Water pollution, the "zones of pollution"

 When discussing water pollution, the "zones of pollution" typically refer to the distinct areas in a river or stream where the level of pollution changes as the water flows downstream, usually including a "zone of clear water," "zone of active decomposition," "zone of degradation," and a "zone of recovery.". 

Explanation of the zones:

Zone of Clear Water:

This is the initial section of the river, with minimal pollution and high levels of dissolved oxygen, where aquatic life thrives.

Zone of Active Decomposition:

Following the point of pollutant entry, this zone experiences rapid bacterial decomposition of organic matter, leading to a significant drop in dissolved oxygen and potential fish kills.

Zone of Degradation:

Here, the water becomes increasingly turbid with sludge deposits, and the oxygen levels remain low due to ongoing decomposition.

Zone of Recovery:

As the water flows further, natural processes like aeration and plant life help to gradually restore the water quality, with a rise in dissolved oxygen and a potential return of some aquatic organisms. 

Key points to remember:

The exact characteristics of each zone can vary depending on the type and amount of pollutant, water flow, and environmental conditions. 

Studying these zones is important for understanding the impact of pollution and for designing strategies to mitigate its effects.