What process is NOT involved in the self-purification of a flowing stream of water?

The self-purification of a flowing stream of water refers to the natural processes that help cleanse the water as it moves. Each of the processes listed contributes in some way to the purification of water, except for dilution.

Sedimentation is the process whereby heavier particles settle to the bottom of the water body, removing solids that can harbor pollutants. Aeration involves the exchange of gases, particularly oxygen, which helps to break down organic matter and supports aerobic organisms that further contribute to the purification process. Infiltration is the process where surface water seeps into the ground, filtering through soil and sediment, which can remove contaminants.

Dilution, on the other hand, refers to the process of mixing a contaminant with a larger volume of water, reducing its concentration. While dilution can temporarily reduce the effects of contaminants, it does not actively purify the water or remove pollutants from it. This makes it distinct from the other processes, which have direct roles in the removal and breakdown of harmful substances, making dilution the correct choice in this context.