20.3 Pollution Revision Notes

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!