When organic waste is discharged into receiving waters, which processes lead to oxygen depletion?

The correct answer describes the process by which bacteria break down organic matter, a critical factor in oxygen depletion in receiving waters. When organic waste enters a water body, it serves as a food source for bacteria. As these bacteria decompose the organic material, they consume dissolved oxygen in the water, a vital resource for aquatic life.

This demand for oxygen during decomposition can lead to significantly lower levels of dissolved oxygen, creating an environment that is less hospitable for fish and other organisms. In severe cases, this can result in hypoxic (low oxygen) or anoxic (no oxygen) conditions, which can be detrimental to aquatic ecosystems.

The other processes listed do not directly contribute to oxygen depletion in the same way. For instance, the decomposition of organic material by plants primarily involves photosynthesis rather than the consumption of oxygen. Evaporation of water might affect water levels but does not directly correlate with oxygen depletion. Chemical reactions in water can involve oxygen but do not typically lead to the same extent of depletion observed during bacterial decomposition of organic waste.