What happens to BOD levels in polluted water?

The correct answer highlights the relationship between biochemical oxygen demand (BOD) and the level of pollutants in water. When a water body is polluted, the presence of organic matter increases, leading to higher BOD levels. BOD measures the amount of oxygen that microorganisms will consume while decomposing organic matter in water. As these microorganisms break down the pollutants, they utilize dissolved oxygen in the water, which causes the BOD levels to rise. This increase signifies a greater demand for oxygen, which can be detrimental to aquatic life because it depletes the oxygen available in the water. Therefore, higher levels of pollution correspond to increased BOD, marking a critical aspect of water quality assessment and monitoring.

The other options do not accurately describe the effects of pollution on BOD levels. For example, nutrient influx tends to promote algal blooms, which can lead to an initial increase in BOD as the decomposing algae consume oxygen, thus making the second choice incorrect. Stating that BOD levels stabilize with better treatment would depend on the effectiveness of the treatment process, and stating they remain unchanged disregards the definite impact pollution has on oxygen demand in a water body.