Self-reoxygenation in polluted streams: In a watercourse receiving industrial wastewater, how is dissolved oxygen (DO) naturally replenished?

Introduction / Context:
When organic-laden effluents enter a stream, microorganisms consume dissolved oxygen, potentially creating an oxygen sag. Natural processes can counterbalance this depletion and are central to stream self-purification models used in environmental design.

Given Data / Assumptions:

• Open stream with sunlight exposure and turbulence.
• Presence of phytoplankton and periphyton typical of natural waters.

Concept / Approach:
Two principal mechanisms restore DO: (1) Reaeration, where oxygen transfers from air to water driven by a concentration gradient, enhanced by turbulence (riffles, falls, wind). (2) Photosynthesis, where algae and aquatic macrophytes produce oxygen during daylight, increasing DO locally and diurnally.

Step-by-Step Solution:
Acknowledge reaeration as a continuous process determined by gas-transfer coefficients and deficit.Recognize photosynthesis as a daylight process adding oxygen and offsetting BOD.Combine both mechanisms as contributors to DO replenishment.

Verification / Alternative check:
Streeter–Phelps models include a reaeration term; field DO profiles show diurnal swings with midday peaks due to photosynthesis and nighttime declines when respiration dominates.

Why Other Options Are Wrong:

• Single-mechanism answers ignore the well-established dual contribution.
• Nitrification: consumes oxygen rather than producing it.

Common Pitfalls:
Ignoring nighttime DO minima and the risk of fish kills when respiration exceeds reaeration.

Final Answer:
Both (a) and (b)