JAMB 2013 Chemistry · Question 23

In a water body, too much sewage leads to

Options

A.a decrease in the temperature of the water which cause in death of aquatic animals
B.an increase in the number of aquatic animals in the water
C.an increase in the bacterial population which reduces the level of oxygen in the water
D.a decrease in the bacterial population which increases the level of oxygen in the water

Answer: C. an increase in the bacterial population which reduces the level of oxygen in the water

Explanation written with AI and checked against the answer key

Analysis of the Question

This question tests your understanding of Water Pollution and its impact on aquatic ecosystems. Specifically, it focuses on the process of eutrophication and the biological oxygen demand (BOD) in water bodies when organic waste (sewage) is introduced.

Concept Explanation

When a large amount of sewage (which is rich in organic matter) is discharged into a water body, the following chain reaction occurs:

Nutrient Enrichment: Sewage contains organic nutrients that act as food for aerobic bacteria.
Bacterial Bloom: Because there is an abundance of food, the population of aerobic bacteria increases rapidly (a "bloom").
Oxygen Depletion: These bacteria require dissolved oxygen to break down the organic matter through respiration. As the bacterial population explodes, they consume the dissolved oxygen in the water at a rate much faster than it can be replenished.
Hypoxia: The level of dissolved oxygen drops significantly, leading to the death of fish and other aquatic organisms that rely on that oxygen to survive.

Step-by-Step Solution

Step 1: Evaluate the effect of sewage. Sewage is organic waste. Its presence provides a massive food source for microorganisms (bacteria).
Step 2: Determine the impact on bacteria. With more food available, the bacterial population will increase, not decrease. This eliminates option D.
Step 3: Determine the impact on oxygen. Since the bacteria are aerobic (oxygen-breathing), their rapid growth leads to the depletion of dissolved oxygen in the water. This eliminates option B.
Step 4: Compare the remaining options. Option A suggests a temperature change, which is not the primary direct effect of sewage. Option C correctly identifies the increase in bacteria and the subsequent reduction in oxygen levels.

Conclusion

The introduction of sewage leads to a surge in decomposers (bacteria) that strip the water of its oxygen, making it impossible for other aquatic life to survive.

Correct Option: C

Exam Tip

When you see questions about "sewage" or "fertilizer runoff" in water, always associate them with "Oxygen Depletion." Remember the sequence: Sewage → More Bacteria → Less Oxygen → Dead Fish. This will help you answer similar questions on Eutrophication quickly!

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