When domestic sewage mixes with river water

Options

A. small animals like rats will die after drinking river water
B. the increased microbial activity releases micronutrients such as iron
C. the increased microbial activity uses up dissolved oxygen
D. the river water is still suitable for drinking as impurities are only about `0.1%`.

Answer

**Step-by-Step Solution:**

1. **Understanding the Problem**: The question asks about the consequences of domestic sewage mixing with river water. Domestic sewage contains harmful substances and microbes that can affect the quality of water.

2. **Analyzing the Options**:
- **Option 1**: "Small animals like rats will die after drinking river water."
- This statement is too absolute. While sewage is harmful, it does not mean that all animals will die. Thus, this option is incorrect.

- **Option 2**: "Increased microbial activities release micronutrients such as iron."
- This option suggests that sewage enhances the release of beneficial nutrients. However, sewage is primarily harmful and does not contribute positively to water quality. Therefore, this option is incorrect.

- **Option 3**: "Increased microbial activity uses dissolved O2."
- This option is accurate. When sewage enters the river, it promotes the growth of microbes that decompose organic matter. This process consumes dissolved oxygen (O2) in the water, which can lead to decreased oxygen levels, affecting aquatic life. Hence, this option is correct.

- **Option 4**: "The river water is still suitable for drinking as impurities are only about 0.1%."
- This statement is misleading. The presence of sewage significantly degrades water quality, making it unsafe for drinking, regardless of the percentage of impurities. Thus, this option is incorrect.

3. **Conclusion**: After analyzing all the options, the correct answer is **Option 3**, which states that increased microbial activity uses dissolved O2.