Pollution

1. Overview

Pollution is the introduction of harmful substances into the environment as a result of human activity. This topic explores how human-driven waste—ranging from chemical runoff to greenhouse gases—disrupts ecosystems, threatens biodiversity, and alters the Earth's climate.

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Key Definitions

• Pollution: The release of substances into the environment that cause harmful effects to living organisms.
• Non-biodegradable: Materials (like certain plastics) that cannot be broken down by the biological action of decomposers like bacteria and fungi.
• Greenhouse Effect: The trapping of the sun's warmth in a planet's lower atmosphere due to the greater transparency of the atmosphere to visible radiation from the sun than to infrared radiation emitted from the planet's surface.
• Eutrophication: The process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients (particularly nitrogen and phosphorus).

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Core Content

Water Pollution: Sewage and Fertilizers

Untreated sewage and excess fertilizers (from farms) contain high levels of nutrients like nitrates and phosphates. When these enter aquatic ecosystems:

• Sewage: Contains pathogens that can cause disease and provides a food source for bacteria, which multiply and use up dissolved oxygen.
• Fertilizers: Lead to rapid growth of aquatic producers, which initiates the process of eutrophication (see Supplement for details).

Non-Biodegradable Plastics

Plastics are synthetic polymers that do not decay easily.

• Aquatic Ecosystems:
  • Animals can become entangled in plastic nets or bags, leading to drowning or restricted movement.
  • Animals mistake plastic for food (e.g., turtles eating plastic bags thinking they are jellyfish). This fills the stomach, causing starvation.
  • Microplastics can absorb toxins and enter the food chain, accumulating in top predators.
• Terrestrial Ecosystems:
  • Plastics take up massive amounts of space in landfills.
  • When burned, they can release toxic gases into the atmosphere.
  • They can block drainage systems, leading to flooding and stagnant water (breeding grounds for mosquitoes).

Air Pollution: Methane and Carbon Dioxide

The Earth's temperature is regulated by the greenhouse effect. However, human activity is causing an enhanced greenhouse effect.

• Carbon Dioxide ($CO_2$):
  • Sources: Burning fossil fuels (coal, oil, gas) and deforestation (fewer trees to absorb $CO_2$).
  • Effect: Increases the thickness of the "blanket" of gases in the atmosphere, trapping more heat.
• Methane ($CH_4$):
  • Sources: Decay of organic waste in landfills, rice paddy fields, and digestive processes of livestock (cattle).
  • Effect: Methane is a much more potent greenhouse gas than $CO_2$, though it exists in lower concentrations.

The Process of Climate Change:

1. Short-wave radiation from the sun passes through the atmosphere.
2. The Earth's surface absorbs this and re-emits it as long-wave infrared radiation (heat).
3. Greenhouse gases ($CO_2$ and $CH_4$) trap this infrared radiation.
4. This leads to Global Warming, resulting in rising sea levels, extreme weather events, and habitat loss.

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Extended Content (Extended Curriculum Only)

The Process of Eutrophication

Eutrophication is a specific sequence of events triggered by nutrient runoff into water bodies. You must be able to explain this step-by-step:

1. Increased Availability of Ions: Nitrates and other ions from untreated sewage or fertilizers leach into rivers or lakes.
2. Increased Growth of Producers: The high nutrient levels cause an "algal bloom" (rapid growth of algae on the water surface).
3. Light Blockage: The thick layer of algae blocks sunlight from reaching plants deeper in the water.
4. Death of Producers: Deeper aquatic plants cannot photosynthesize and die.
5. Increased Decomposition: Aerobic bacteria decompose the dead plant matter.
6. Increased Aerobic Respiration: The decomposers (bacteria) multiply rapidly and use up the dissolved oxygen in the water for aerobic respiration.
7. Reduction in Dissolved Oxygen: The concentration of oxygen in the water drops significantly (anoxia).
8. Death of Organisms: Fish and other aquatic organisms that require dissolved oxygen die of suffocation.

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Key Equations

While there are no specific mathematical formulas for pollution, you should understand the relationship between gas concentrations:

• Net Carbon Flux: $\text{Total } CO_2 \text{ released} - \text{Total } CO_2 \text{ absorbed} = \text{Net change in atmospheric } CO_2$.
• Oxygen Concentration: As Decomposer Population $\uparrow$, Dissolved Oxygen $\downarrow$.

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Common Mistakes to Avoid

• ❌ Wrong: Thinking the greenhouse effect is caused by the "hole in the ozone layer."
• ✓ Right: The greenhouse effect is caused by gases like $CO_2$ trapping heat; the ozone layer is a separate issue involving UV radiation.
• ❌ Wrong: Stating that fertilizers are "poisons" or "toxins" that kill fish directly.
• ✓ Right: Fertilizers cause algae to grow; it is the lack of oxygen caused by bacteria that eventually kills the fish.
• ❌ Wrong: Saying "the water runs out of air."
• ✓ Right: Use the specific term: "reduction in dissolved oxygen."

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Exam Tips

• Command Words:
  • If the question says "State," give a brief name or factor (e.g., "State two greenhouse gases").
  • If it says "Explain," you must give a reason "because..." (e.g., Explain why fish die in eutrophic water).
• Contexts: You will often see data tables showing oxygen levels at different points along a river. Look for the point where sewage enters; the oxygen will drop immediately downstream because of bacterial activity.
• Typical Values: Be prepared to interpret graphs where $CO_2$ levels are measured in "parts per million" (ppm). Current levels are approximately 415-420 ppm.
• Step-by-Step: For Eutrophication questions, always use a numbered list or clear sequence in your head to ensure you don't skip the "bacteria" or "respiration" steps—these are the most common marks missed!

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0610 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

A farmer uses a large amount of fertiliser on their crops. Rainwater washes some of the fertiliser into a nearby lake.

(a) State the name of the process that can occur in the lake as a result of the fertiliser runoff. [1]

(b) Describe how the increased availability of nitrate ions from the fertiliser leads to a reduction in the dissolved oxygen concentration in the lake. [4]

Worked Solution:

(a)

1. Eutrophication [Direct recall of the term describing the process]

How to earn full marks:

• Correct spelling of "eutrophication" is essential.

(b)

1. The increased nitrate concentration promotes excessive growth of algae. [Algae respond to the increased nutrients]

2. This algal bloom blocks sunlight from reaching aquatic plants below the surface. [Sunlight is needed for photosynthesis]

3. The aquatic plants die due to lack of light for photosynthesis. [Photosynthesis stops, plants die]

4. Decomposers break down the dead algae and plants, using aerobic respiration. This respiration uses up the dissolved oxygen in the water. [Decomposers deplete oxygen during respiration]

How to earn full marks:

• Mention "algae" or "producers" for mark 1.
• Link reduced light to death of plants for marks 2 and 3.
• State that decomposers use up oxygen during respiration for mark 4.

Common Pitfall: Make sure you understand the sequence of events in eutrophication. It's not enough to just mention the terms; you need to explain how each step leads to the next, ultimately causing oxygen depletion.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

A town's sewage treatment plant malfunctions, and untreated sewage is released into a river.

(a) State two potential effects of this untreated sewage on the river ecosystem. [2]

(b) Explain why the release of untreated sewage into the river can lead to the death of fish. [4]

Worked Solution:

(a)

1. Increase in bacterial population/increase in pathogens [Sewage carries bacteria]

2. Decrease in dissolved oxygen levels/eutrophication [Sewage leads to oxygen depletion]

How to earn full marks:

• Any two valid effects of untreated sewage are accepted.
• Must be distinct and specific effects.

(b)

1. Untreated sewage contains organic matter that is decomposed by bacteria. [Sewage decomposition consumes oxygen]

2. The bacteria respire aerobically, using up dissolved oxygen in the water. [Aerobic respiration by decomposers]

3. This reduces the concentration of dissolved oxygen available for fish to respire. [Fish need oxygen]

4. As oxygen levels decrease, fish struggle to respire and eventually die due to lack of oxygen. [Lack of oxygen leads to death]

How to earn full marks:

• Relate sewage to increased decomposers (bacteria) for mark 1.
• Link decomposers to reduced dissolved oxygen levels for marks 2 and 3.
• State that fish die because of lack of oxygen for mark 4.

Common Pitfall: Be specific when describing the effects of sewage. Don't just say "pollution"; mention specific consequences like decreased oxygen or increased bacteria. Also, remember that fish need dissolved oxygen for respiration.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

A scientist is investigating the effects of different types of plastic on soil ecosystems. They bury three different types of plastic (A, B, and C) in separate plots of soil. After one year, they measure the number of earthworms in each plot and the concentration of microplastics in the soil. The results are shown below.

Plastic Type	Number of Earthworms per m²	Microplastic Concentration (mg/kg soil)
A	5	10
B	12	2
C	2	25

(a) Describe the likely effects of non-biodegradable plastics on terrestrial ecosystems, referring to the data above. [4]

(b) Suggest two reasons why the presence of microplastics might affect the number of earthworms in the soil. [4]

Worked Solution:

(a)

1. Non-biodegradable plastics can persist in the environment for long periods, breaking down into microplastics. [Describing the breakdown process]

2. The data shows that Plastic C, which has the highest microplastic concentration (25 mg/kg), has the lowest number of earthworms (2 per m²). [Linking plastic C to low earthworm counts]

3. This suggests that high concentrations of microplastics may be harmful to earthworms, potentially reducing their populations. [Inferring the impact on earthworm populations]

4. Plastic B, with the lowest microplastic concentration (2 mg/kg), has the highest earthworm population (12 per m²), indicating a possible correlation between low microplastic levels and healthy earthworm populations. [Linking plastic B to high earthworm counts]

How to earn full marks:

• State that plastics break down into microplastics for mark 1.
• Link Plastic C and its high microplastic concentration to low earthworm counts for mark 2 and 3.
• Link Plastic B and its low microplastic concentration to high earthworm counts for mark 4.

(b)

1. Microplastics may physically block the earthworms' digestive systems, preventing them from absorbing nutrients from the soil. [Physical blockage of digestive system]

2. Microplastics can leach toxic chemicals into the soil, which are then absorbed by the earthworms, leading to poisoning or death. [Chemical toxicity of microplastics]

3. Microplastics may alter the soil structure, making it more difficult for earthworms to burrow and move through the soil, reducing their access to food and oxygen. [Altered soil structure]

4. Microplastics may contaminate the earthworms' food sources, such as decaying organic matter, reducing the nutritional value of their diet. [Contamination of food sources]

How to earn full marks:

• Any two valid reasons for the effect of microplastics on earthworms are accepted.
• Each reason should be biologically plausible.

Common Pitfall: When answering data-based questions, always refer directly to the data provided in your answer. Don't just make general statements; use the numbers to support your claims. Also, remember that microplastics can have both physical and chemical effects on organisms.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

Methane ($CH_4$) and carbon dioxide ($CO_2$) are both greenhouse gases that contribute to climate change.

(a) State one major source of methane in the atmosphere. [1]

(b) Describe how increased concentrations of methane and carbon dioxide in the atmosphere lead to the enhanced greenhouse effect and climate change. [5]

(c) A scientist measures the concentration of carbon dioxide in a sealed container containing a sample of soil. Initially, the concentration is 400 ppm (parts per million). After 24 hours, the concentration has increased to 412 ppm. The volume of the container is 10 litres.

(i) Calculate the increase in the volume of carbon dioxide in the container, in litres, after 24 hours. Note that 1 ppm is equivalent to 1 volume part per million volume parts. [2]

(ii) Suggest one process occurring in the soil that could account for the increase in carbon dioxide concentration. [1]

Worked Solution:

(a)

1. Rice paddies / livestock farming / decomposition in landfills / natural gas leaks / melting permafrost. [Direct recall of a major methane source]

How to earn full marks:

• Accept any one valid source of methane.

(b)

1. Methane and carbon dioxide are greenhouse gases that absorb infrared radiation (heat) emitted from the Earth's surface. [Absorption of infrared radiation]

2. This absorption prevents the heat from escaping into space, trapping it in the atmosphere. [Trapping heat in the atmosphere]

3. This trapped heat causes the average temperature of the Earth's atmosphere to increase. [Increasing global temperature]

4. This increase in global temperature leads to climate change, including changes in weather patterns, rising sea levels, and melting glaciers. [Climate change consequences]

5. The increased concentration of these gases enhances the greenhouse effect, leading to a greater amount of heat being trapped and a more significant rise in global temperatures. [Enhanced greenhouse effect]

How to earn full marks:

• State that greenhouse gases absorb infrared radiation for mark 1.
• Explain that this prevents heat from escaping for mark 2.
• Link the trapped heat to increased global temperatures for mark 3.
• Mention specific consequences of climate change for mark 4.
• State that increased gas concentration enhances the effect for mark 5.

(c) (i)

1. Increase in $CO_2$ concentration = 412 ppm - 400 ppm = 12 ppm. [Calculate the change in concentration]

2. Volume of $CO_2$ increase = (12 / 1,000,000) * 10 litres = $1.2 \times 10^{-4}$ litres. [Convert ppm to volume fraction, then multiply by total volume] $\boxed{1.2 \times 10^{-4} \text{ litres}}$

How to earn full marks:

• Correctly calculate the change in $CO_2$ concentration (12 ppm) for mark 1.
• Correctly calculate the volume increase using the ppm ratio and total volume, with correct units for mark 2.

(ii)

1. Respiration by soil organisms (e.g., bacteria, fungi, earthworms). [Identifying a process that releases carbon dioxide]

How to earn full marks:

• State respiration by soil organisms.
• Accept decomposition by soil organisms.

Common Pitfall: Remember that the greenhouse effect is a natural process that keeps the Earth warm enough to support life. The enhanced greenhouse effect, caused by increased greenhouse gas concentrations, is what leads to climate change. Also, pay close attention to units in calculations, especially when dealing with ppm.