Typical dissolved oxygen (DO) in river water In a healthy, well-aerated river reach, the dissolved oxygen concentration is commonly around how many parts per million (ppm)?
-
A5
-
B100
-
C250
-
D500
Answer
Correct Answer: 5
Explanation
Introduction / Context: Dissolved oxygen (DO) is a core indicator of aquatic ecosystem health. Adequate DO supports fish respiration, aerobic biodegradation, and overall riverine biodiversity.
Given Data / Assumptions:
- We consider typical temperate conditions with moderate temperature.
- Values are in ppm, interchangeable with mg/L for dilute aqueous solutions.
- Water is not supersaturated and is free from extreme organic loading.
Concept / Approach: Oxygen solubility in water at ambient conditions is on the order of single-digit mg/L. Natural rivers often have DO around 5 to 9 mg/L depending on temperature, turbulence, and biological activity. Values in the hundreds of ppm are physically unrealistic for oxygen solubility at normal pressure.
Step-by-Step Solution:
Recall oxygen solubility at 20°C ~ 9 mg/L; warmer water holds less.Healthy river reaches typically maintain > 5 mg/L to avoid fish stress.Among the options, 5 ppm is the only realistic magnitude.Verification / Alternative check: Field measurements and standards (often DO > 5 mg/L for aquatic life) align with this range; environmental guidelines use similar thresholds.
Why Other Options Are Wrong:
100, 250, 500 ppm are orders of magnitude above oxygen solubility at 1 atm and cannot occur in natural rivers.Common Pitfalls: Confusing DO with BOD or COD values that may be larger, and misinterpreting ppm scales across different pollutants.
Final Answer: 5