To counterbalance the consumption of D.O. due to de-oxygenation atmosphere supplies oxygen to the water. This is known as re-Oxygenation. It Increases with the time of flow from the point of pollution.

Re-oxygenation curve of a polluted stream

The re-oxygenation curve of a polluted stream refers to the pattern of changes in dissolved oxygen levels over time as the stream recovers from pollution. It provides valuable information about the stream's ability to naturally restore oxygen levels and recover from pollution.

Explanation:

The correct answer is option 'B' - the re-oxygenation curve of a polluted stream increases with time. This means that as time passes, the dissolved oxygen levels in the stream gradually increase.

Factors influencing re-oxygenation:

Several factors contribute to the increase in dissolved oxygen levels over time in a polluted stream:

1. Natural processes: Natural processes such as photosynthesis, diffusion, and mixing with oxygen-rich water from upstream sources play a crucial role in re-oxygenation. These processes gradually replenish the oxygen levels in the stream.

2. Biological activity: Aquatic organisms, such as algae and plants, can contribute to re-oxygenation through photosynthesis. During photosynthesis, these organisms consume carbon dioxide and release oxygen, thereby increasing the dissolved oxygen levels in the water.

3. Physical conditions: Factors like temperature, flow rate, and turbulence in the stream can affect the re-oxygenation process. Higher temperatures generally reduce the water's ability to hold dissolved oxygen, while increased flow rates and turbulence enhance oxygen transfer from the atmosphere to the water.

4. Decomposition of organic matter: Polluted streams often contain high levels of organic matter, such as sewage or agricultural runoff. As these organic materials decompose, bacteria and other microorganisms consume oxygen during the decomposition process. Once the organic matter is depleted, the demand for oxygen decreases, allowing dissolved oxygen levels to recover.

Conclusion:

In conclusion, the re-oxygenation curve of a polluted stream increases with time. This indicates that as natural processes, biological activity, and decomposition of organic matter occur, the stream gradually recovers and replenishes its dissolved oxygen levels. Monitoring and understanding the re-oxygenation curve can help assess the health and recovery of a polluted stream and guide appropriate management and restoration efforts.