Anaerobic conditions in a natural water system are most likely to occur in which scenario?

Anaerobic conditions in a natural water system occur when there is a lack of dissolved oxygen, which is essential for aerobic organisms that require oxygen for respiration. In the scenario where there are low levels of dissolved oxygen combined with high biological oxygen demand (BOD) contaminants, the situation becomes conducive for anaerobic processes.

High BOD levels indicate the presence of a significant amount of organic material that bacteria can decompose. This decomposition process consumes oxygen. When the BOD is high in conjunction with low levels of dissolved oxygen, it means that the demand for oxygen by decomposer bacteria exceeds the available supply of dissolved oxygen. Consequently, the bacteria switch from aerobic respiration to anaerobic respiration as they decompose the organic matter. This shift can lead to the production of methane, hydrogen sulfide, and other byproducts typical of anaerobic environments.

In contrast, scenarios with high levels of dissolved oxygen would support aerobic decomposition and prevent the establishment of anaerobic conditions, regardless of BOD levels. Therefore, the combination of low dissolved oxygen and high BOD contaminants directly facilitates the development of anaerobic conditions in natural water systems.