it is most often confused with. Infiltration is water soaking into the soil from the surface, so it is also a transfer, but it happens after the water has landed. Ground water is not a transfer at all: it is one of the stores listed in 3.1.1(a), alongside ice sheets and glaciers, the atmosphere, lakes and rivers, and the oceans. Ground water flow is the transfer; ground water itself is the store.
6 · Three source-led practice sets▶
Three complete source-led sets, of the kind Paper 2 is made of — a bar chart, a line graph and a map. Work each one on paper before you open the model answers; reading a model you have not attempted teaches almost nothing. Use the five moves from section 4: units, figures, gaps, both sides, verdict.
Practice set 1 · 12 marks · AO3
Figure 1 — urban and rural access to a safely managed drinking-water supply
Five regions were surveyed. For each one, the percentage of the urban population and the percentage of the rural population served by a safely managed drinking-water supply were recorded in the same year.
Figure 1. Constructed for practice, not measurements from named countries. Read the key before you read the question: at each region there are two bars, and the vertical axis is a percentage of that population, not a number of people.
(a)[1]
State the percentage of the rural population served in Region D.(b)[3]
Describe how the difference between urban and rural access varies across the five regions. Use figures from Figure 1.(c)[2]
Calculate the rural figure for Region E as a percentage of the urban figure for Region E. Show your working.(d)[3]
Explain why a rural population is often less likely than an urban one to have a safely managed drinking-water supply.(e)[3]
A politician says Figure 1 proves that Region E should spend its money on desalination plants. Using Figure 1, evaluate that claim.Model answers — set 1Figure 1 — the full set of answers
(a) [1]
34%. The unit is the per cent sign, and it is part of the answer.
(b) [3]
The urban figure is higher than the rural figure in every region (1). The gap widens as overall access falls: it is only 8 percentage points in Region A (98% urban against 90% rural) but 38 percentage points in Region E (60% against 22%) (1). Reading across the five regions the gaps are 8, 18, 30, 37, 38 percentage points, so the widening is large at first and then almost levels off between Regions D and E (1).
(c) [2]
Working: 22 ÷ 60 × 100 (1 — method). Answer: 36.7% (accept 37%) (1). In words, barely more than a third of Region E's rural population is served compared with its urban population.
(d) [3]
Rural homes are spread out, so a piped network has to cover a long distance for very few users and costs far more per household to build and maintain (1). Treatment works are expensive to build and to run and need trained staff and a reliable power supply, so they are placed where the population is concentrated — in the towns (1). Rural households are therefore left using untreated surface water or shallow wells, which may be contaminated by sewage or by agricultural run-off, and the same inequality applies to sewage treatment, so the water available to them is more likely to carry pathogens (1).
(e) [3]
Supporting the claim: Region E has the lowest access of the five, 60% urban and 22% rural, so it clearly has the greatest need for a new source of potable water (1). Against it: Region E also has the largest urban–rural gap, 38 percentage points, and desalination plants must be on a coast with the water piped from there — which is exactly the kind of network that has failed to reach the rural population already. So a desalination plant would very likely widen the gap rather than close it (1). Limits of the data: Figure 1 gives percentages for one year only; it does not say whether these regions are coastal, how much rainfall or ground water they have, what the shortage is caused by, or what any of the options would cost (1). A judgement such as "the figure identifies Region E as the priority but says nothing about which solution to choose, and the rural gap suggests distribution rather than supply may be the real problem" is the sort of ending that earns the evaluation.
Notice (e): the data can identify a priority without justifying a particular solution. Saying so is the evaluation.
Practice set 2 · 13 marks · AO3
Figure 2 — a river downstream of a discharge pipe
Untreated waste is discharged into a river from a single pipe. Water samples were taken at intervals downstream of the pipe on one day. Upstream of the pipe, nitrate was 2 mg per dm³ and dissolved oxygen was 9.5 mg per dm³.
Figure 2. Constructed for practice, not measurements from a named river. Both lines use the same vertical axis because both are concentrations in mg per dm³. Check the key before you read a value.
(a)[1]
State the distance downstream at which the dissolved oxygen concentration is lowest.(b)[3]
Describe how the dissolved oxygen concentration changes with distance downstream of the pipe. Use figures from Figure 2.(c)[2]
Calculate the percentage decrease in nitrate concentration between the pipe and 20 km downstream. Show your working.(d)[4]
Explain why the dissolved oxygen concentration falls close to the pipe and then rises again further downstream.(e)[3]
The company operating the pipe claims Figure 2 shows the river has fully recovered by 20 km. Using Figure 2, evaluate that claim.Model answers — set 2Figure 2 — the full set of answers
(a) [1]
2 km downstream, where dissolved oxygen is 2.0 mg per dm³.
(b) [3]
Dissolved oxygen falls at first, from 3.0 mg per dm³ at the pipe to a minimum of 2.0 mg per dm³ at 2 km (1). It then rises steadily with distance, reaching 5.5 mg per dm³ at 8 km and 9.5 mg per dm³ at 20 km (1). The total rise from the minimum to 20 km is 7.5 mg per dm³, and by 20 km the concentration has returned to the upstream value of 9.5 mg per dm³ (1).
(c) [2]
Working: (19 − 3) ÷ 19 × 100 = 16 ÷ 19 × 100 (1 — method). Answer: 84.2% (accept 84%) (1). A percentage change has no unit; a common slip here is to write 16 mg per dm³, which is the difference, not the percentage.
(d) [4]
The waste adds a large amount of organic material and nitrate to the river at the pipe — nitrate at the pipe is 19 mg per dm³ against 2 upstream (1). Bacteria decompose that organic material, and they respire as they do so, using up the dissolved oxygen in the water, which is why the oxygen reaches its minimum just downstream of the pipe (1). The added nitrate also allows algae to grow rapidly; the algae block the light, plants beneath them die, and decomposing those adds still further to the oxygen demand — this is eutrophication (1). Further downstream the waste has been diluted and largely decomposed, so there is less material left for the bacteria to work on, while oxygen dissolves back into the water from the air as the river flows and is mixed by turbulence — so the concentration rises again (1).
(e) [3]
Supporting the claim: dissolved oxygen at 20 km is 9.5 mg per dm³, which is the same as the upstream value of 9.5, so on that measure the river has returned to its previous condition (1). Against it: nitrate at 20 km is still 3 mg per dm³ against 2 mg per dm³ upstream, so it has fallen a long way but has not returned to the upstream level — and the graph stops at 20 km, so it cannot show what happens beyond that (1). Limits of the data: samples from a single day, only two measurements taken, and nothing at all about the organisms living in the river, which is what "recovered" would really mean; the stretch that was left short of oxygen may have lost its animals whether or not the chemistry has returned to normal (1). A judgement such as "the oxygen has recovered but the nitrate has not, so the claim is only partly supported" is the sort of ending that earns the evaluation.
Two measurements, two different verdicts. When a source gives you more than one variable, check whether they agree before you accept a claim built on one of them.
Practice set 3 · 10 marks · AO2 and AO3
Map 1 — a bay with a marine aquaculture site
A company has set up a marine aquaculture site in a shallow bay. The map shows the town and river mouth, the cages, a coral reef, a protected area and the direction of the prevailing current. Use the scale bar and the key.
Map 1. Constructed for practice, not a real coastline. The scale bar shows 2 km; the current arrow shows the direction the water moves for most of the year.
(a)[1]
Using the scale, estimate the distance from the aquaculture site to the coral reef.(b)[2]
Using Map 1, suggest two reasons why this is a poor position for the aquaculture site.(c)[4]
Describe and explain how waste from the cages could affect the coral reef.(d)[3]
Suggest one change the company could make to reduce the impact on the reef, and justify your choice using Map 1.Model answers — set 3Map 1 — the full set of answers
(a) [1]
About 6.0 km. Method: the gap between the site and the reef is three times the length of the scale bar, and the scale bar is 2 km, so 3 × 2 = 6.0 km. Accept a sensible range around that figure — an estimate from a scale bar is expected to be approximate, but it must carry its unit.
(b) [2]
Any two of: the site sits up-current of the coral reef, so anything released from the cages is carried towards it (1); it is close to the river mouth and the town, so the water there already receives sewage and run-off and the farm adds to an existing load (1); it lies between the town and the protected area, so waste can reach a stretch of water that has been set aside for conservation (1).
(c) [4]
Describe: uneaten food and the farmed animals' faeces fall out of the cages and add organic material and dissolved nutrients — nitrate and phosphate — to the water (1). Explain: the prevailing current runs from the site towards the reef, which is only about 6.0 km away, so the enriched water is carried over the reef rather than being dispersed out to sea (1). The added nutrients allow algae to grow rapidly on and above the reef; the algae block the light the reef organisms depend on (1). As the algae and the organic waste die and are decomposed by bacteria, the bacteria respire and use up the dissolved oxygen, so the animals of the reef are left short of oxygen and the reef is damaged (1). Also creditable: disease and parasites spreading from the high density in the cages, or escaped animals competing with the wild populations of the reef food web.
(d) [3]
A defensible change is to move the cages to the other side of the reef — that is, down-current of it — or further out of the bay (1 — a clear choice). Justification from the map: the current runs from west to east, so a site placed east of the reef would carry its waste away from the reef instead of over it, and moving out of the shallow bay would give deeper, better-mixed water in which the waste is diluted more quickly (1). The new position would have to stay outside the protected area, and the further the cages are from the town the more it costs the company to service them — so this is a trade-off rather than a free improvement (1). Also creditable with justification: reducing stocking density, feeding more carefully so less food is uneaten, or fallowing the site periodically.
What Map 1 does not show
No depth, no tide, no seasonal change in the current, no measurements of anything. A map tells you about position and nothing about quantity, so an answer that claims the reef "will be destroyed" has gone well beyond the evidence. Say what the position makes likely, and say what you would need to measure to be sure.
On a map question, the marks are in the relationships: what is up-current of what, what is between what, and how far apart things are.
Checkpoint — the practice sets
Three questions on what the sources do and do not say.
Score: 0 / 3
Question 1
In Figure 1, the urban–rural gaps across the five regions are 8, 18, 30, 37, 38 percentage points. Which statement is fully supported by the figure?
The gaps rise from 8 percentage points in Region A to 38 in Region E while the overall level falls, so the widest-gap-where-access-is-lowest statement is the pattern the figure actually shows. The half-of-urban claim is arithmetically wrong — in Region A rural access is about 92% of urban, nowhere near half. The claim about Region A over-reads the bars: 90% rural access is high, but it is not 100%, so roughly one rural household in ten is still unserved. And the claim about next time is a prediction; a single year's data cannot support one, and reading beyond the range of a data set is exactly what an evaluation question wants you to avoid.
Question 2
In Figure 2, why does the dissolved oxygen reach its minimum a short distance downstream of the pipe rather than exactly at the pipe itself?
Decomposition is not instant. The bacteria multiply and respire over a period of time, and during that time the water they are in has been carried downstream — so the greatest oxygen demand shows up some distance below the outfall rather than at it. The nitrate answer states something true — nitrate is 19 mg per dm³ at the pipe, the highest value on the graph — but that is not a reason for where the oxygen minimum sits. The depth answer invents information the figure does not give: there is nothing about depth anywhere on it. And waste water arriving with extra oxygen in it is the opposite of what happens.
Question 3
On Map 1, which single feature does most to make the aquaculture site a risk to the coral reef?
Direction of flow is the key relationship on this map, and it is the sort of thing a Paper 2 map is put there to test. The protected area is drawn separately from the reef, to the south-east of it, so the reef is not inside it. The shallow bay is a genuine contributing factor, because shallow water disperses waste less well, but on its own it would matter far less if the current ran the other way. The position of the town matters for the water in the bay generally, but it is not what puts this particular site upstream of this particular reef.