What is a DO sag and why does it occur downstream of wastewater discharges?

• A. A drop in dissolved oxygen caused by microbial degradation of organic matter.
• B. A rise in dissolved oxygen due to photosynthesis.
• C. A stable DO level in the stream.
• D. DO sag occurs only in marine environments.

Answer: (A)

A DO sag is a localized drop in dissolved oxygen as water flows downstream from a wastewater discharge. This happens because wastewater adds a lot of organic material, which microbes consume as they break it down. The microbial respiration uses up the dissolved oxygen in the water faster than it can be replenished by mixing with the air and dilution with cleaner water, creating a noticeable dip in DO downstream. Over time, reaeration and dilution help restore oxygen levels, but the initial sag reflects the oxygen demand imposed by the organic matter in the discharge.

The other ideas don’t fit: a rise in DO from photosynthesis doesn’t describe what occurs right after a wastewater input, DO levels aren’t typically stable in such zones, and DO sag is a common issue in freshwater systems receiving wastewater, not restricted to marine environments.