4.4 Water pollutionIB Environmental Systems and Societies HL: Subtopic test

1. 1
   Western Lake Erie is a shallow lake bordered by farmland and cities in the USA and Canada. Fertiliser and manure containing phosphorus wash from fields into rivers that drain into the lake. In recent summers the surface water has turned green with dense phytoplankton growth, and in late summer parts of the lake bottom have been found to contain very little dissolved oxygen.

   (a)

   Which statement best describes eutrophication?

   [1 mark]

   • AInputs of mineral nutrients, especially nitrates and phosphates, to water, often causing excessive phytoplankton growth
   • BA rise in water temperature caused by cooling water from power stations
   • CThe accumulation of plastic debris in water
   • DAn increase in the salt content of a lake

   (b)

   Which is the correct sequence of changes in the lake after nutrient inputs increase?

   [1 mark]

   • APhytoplankton die off, nutrients fall, fish numbers rise
   • BDissolved oxygen rises, fish numbers rise, water becomes clearer
   • CPhytoplankton bloom, light is blocked, decomposers multiply and BOD rises, dissolved oxygen falls, fish die
   • DDecomposers multiply, dissolved oxygen rises, phytoplankton decline

   (c)

   Explain why dissolved oxygen is very low at the bottom of the lake in late summer.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A town's sewage treatment works receives wastewater from homes, businesses and a food factory. At the works the wastewater first passes through screens and settling tanks. It then flows into tanks where air is bubbled through it and microorganisms break down organic matter. A final stage removes nitrates and phosphates before the water is released into a river.

   (a)

   Which stage of sewage treatment is described by the screens and settling tanks?

   [1 mark]

   • ATertiary treatment
   • BPrimary treatment
   • CSecondary treatment
   • DDisinfection only

   (b)

   What is the main purpose of the tanks with air bubbled through the wastewater and microorganisms?

   [1 mark]

   • ATo remove plastics and large objects
   • BTo remove nitrates and phosphates chemically
   • CTo kill all pathogens with ultraviolet light
   • DTo break down dissolved and fine organic matter and so lower BOD

   (c)

   Explain why the final stage, which removes nitrates and phosphates, is important for the river.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A river is sampled upstream and downstream of the point where a sewage works discharges. Upstream: biochemical oxygen demand (BOD) 2 mg of oxygen per dm³, dissolved oxygen 9 mg per dm³, mayfly and stonefly nymphs common, sludge worms absent. Downstream: BOD 9 mg of oxygen per dm³, dissolved oxygen 3 mg per dm³, mayfly and stonefly nymphs absent, sludge worms (Tubifex) abundant.

   (a)

   Calculate the percentage increase in BOD downstream of the discharge and explain why BOD is higher downstream.

   [3 marks]

   (b)

   Explain how the invertebrates recorded can be used to assess water quality, and state one limitation of using a biotic index.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   The Baltic Sea is a shallow, almost enclosed sea fed by rivers that drain farmland and cities in nine surrounding countries. Water exchange with the North Sea is limited, and a layer of less salty surface water lies above denser, saltier water, which reduces mixing. Nitrogen and phosphorus from fertiliser, livestock and sewage have caused large areas of the seabed to contain almost no oxygen, and in summer blooms of cyanobacteria cover large areas. In 2007 the surrounding countries and the European Union adopted the Baltic Sea Action Plan, run by the Helsinki Commission (HELCOM), to reduce nutrient inputs.

   (a)

   Evaluate the three levels of management that could be used to address eutrophication in the Baltic Sea.

   [6 marks]

   (b)

   Discuss why hypoxic (low-oxygen) waters are likely to become more frequent in seas such as the Baltic, and the risks that algal blooms pose to people and other animals.

   [6 marks]

   Total for question 4: 12 marks

End of questions