What is an oxygen sag curve? What changes in a river cause this effect?
The Correct Answer and Explanation is:
An oxygen sag curve is a graph that depicts the fluctuation in dissolved oxygen levels in a river or stream as it moves downstream from a pollution source. This curve typically has a characteristic “sag” shape, where the oxygen concentration drops sharply near the point of contamination and then recovers downstream.
This phenomenon occurs when a large amount of organic matter, such as sewage or industrial waste, is introduced into the river. The bacteria and microorganisms in the water begin to break down the organic material through a process called biological oxygen demand (BOD). As these organisms decompose the organic waste, they consume oxygen in the process, causing a significant decrease in dissolved oxygen levels. This creates the initial dip in the oxygen sag curve.
Several factors contribute to changes in a river that lead to the oxygen sag effect:
- Pollution Input: The introduction of organic pollutants, like sewage, agricultural runoff, or industrial effluents, increases the biological oxygen demand (BOD) of the water. The higher the BOD, the greater the oxygen consumption by microorganisms during decomposition.
- Water Temperature: Warmer water holds less oxygen than cooler water. In the summer, for example, elevated temperatures can exacerbate the oxygen sag, as the decomposition rate increases and the water’s oxygen-holding capacity decreases.
- Flow Rate: Low flow rates or stagnant conditions mean that the water has less movement, which can result in insufficient mixing and slower oxygen replenishment. Higher flow rates can help dilute pollutants and spread oxygen more evenly, mitigating the sag.
- Self-purification: As the river moves downstream, oxygen concentrations typically begin to recover as natural processes like aeration, mixing, and photosynthesis by aquatic plants restore oxygen levels. This results in the upward recovery part of the oxygen sag curve.
In summary, the oxygen sag curve is a critical indicator of the health of aquatic ecosystems, showing how pollution affects oxygen levels and the ability of a river to recover naturally.
