IB DP Environmental Systems and Societies – Water systems, security, aquatic food production and pollution Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – the IB holds copyright in its own papers. Use these alongside the official past papers available through your school or the IB store.

This practice set covers Topic 4, Water, of IB Diploma Programme Environmental Systems and Societies: syllabus sections 4.1 to 4.4 (water systems; water access, use and security; aquatic food production systems; water pollution). All questions suit SL and HL, except Question 11, which uses an HL-only lens and is labelled HL. It follows the IB ESS subject brief for first assessment 2026 — the course examined in the May and November 2026, 2027 and 2028 sessions.

Learn the content first in the Topic 4 study guide and Topic 4 revision notes. See also the IB DP ESS course hub and printable ESS checklist. All data are fictional. Points in the answers marked [1] are indicative; your teacher will share the IB assessment criteria for extended answers.

Questions

1. Define residence time. A reservoir holds 2.4 km³ of water, and 0.3 km³ flows out each year at steady state. Calculate its residence time. [3]

2. Classify each flow as a transfer or a transformation: (i) evaporation, (ii) infiltration, (iii) condensation, (iv) surface runoff. [4]

3. Explain how replacing farmland with a housing estate changes flows in a river’s drainage basin. [3]

4. The table shows fictional data for three countries.

Country	Renewable freshwater (km³ per year)	Population (millions)
P	64	20
Q	27	18
R	6.3	14

Country P has 3,200 m³ per person per year.

(a) Calculate the water available per person per year for Q and R. [2] (b) Using the thresholds 1,700, 1,000 and 500 m³ per person, classify Q and R. [2] (c) Suggest two reasons why people in Country P may still lack water security. [2]

5. Evaluate desalination as a way to improve water security in a dry coastal city. [4]

6. The table shows fictional data for a demersal fishery.

Year	Catch (t)	Effort (boat-days)
2012	52,000	10,400
2016	54,000	13,500
2020	50,400	16,800
2024	41,600	20,800

(a) Calculate the catch per unit effort (CPUE) for 2020 and 2024. [2] (b) Calculate the percentage change in CPUE from 2012 to 2024. [2] (c) With reference to maximum sustainable yield, explain what the data suggest about the fish stock. [2] (d) Outline two management measures that could allow the stock to recover. [2]

7. A salmon farm stocks 150 t of young fish. After one growing cycle it harvests 3,150 t of fish, having used 3,900 t of feed.

(a) Calculate the feed conversion ratio. [2] (b) Explain two environmental impacts of open-net salmon farming on the surrounding marine ecosystem. [4]

8. A fictional survey measured dissolved oxygen (DO) and BOD along a river with a sewage outfall at 0 km.

Distance from outfall (km)	−1 (upstream)	0.5	2	5	10	20
DO (mg/L)	9.2	6.1	3.4	4.8	7.5	9.0
BOD (mg/L)	2	14	9	6	3	2

(a) Calculate the percentage decrease in DO from the upstream site to the lowest value. Give your answer to 3 significant figures. [2] (b) Explain the fall in DO, using the BOD data. [2] (c) Explain why DO recovers further downstream. [2] (d) State one advantage of also sampling indicator species at these sites. [1]

9. In a fictional lake, a persistent pesticide was measured at 0.05 ppm in plankton, 0.4 ppm in small fish, 3.2 ppm in predatory fish and 16 ppm in fish-eating birds.

(a) Calculate how many times more concentrated the pesticide is in the birds than in the plankton. [1] (b) Explain why the concentration rises along the food chain. [3]

10. To what extent can eutrophication of a lake be managed successfully? Refer to the three levels of pollution management. [9]

11. (HL) Using the lens of environmental and ecological economics, evaluate charging farmers by volume for irrigation water as a way to improve water security. [6]

Answers

1. Residence time is the average time a water molecule spends in a storage [1]. Residence time = 2.4 ÷ 0.3 [1] = 8 years [1] [3] Examiner insight: A definition without “average” can miss the mark; show the division and give units.

2. (i) evaporation: transformation [1]; (ii) infiltration: transfer [1]; (iii) condensation: transformation [1]; (iv) surface runoff: transfer [1] [4] Examiner insight: The test is a change of state; a one-word answer is enough, but writing both words or hedging scores nothing.

3. Impermeable surfaces reduce infiltration, so more rain becomes surface runoff [1]. Drains carry water quickly to the river, so lag time shortens and peak discharge rises [1]. Less water percolates to groundwater, so dry-season baseflow can fall [1] [3] Examiner insight: Each point needs cause and effect; “more flooding” alone describes an outcome without naming the flow that changed.

4. (a) Q: 27 × 10⁹ ÷ 18 × 10⁶ = 1,500 m³ [1]; R: 6.3 × 10⁹ ÷ 14 × 10⁶ = 450 m³ per person per year [1] (b) Q is under water stress (1,000 to 1,699) [1]; R has absolute scarcity (below 500) [1] (c) Any two: water may be unevenly shared between regions or seasons [1]; people may lack pipes, treatment or money to reach it (economic scarcity) [1]; the water may be polluted; much may be used by agriculture or industry [6] Examiner insight: Unconverted km³ loses both accuracy marks in (a), but (b) can still earn follow-through marks from your own values.

5. Benefit: gives a reliable supply that does not depend on rainfall [1]. Cost: needs large amounts of energy, often from fossil fuels, so it is expensive and adds to emissions [1]. Cost: the very salty brine returned to the sea can harm marine life near the outlet [1]. Judgement: suits a wealthy coastal city with renewable energy; elsewhere, cut demand first [1] [4] Examiner insight: “Evaluate” needs both sides and a conclusion; a list of costs with no judgement cannot reach the final mark.

6. (a) 2020: 50,400 ÷ 16,800 = 3.0 t per boat-day [1]; 2024: 41,600 ÷ 20,800 = 2.0 t per boat-day [1] (b) CPUE 2012 = 52,000 ÷ 10,400 = 5.0; (2.0 − 5.0) ÷ 5.0 × 100 [1] = −60% (a 60% decrease) [1] (c) Effort doubled while CPUE fell every period, so fewer fish are available per unit of effort: the stock is shrinking [1]. Catches have been above MSY, so harvest exceeds the stock’s natural growth; the fall in catch after 2016 suggests the stock is overfished and at risk of collapse [1] (d) Any two: a lower total allowable catch set below MSY [1]; larger mesh size so young fish escape [1]; closed season during spawning; a no-take marine protected area [8] Examiner insight: In (c), credit comes from linking the CPUE trend to stock size, not from repeating the catch figures.

7. (a) Increase in mass = 3,150 − 150 = 3,000 t [1]; FCR = 3,900 ÷ 3,000 = 1.3 [1] (b) Uneaten feed and faeces add nutrients and organic matter to the water [1], which can raise BOD and cause local eutrophication or low oxygen on the seabed below the pens [1]. Dense stocking spreads parasites and disease such as sea lice [1] to wild fish swimming past the farm, reducing wild populations [1] [6] Examiner insight: Using the harvest mass (3,150 t) instead of the gain loses the method mark; in (b) each impact needs the mechanism and the effect for both marks.

8. (a) (3.4 − 9.2) ÷ 9.2 × 100 [1] = 63.0% decrease [1] (b) Sewage adds organic matter, so BOD rises sharply to 14 mg/L just below the outfall [1]; bacteria decomposing it use dissolved oxygen, so DO falls to 3.4 mg/L at 2 km [1] (c) Organic matter is used up, so BOD falls back to 2 mg/L by 20 km [1]; oxygen enters from the air and from photosynthesis faster than it is used, so DO returns to 9.0 mg/L [1] (d) Indicator species show conditions over a longer period, not just at the moment of sampling [1] [7] Examiner insight: “63%” is only 2 significant figures when 3 are asked for; in (b) and (c), quoting values from the table is what turns a general answer into a creditworthy one.

9. (a) 16 ÷ 0.05 = 320 times [1] (b) The pesticide is persistent and fat-soluble, so it is not broken down or excreted [1]. It builds up in each organism’s tissues over its life (bioaccumulation) [1]. Each consumer eats many organisms from the level below, and energy is lost between levels while the pesticide is kept, so concentration rises at each trophic level (biomagnification) [1] [4] Examiner insight: The mark for biomagnification needs the link to eating many prey; naming the term alone rarely earns credit.

10. Indicative points (the real exam marks extended answers holistically against IB criteria your teacher will share, not point by point):

• Eutrophication is caused by excess nitrates and phosphates, mainly from fertiliser runoff and sewage [1].
• Level 1, changing human activity: less fertiliser, applied away from rain; phosphate-free detergents [1]. This tackles the cause but relies on behaviour change [1].
• Level 2, controlling release: buffer strips of vegetation along streams, and nutrient removal at sewage works [1]. Effective for point sources; runoff from many farms is a non-point source and harder to control [1].
• Level 3, clean-up and restoration: dredging sediment, aerating the lake, restoring reed beds [1]. Quick results, but these treat symptoms, are costly and often need repeating [1].
• Phosphate stored in sediment can keep releasing nutrients for years, so recovery is slow even after inputs stop [1].
• Judgement: success needs all three levels combined, with most weight on Levels 1 and 2 across the whole catchment [1] [9] Examiner insight: “To what extent” requires a judgement; describing each level without weighing its limits and concluding cannot earn the final evaluative credit.

11. Indicative points, a guide only as in Question 10:

• Cheap or free water means farmers do not pay its full cost and overuse it [1].
• Charging by volume gives a price signal, encouraging drip irrigation and less water-hungry crops [1].
• Revenue can fund repairs that reduce leakage [1].
• Poorer farmers may not afford the charge, reducing income and food security; subsidies or a free basic allowance may be needed [1].
• Metering is costly, and illegal wells can bypass charges, so enforcement matters [1].
• Judgement: pricing can improve water security if it is fair and enforced, but it works best alongside regulation and support for efficient irrigation [1] [6] Examiner insight: HL answers gain credit by using the lens explicitly (price signals, full cost, who pays), not by repeating an SL list of demand-reduction methods.

Where marks are usually lost

• Not converting km³ to m³ before dividing by population.
• Judging a fishery on catch alone and ignoring effort and CPUE.
• Using harvest mass instead of mass gained when calculating FCR.
• Giving percentages to fewer significant figures than the question asks.
• Describing river data with no values or distances quoted.
• Blaming algae, rather than decomposers, for oxygen depletion.
• Listing management measures without evaluating them or reaching a judgement.

Next steps

• Topic 4 Water revision notes
• Topic 4 Water study guide
• Topic 2 Ecology practice questions
• IB DP ESS course hub
• Printable ESS checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

International Baccalaureate Organization, Diploma Programme Subject Brief, Environmental systems and societies, first assessment 2026. This set covers Topic 4, Water: sections 4.1, 4.2, 4.3 and 4.4. The brief lists subtopics for Topic 1 only; the subtopic numbering on this page follows the printable ESS checklist.

The brief gives Topic 4 Water 12 teaching hours at SL and 25 at HL. It does not list the subtopics or learning outcomes for this topic, so the numbered subtopics and outcomes on this page follow the syllabus numbering used in the printable ESS checklist, not the brief itself.