Stream pollution zones: If dissolved oxygen (D.O.) in a natural stream falls to essentially zero, the affected reach corresponds to which classical zone of a polluted stream?

Introduction / Context:
After a wastewater discharge enters a river, the receiving water experiences characteristic changes in dissolved oxygen (D.O.) and quality along the flow path. Recognizing these classical zones helps diagnose pollution severity and the self-purification process.

Given Data / Assumptions:

• D.O. concentration is observed to drop to ~0 mg/L.
• Typical river self-purification model with zones: degradation, active decomposition (septic), recovery, and clean water.
• Steady discharge and conservative understanding of oxygen demand.

Concept / Approach:
In the septic or active decomposition zone, biochemical oxygen demand (BOD) far exceeds oxygen reaeration, driving D.O. to near zero. Putrefaction and anaerobic processes dominate until downstream reaeration and reduced BOD allow D.O. to recover.

Step-by-Step Reasoning:
Immediately after discharge: degradation begins; D.O. starts falling.In the active decomposition zone: oxygen deficit peaks; D.O. ~ 0 mg/L.Farther downstream: reaeration and oxidation reduce deficit; D.O. increases (recovery zone).

Verification / Alternative check:
The D.O. sag curve analysis (Streeter–Phelps framework) mirrors these zones, with the minimum D.O. typically inside or near the active decomposition zone depending on kinetics.

Why Other Options Are Wrong:
Degradation zone: D.O. is declining but not necessarily zero.Recovery/cleaner water zones: D.O. is rising or near saturation, not zero.None: incorrect because a standard zone fits the observation.

Common Pitfalls:

• Equating first drop in D.O. with zero; true zero is characteristic of the septic zone.
• Ignoring reaeration effects which eventually restore D.O.

Final Answer:
Zone of active decomposition (septic zone)