Question 1
A river receives fertilizer washed from many fields and acidic wastewater released through a factory pipe.
Distinguish between the two pollution sources using the terms point source and non-point source.
[2]
State one impact that the acidic wastewater may have on the river ecosystem.
[1]
Question 2
Explain how plastic waste released near a coast may accumulate in an oceanic gyre and harm marine organisms.
[3]
Question 3
A water-quality index combines measurements of dissolved oxygen, pH, temperature, turbidity, nitrate and phosphate into one score.
Outline one advantage of using a water-quality index.
[1]
Outline one limitation of using only the index score to inform pollution management.
[1]
State one reason why measurements should be repeated at different times.
[1]
Question 4
A town discharges treated sewage into the River Luma. Water-quality measurements were made upstream of the outfall and at two downstream sites during dry weather.
River Luma water-quality measurements; each value is the mean of three measurements.
| Site | Dissolved oxygen / mg L | Nitrate / mg L | Phosphate / mg L | Turbidity / NTU | Initial dissolved oxygen in BOD bottle / mg L | Dissolved oxygen after 5 days / mg L |
|---|---|---|---|---|---|---|
| U | 8.8 | 0.7 | 0.04 | 6 | 8.6 | 7.4 |
| D1 | 5.7 | 4.8 | 0.62 | 25 | 8.4 | 3.1 |
| D5 | 7.0 | 3.1 | 0.31 | 14 | 8.5 | 5.5 |
Calculate the five-day biochemical oxygen demand at site D1.
[2]
Describe the changes in water quality between sites U and D1.
[3]
Explain the relationship between sewage-derived organic matter, biochemical oxygen demand and dissolved oxygen at D1.
[3]
Evaluate the usefulness of these measurements for determining whether the sewage-treatment plant is the main cause of pollution in the river.
[4]
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Question 5
Figure 1 shows water-quality measurements along the River Elan. A sewage outfall enters the river between 1 km and 3 km downstream from the first sampling site.
Identify the distance downstream at which dissolved oxygen is lowest.
[1]
Calculate the percentage increase in BOD between the 1 km and 3 km sampling sites.
[2]
Explain the changes in BOD and dissolved oxygen immediately downstream from the sewage outfall.
[3]
Suggest one reason why dissolved oxygen partially recovers farther downstream.
[1]
Question 6
Water samples were collected from three locations near an organic-waste discharge. Dissolved oxygen was measured before and after five days of incubation at .
Dissolved oxygen in water samples before and after five days of incubation.
| Sampling location | Initial dissolved oxygen / mg L | Dissolved oxygen after 5 days / mg L |
|---|---|---|
| Upstream | 8.6 | 7.8 |
| At discharge | 8.1 | 1.9 |
| 4 km downstream | 7.9 | 4.4 |
Calculate the five-day BOD of the sample collected at the discharge.
[1]
Calculate the five-day BOD of the upstream and downstream samples.
[2]
Explain why the bottles were airtight and incubated in darkness.
[2]
Interpret what the BOD results indicate about the spatial effect of the discharge.
[1]
Question 7
Figure 1 shows a five-day biochemical oxygen demand (BOD) test performed on an undiluted water sample.
Calculate the five-day BOD of the sample.
[2]
Explain why the bottle is incubated in darkness.
[1]
Explain why the bottle is completely filled and sealed.
[1]
Question 8
A eutrophic lake receives nutrients from farms and a wastewater-treatment plant. Fish catches have declined and surface scums discourage recreation.
Outline one management strategy at each of the three levels of pollution management.
[3]
Outline why reducing pollution at its source is generally more sustainable than removing it from the lake.
[1]
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Question 9
Distinguish between the impacts of tributyltin and PCBs as water pollutants.
[2]
State one ecological effect of thermal pollution.
[1]
Question 10
Lake Nembi supports a commercial fishery, swimming beaches and a drinking-water supply. Most of its catchment is used for maize farming. A restoration programme began in 2020.
Annual summer water-quality indicators and commercial fish catch in Lake Nembi, 2016–2024.
| Year | Mean summer phosphate / | Chlorophyll- / | Bottom-water dissolved oxygen / | Commercial fish catch / tonnes |
|---|---|---|---|---|
| 2016 | 0.11 | 38 | 4.1 | 104 |
| 2017 | 0.13 | 45 | 3.6 | 97 |
| 2018 | 0.15 | 53 | 2.9 | 90 |
| 2019 | 0.18 | 62 | 2.1 | 84 |
| 2020 | 0.17 | 58 | 2.4 | 87 |
| 2021 | 0.14 | 48 | 3.1 | 95 |
| 2022 | 0.12 | 41 | 3.7 | 103 |
| 2023 | 0.10 | 35 | 4.3 | 110 |
| 2024 | 0.09 | 31 | 4.8 | 116 |
Calculate the percentage decrease in mean summer phosphate concentration between 2019 and 2024.
[2]
Explain how the changes in phosphate concentration could have caused the changes in bottom-water dissolved oxygen.
[4]
Suggest how the observed changes may affect two ecosystem services provided by Lake Nembi.
[3]
Evaluate the sustainability of the Lake Nembi restoration programme.
[4]
Question 11
The island state of Pelara lies beside a subtropical oceanic gyre. Scientists investigated plastic pollution along a river-to-ocean pathway and in a marine food chain.
Identify the location with the greatest mean microplastic abundance.
[1]
Explain why plastic entering the sea near Pelara accumulates in the offshore gyre.
[3]
Distinguish between bioaccumulation and biomagnification, using Figure 3(c).
[3]
Evaluate a proposal that Pelara should prioritize collecting plastic from the gyre rather than preventing plastic from entering the river.
[5]
Question 12
Figures 2(a) and 2(b) show the movement and occurrence of plastic pollution in a subtropical ocean basin.
Mean plastic pollution measurements across trophic levels in a subtropical ocean basin.
| Organism | Trophic level | Microplastic abundance / particles g tissue | Plastic-associated PCBs / ng g |
|---|---|---|---|
| Zooplankton | Lowest | 12 | 3 |
| Sardines | Low–medium | 28 | 9 |
| Tuna | High | 84 | 38 |
| Seabirds | Highest | 126 | 76 |
Calculate how many times greater the mean microplastic abundance is in seabirds than in zooplankton.
[1]
Describe the relationship between trophic level and the two measurements shown in Figure 2(b).
[2]
Explain why floating plastic accumulates in the central zone shown in Figure 2(a).
[2]
Explain how plastic-associated PCBs may reach the concentrations shown in seabirds.
[2]
Suggest one management strategy that would address the main pathway of plastic input shown in Figure 2(a).
[1]
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Question 13
Explain how fertilizer run-off can cause hypoxia in a lake, including one positive feedback process.
[4]
Question 14
Harmful algal blooms occur in both freshwater and coastal marine environments.
Explain one pathway by which a freshwater cyanobacterial bloom can affect human health.
[2]
Explain one pathway by which a coastal dinoflagellate bloom can affect human health.
[2]
Question 15
Figure 2 shows the main stages of a sewage-treatment plant.
Describe how primary treatment removes material from sewage.
[1]
Explain why aeration during secondary treatment lowers the BOD of the effluent.
[2]
State one process used during tertiary treatment.
[1]
Question 16
Figure 3 compares biological and chemical observations at two similar stream sites.
Biological and chemical observations at two similar stream sites.
| Stream site | Dissolved oxygen / | Mayfly nymphs per kick sample | Pollution-tolerant worms per kick sample |
|---|---|---|---|
| Site A | 9.2 | 36 | 4 |
| Site B | 3.4 | 2 | 58 |
Identify the site showing stronger evidence of organic pollution.
[1]
Using Figure 3, outline two pieces of evidence supporting your answer to (a).
[2]
State one limitation of concluding that pollution alone caused the difference in organism abundance.
[1]
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Question 17
A rapidly growing town plans to install an advanced centralized sewage-treatment plant, but many households lack sewer connections and electricity supplies are unreliable.
Explain two challenges to the equitable implementation of the proposed treatment system.
[2]
Outline one feature of a more appropriate treatment strategy for this town.
[1]
Question 18
In 2022, the tanker Ardent Sea grounded near the fictional Maruva coast and released crude oil. The coast contains sandy beaches, mangrove wetlands and offshore seagrass beds used as fish nursery grounds.
Calculate the percentage decrease in visible oil cover on the sandy beach between one and twelve months after the spill.
[2]
Explain two ways in which crude oil may harm organisms in the affected habitats.
[4]
Analyse the environmental and socioeconomic recovery shown by the data.
[3]
Evaluate the management response to the Ardent Sea spill.
[4]
Question 19
The River Arvo receives cooling water from a power station and effluent from an electronics factory. Scientists sampled three sites with similar channel habitat monthly over a four-month period beginning in late summer.
Mean values from monthly samples over four months; the same kick-sampling effort was used at each site.
| Site | Position | Temperature / | Dissolved oxygen / | PCB / | Mayfly nymphs / kick sample | Aquatic worms / kick sample |
|---|---|---|---|---|---|---|
| A | Upstream | 17 | 9.4 | 0.02 | 42 | 3 |
| B | Below power station | 25 | 6.3 | 0.03 | 15 | 24 |
| C | Below electronics factory | 23 | 4.8 | 0.46 | 2 | 61 |
Describe the spatial pattern in mayfly nymph and aquatic worm abundance.
[2]
Explain how thermal pollution may contribute to the biological pattern at site B.
[3]
Suggest why PCBs may pose a long-term risk to organisms downstream of site C even if the factory discharge is stopped.
[4]
Evaluate the use of mayfly nymphs and aquatic worms, together with the abiotic measurements, to identify the causes of pollution in the River Arvo.
[5]
Question 20
Health agencies monitored two harmful algal blooms during the same year: a freshwater cyanobacterial bloom in Lake Orin and a coastal dinoflagellate bloom in Maren Bay.
Weekly monitoring data for harmful algal blooms in Lake Orin and Maren Bay.
| Week | Lake Orin cells / | Lake toxin / (action 1.0) | Maren Bay cells / | Shellfish toxin / (action 80) |
|---|---|---|---|---|
| 1 | 32,000 | 0.15 | 80,000 | 10 |
| 2 | 55,000 | 0.22 | 150,000 | 14 |
| 3 | 110,000 | 0.50 | 320,000 | 22 |
| 4 | 220,000 | 1.10 | 550,000 | 40 |
| 5 | 410,000 | 1.90 | 850,000 | 62 |
| 6 | 650,000 | 3.10 | 1,100,000 | 96 |
| 7 | 780,000 | 3.60 | 1,320,000 | 118 |
| 8 | 690,000 | 3.00 | 1,400,000 | 125 |
| 9 | 500,000 | 2.50 | 1,150,000 | 130 |
| 10 | 300,000 | 1.50 | 800,000 | 102 |
| 11 | 120,000 | 0.65 | 420,000 | 68 |
| 12 | 45,000 | 0.28 | 180,000 | 35 |
State which monitored toxin concentration exceeded its action threshold by the greater factor.
[2]
Compare the pathways by which the two blooms may affect human health.
[3]
Explain why cell density or water colour alone cannot determine whether a bloom is safe.
[3]
Evaluate the effectiveness of monitoring and temporary use restrictions for managing the two harmful algal blooms.
[5]
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Question 21
Figure 4 shows changes in Lake Nera before and after vegetated buffer strips and improved wastewater treatment were introduced at the start of 2020.
Annual Lake Nera indicators; vegetated buffer strips and improved wastewater treatment introduced at the start of 2020.
| Year | Management period | Total phosphorus / | Summer chlorophyll-a / | Minimum bottom-water dissolved oxygen / | Commercial fish catch / tonnes |
|---|---|---|---|---|---|
| 2017 | Before intervention | 80 | 48 | 2.1 | 64 |
| 2018 | Before intervention | 86 | 53 | 1.8 | 58 |
| 2019 | Before intervention | 92 | 61 | 1.2 | 45 |
| 2020 | After intervention | 74 | 47 | 2.4 | 50 |
| 2021 | After intervention | 55 | 33 | 3.8 | 62 |
| 2022 | After intervention | 42 | 24 | 5.0 | 73 |
| 2023 | After intervention | 36 | 19 | 5.7 | 79 |
Calculate the percentage decrease in total phosphorus concentration from 2019 to 2023.
[2]
Describe two changes in the lake after the management measures were introduced.
[2]
Explain the relationship between phosphorus, chlorophyll-a and minimum dissolved oxygen shown before the intervention.
[3]
Suggest one ecosystem service that improved following the intervention, using evidence from Figure 4.
[1]
Question 22
A power station releases heated cooling water into a river. Figure 5 shows physical, chemical and biological measurements around the discharge.
Physical, chemical and biological measurements at river sites around a cooling-water outfall.
| Site (distance from outfall) / | Water temperature / | Dissolved oxygen / | Stonefly nymphs / standard sample | Aquatic worms / standard sample |
|---|---|---|---|---|
| Upstream () | 18 | 8.7 | 24 | 4 |
| Outfall () | 29 | 6.1 | 2 | 38 |
| Downstream () | 27 | 6.4 | 5 | 31 |
| Downstream () | 23 | 7.4 | 14 | 15 |
| Downstream () | 20 | 8.2 | 21 | 7 |
Calculate the increase in water temperature between the upstream site and the outfall.
[1]
Describe the relationship between temperature and dissolved oxygen shown in Figure 5.
[2]
Explain the changes in the abundance of stonefly nymphs and aquatic worms near the outfall.
[3]
Analyse one strength and one limitation of using the two organisms as indicators of pollution at this site.
[2]
Question 23
Figure 6 compares harmful algal blooms in freshwater Lake Aster and coastal Pelican Bay using monthly data from April to September. The public-health thresholds shown are the values at which authorities issue a warning or impose a closure.
Monthly organism abundance and toxin concentrations from April to September, with public-health thresholds.
| Month | Lake Aster cyanobacteria / | Lake Aster microcystin / | Pelican Bay dinoflagellates / | Pelican Bay brevetoxin / |
|---|---|---|---|---|
| Public-health threshold | — | 1.0 (drinking-water warning) | — | 15 (shellfish-harvest closure) |
| April | 0.2 | 0.1 | 2 | 0.5 |
| May | 0.8 | 0.4 | 8 | 2 |
| June | 2.5 | 1.2 | 25 | 7 |
| July | 6.0 | 3.8 | 70 | 18 |
| August | 8.5 | 5.6 | 95 | 26 |
| September | 3.0 | 1.7 | 30 | 9 |
Identify the first month in the period shown in which each public-health threshold was exceeded.
[2]
Compare the patterns of organism abundance and toxin concentration from April to September in the two water bodies.
[2]
Explain why brevetoxin may reach a hazardous concentration in shellfish even when the shellfish remain alive.
[2]
Suggest one appropriate short-term management response for each water body when its threshold is exceeded.
[2]
Question 24
Outline four ways in which oil may enter and spread through aquatic systems.
[4]
Explain the environmental and socioeconomic impacts of a named major oil spill and the strategies used to manage it.
[7]
Using named examples, evaluate the claim that preventing water pollution is more sustainable than cleaning it up after release.
[9]
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Question 25
Explain how plastic waste released near coastlines may accumulate in subtropical oceanic gyres.
[4]
Explain how microplastics and their associated chemicals may move through marine food chains and affect organisms.
[7]
Using named examples, evaluate strategies for reducing plastic pollution in marine environments.
[9]
Question 26
An estuary receives nutrient-rich river water and sewage. During summer, a warm, low-salinity surface layer forms above denser bottom water.
Explain how interacting processes may produce a bottom-water dead zone in this estuary.
[4]
Question 27
The Sava Estuary receives freshwater and nutrients from an agricultural river. Scientists examined the increasing occurrence of a summer bottom-water dead zone.
Annual summer conditions in the Sava Estuary, 2005–2025.
| Year | Surface temperature / | Strongly stratified days / days | Maximum dead-zone area / |
|---|---|---|---|
| 2005 | 21.0 | 38 | 115 |
| 2006 | 21.2 | 40 | 126 |
| 2007 | 21.1 | 39 | 121 |
| 2008 | 21.4 | 45 | 143 |
| 2009 | 21.3 | 43 | 136 |
| 2010 | 21.6 | 48 | 157 |
| 2011 | 21.8 | 51 | 165 |
| 2012 | 21.7 | 49 | 160 |
| 2013 | 22.0 | 55 | 184 |
| 2014 | 22.1 | 56 | 192 |
| 2015 | 22.3 | 61 | 211 |
| 2016 | 22.2 | 58 | 203 |
| 2017 | 22.6 | 65 | 229 |
| 2018 | 22.8 | 68 | 245 |
| 2019 | 22.7 | 66 | 238 |
| 2020 | 23.1 | 71 | 264 |
| 2021 | 23.3 | 73 | 274 |
| 2022 | 23.2 | 70 | 261 |
| 2023 | 23.6 | 76 | 286 |
| 2024 | 23.8 | 78 | 298 |
| 2025 | 24.0 | 79 | 310 |
Calculate the percentage increase in maximum dead-zone area between 2005 and 2025.
[2]
Explain how nutrient enrichment and stratification interact to produce bottom-water hypoxia.
[4]
Analyse how global warming may amplify dead-zone formation in the Sava Estuary.
[3]
Evaluate the conclusion that agricultural nitrate is the main cause of the increasing dead-zone area.
[5]
Question 28
The rapidly growing city of Darsana is deciding how to upgrade sewage treatment. The existing plant provides primary and secondary treatment. Untreated sewage from some unconnected neighbourhoods also enters the River Ilya.
Wastewater quality at successive treatment stages and local discharge standards for the River Ilya.
| Indicator | Influent | After primary | After secondary | After tertiary | Local standard |
|---|---|---|---|---|---|
| Biochemical oxygen demand / | 240 | 165 | 22 | 6 | 20 |
| Total suspended solids / | 280 | 125 | 24 | 7 | Not specified |
| Nitrate / | 18 | 17 | 15 | 3 | 5 |
| Phosphate / | 7.0 | 6.2 | 5.4 | 0.7 | 1.0 |
| Faecal indicator bacteria / | 2,400,000 | 1,500,000 | 190,000 | 400 | 1,000 |
Calculate the percentage reduction in biochemical oxygen demand achieved by secondary treatment relative to the influent.
[2]
Explain how primary and secondary treatment produce the observed changes in biochemical oxygen demand and total suspended solids.
[4]
Analyse why tertiary treatment is required for discharge to the River Ilya.
[4]
Evaluate which treatment strategy would provide the most equitable and environmentally effective sewage management for Darsana.
[5]
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Question 29
Figures 7(a) and 7(b) show seasonal conditions in the stratified Koro Estuary. Hypoxia is defined in this study as dissolved oxygen below .
Monthly surface-to-bottom salinity difference, bottom-water dissolved oxygen and surface-water temperature in the Koro Estuary.
| Month | Salinity difference / PSU | Bottom-water dissolved oxygen / | Surface-water temperature / |
|---|---|---|---|
| January | 1 | 7.8 | 12 |
| February | 1 | 7.5 | 13 |
| March | 2 | 6.8 | 15 |
| April | 5 | 5.2 | 18 |
| May | 9 | 3.1 | 22 |
| June | 13 | 1.8 | 25 |
| July | 15 | 0.9 | 27 |
| August | 14 | 0.6 | 27 |
| September | 10 | 1.5 | 24 |
| October | 6 | 3.4 | 20 |
| November | 3 | 5.9 | 16 |
| December | 1 | 7.2 | 13 |
Determine the number of months during which bottom water was hypoxic.
[1]
Describe the relationship between the salinity difference and bottom-water dissolved oxygen.
[2]
Explain how the conditions shown in Figures 7(a) and 7(b) produce the summer dead zone.
[4]
Predict one way climate change could increase the duration of hypoxia in this estuary.
[1]
Question 30
A regional authority is considering sewage-treatment improvements for a rapidly growing settlement. Figure 8(a) shows the existing treatment process. Figure 8(b) gives mean water-quality results, and Figure 8(c) compares two upgrade options.
Mean water-quality results at each treatment stage.
| Parameter | Raw sewage | After primary | After secondary | After tertiary |
|---|---|---|---|---|
| BOD / mg L | 220 | 150 | 18 | 7 |
| Total suspended solids / mg L | 260 | 120 | 20 | 5 |
| Total nitrogen / mg L | 42 | 39 | 31 | 8 |
| Total phosphorus / mg L | 7.0 | 6.0 | 5.0 | 0.8 |
| E. coli / CFU per 100 mL | 2 400 000 | 1 900 000 | 180 000 | 180 |
Comparison of tertiary-treatment upgrade options. Suitable public land is available; the electricity supply is interrupted frequently.
| Option | Operating cost / USD | removal / % | removal / % | Electricity | Skilled staff | Land | Reliability / % |
|---|---|---|---|---|---|---|---|
| Centralized mechanical plant | 0.64 | 88 | 78 | High | High | Low | 82 |
| Constructed wetland + UV | 0.29 | 68 | 64 | Low | Low | High | 96 |
Calculate the percentage reduction in BOD achieved by secondary treatment relative to the water entering that stage.
[2]
Identify the treatment stage responsible for the greatest reductions in total nitrogen and total phosphorus.
[1]
Explain how the secondary and tertiary stages produce the changes shown in Figure 8(b).
[3]
Evaluate which upgrade option is more appropriate for this settlement.
[2]
Question 31
Draw a systems model showing how nutrient enrichment may cause eutrophication, including one positive feedback loop.
[4]
Analyse how eutrophication may affect ecosystem services provided by a lake.
[7]
Using named examples, evaluate strategies at the three levels of pollution management for restoring eutrophic water bodies.
[9]
Question 32
Compare harmful algal blooms in freshwater and coastal marine environments.
[4]
Explain the pathways by which toxins from harmful algal blooms may affect humans and other animals.
[7]
Using one named freshwater example and one named marine example, evaluate strategies for managing harmful algal blooms and their risks.
[9]
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Question 33
Figure 2 shows the treatment of sewage before the final effluent is discharged to a river.
Distinguish between the main functions of primary, secondary and tertiary sewage treatment shown in Figure 2.
[4]
Analyse how the processes shown in Figure 2 reduce environmental risks without simply transferring pollution to another location.
[7]
Using named examples, evaluate strategies for providing sewage treatment equitably in rapidly growing settlements.
[9]
Question 34
Figure 1 shows processes contributing to a seasonal dead zone in a generalized estuary.
Explain how the interacting processes shown in Figure 1 may produce a bottom-water dead zone.
[4]
Analyse the value and limitations of using indicator species together with abiotic measurements to monitor the dead zone.
[7]
Using named examples, to what extent is global warming responsible for the increasing frequency and extent of aquatic dead zones?
[9]
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