What best describes a DO sag in a stream receiving wastewater?

• A. A drop in DO caused by microbial degradation of organic matter.
• B. A temporary rise in DO due to aeration.
• C. A stable DO level as water flows downstream.
• D. DO sag occurs only in marine environments.

Answer: (A)

A DO sag is a drop in dissolved oxygen caused by microbial degradation of organic matter introduced by wastewater. When wastewater enters a stream, it brings a lot of organic material. The microbes that break down this material consume oxygen in the process, so the oxygen available in the water decreases downstream of the discharge. This creates the characteristic dip in DO as the water mixes and flows away from the source.

Downstream, DO can recover as the water is aerated from the air, diluted by cleaner stream water, and sometimes supplemented by oxygen produced during daylight from photosynthetic activity of aquatic plants and algae. The other scenarios don’t fit a sag: a rise in DO due to aeration would reverse the sag; a stable DO level indicates no deficit; and DO sag is not limited to marine environments—it commonly occurs in freshwater streams receiving wastewater.