River water quality for aquatic life: The minimum dissolved oxygen (DO) concentration typically prescribed for the survival of fish in a river is approximately:

Introduction / Context:

Dissolved oxygen (DO) is critical for aquatic organisms. Rivers receiving treated effluents must maintain adequate DO to sustain fish and benthic life. Regulatory targets and ecological studies establish minimum thresholds.

Given Data / Assumptions:

• Question asks for the minimum DO for fish survival, not the desirable or optimum DO.
• Units ppm and mg/L are equivalent for dilute aqueous solutions.

Concept / Approach:

While 5–6 mg/L is often cited as desirable for healthy fisheries, a minimum around 4 mg/L is commonly taken as a survival threshold for many freshwater species. Below this level, stress and mortality risks rise rapidly, especially for sensitive species and at higher temperatures.

Step-by-Step Solution:

Verification / Alternative check:

Standard environmental guidelines and fisheries literature frequently recommend maintaining river DO ≥ 4 mg/L at all times, with targets ≥ 5 mg/L preferred.

Why Other Options Are Wrong:

• 3 ppm or 2 ppm: Too low; severe stress and fish kills can occur.
• 5 ppm or 10 ppm: Desirable/healthy levels, but the question asks the minimum for survival.

Common Pitfalls:

• Confusing minimum survival with recommended design or regulatory targets.

Final Answer:

4 ppm