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Which one of the following statements is true for the trends of Dissolved Oxygen (DO) and Biological Oxygen Demand (BOD) in a water stream receiving pollutants from a point source?
In the septic zone, both DO and BOD levels remain stationary.
In the recovery zone, both DO and BOD levels increase rapidly.
In a decomposition zone, DO level drops rapidly, whereas BOD level remains more or less stable.
In the septic zone, DO level decreases and BOD level increases whereas in the recovery zone DO increases and BOD decreases.
Answers (1)
The statement that is true for the trends of Dissolved Oxygen (DO) and Biological Oxygen Demand (BOD) in a water stream receiving pollutants from a point source is:
"In a decomposition zone, DO level drops rapidly, whereas BOD level remains more or less stable."
When pollutants from a point source enter a water stream, they can contribute to an increase in BOD. BOD refers to the amount of oxygen required by microorganisms to decompose organic matter in the water. As pollutants enter the water, they provide a food source for microorganisms, leading to increased decomposition and microbial activity. This increased activity leads to higher BOD levels.
On the other hand, the introduction of pollutants and the subsequent microbial decomposition can deplete the dissolved oxygen levels in the water. Microorganisms utilize oxygen during the breakdown of organic matter, leading to a drop in DO concentration.
In the decomposition zone downstream of a point source, where the concentration of pollutants is high and microbial activity is increased, the DO level drops rapidly as oxygen is consumed by microorganisms during organic matter decomposition. At the same time, the BOD level remains more or less stable or may even increase due to the continued influx of pollutants and the ongoing microbial decomposition.
Therefore, in a water stream receiving pollutants from a point source, the statement that the DO level drops rapidly in the decomposition zone while the BOD level remains more or less stable is accurate. Hence, the correct answer is option 3.
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