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IB ESS HL 4.4 Water Pollution Questions

Practise IB ESS HL 4.4 by analysing direct tests, biotic indices and WQI evidence and evaluating pollution controls and coordinated action.

Syllabus
First assessment 2026
Course
ESS HL
Level
HL

Exam points

  • Analyse point and non-point sources using discharge, runoff and waste evidence.
  • Evaluate pH, dissolved oxygen, turbidity and BOD measurements across impacted sites.
  • Analyse eutrophication from nutrient enrichment through oxygen loss and assess controls.
  • Compare biotic indices and WQI with direct tests, including what each measures and misses.
  • Evaluate citizen action through consumption change, monitoring, lobbying and coordinated removal.

Covered syllabus points

4.4.1—Water pollution sources4.4.2—Plastic debris accumulation4.4.3—Water quality measurement4.4.4—Biochemical oxygen demand (BOD)4.4.5—Eutrophication4.4.6—Eutrophication sequence4.4.7—Eutrophication and ecosystem services4.4.8—Eutrophication management levels4.4.9 (HL)—Water pollutants4.4.10 (HL)—Harmful algal blooms4.4.11 (HL)—Anoxic and hypoxic waters4.4.12 (HL)—Sewage treatment stages4.4.13 (HL)—Indicator species4.4.14 (HL)—Biotic index4.4.15 (HL)—Water Quality Index (WQI)4.4.16 (HL)—Water quality guidelines4.4.17 (HL)—Citizen action

Question 1

[Maximum number: 6]

Question (a)

(a)

A small lake is going through the process of eutrophication. State one possible point source and one possible non-point source of nutrient pollution in the lake.
Point source:
Non-point source:

[ 2 ]

Question (b)

(b)

Define the term biochemical oxygen demand (BOD).

[ 1 ]

Question (c)

(c)
pH-tolerance ranges of freshwater organisms

Figure:pH-tolerance ranges of freshwater organisms

The pH-tolerance chart shows the ranges over which aquatic organisms in a small lake can survive.

[ 3 ]

Question (i)

(i)

Use the pH-tolerance ranges of freshwater organisms: the pH tolerance ranges of trout, bass, perch, frogs, salamanders, clams, crayfish, snails and mayfly in the small lake.

State which organism(s) would still be present if the pH of the lake was reduced to 5.0.

[ 1 ]

Question (ii)

(ii)

Use the pH-tolerance ranges of freshwater organisms: the pH tolerance ranges of trout, bass, perch, frogs, salamanders, clams, crayfish, snails and mayfly in the small lake.

Explain why the populations of remaining organisms in the lake may decline in the long term.

[ 2 ]

Question 2

[Maximum number: 5]

Many countries produce large amounts of solid domestic waste. Pacific Ocean currents concentrate and trap solid domestic waste from the Pacific Rim countries and have created the Great Pacific Garbage Patch (GPGP) in the North Pacific Gyre (Figure 5).

Figure 6: Consumption and disposal of plastic waste for India and selected Pacific Rim countries

Figure 6:Consumption and disposal of plastic waste for India and selected Pacific Rim countries

* Estimated for 2017 based on 10% growth.

Question (a)

(a)

Identify one reason why most plastics may be considered more serious pollutants than other forms of solid domestic waste.

[ 1 ]

Question (b)

(b)

Explain why the data in Figure 6 show no correlation between plastic consumption and plastic waste deposited in ocean for the countries listed.

[ 4 ]

Question 3

[Maximum number: 6]

Question (a)

(a)
Coliform bacteria one day after untreated sewage release

Figure:Coliform bacteria one day after untreated sewage release

Using Figure 11(b), estimate the highest concentrations of coliform bacteria (in units/100 mL) found in the St Lawrence River one day after the untreated sewage was released.

[ 1 ]

Question (b)

(b)

Outline an environmental problem that may result from the release of untreated sewage into a river.

[ 2 ]

Question (c)

(c)
Coliform bacteria before untreated sewage release

Figure:Coliform bacteria before untreated sewage release

Figure:Untreated sewage release into the St Lawrence River

Montreal:
In November 2015, the City of Montreal discharged between 5 and 8 billion litres of untreated sewage into the St Lawrence River.
• It was called “FlushGate” in the Canadian news.
• Citizens were warned to avoid contact with the water.
• River pollution levels returned to normal within 4 to 10 days.
• The Mayor of Montreal described it as the “most environmentally-friendly solution”.

Quebec:
In November 2016, the City of Quebec discharged 110 million litres of untreated sewage into the St Lawrence River.

Nationally:
• 25 % of Canadians do not have access to sewage or wastewater treatment centres.
• 205 billion litres of untreated sewage are released into Canadian rivers and oceans each year.

With reference to Figures 10, 11(a) and 11(b), describe a method to monitor the impact of the release of untreated sewage into the St Lawrence River ecosystem.

[ 3 ]
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