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

Anaerobic conditions occur when there is a significant lack of oxygen in a water system, which is often exacerbated by the presence of high biological oxygen demand (BOD) contaminants. When BOD is high, it indicates that a large amount of organic matter is present in the water. As microorganisms decompose this organic matter, they consume dissolved oxygen in the process. If the demand for oxygen exceeds the availability of dissolved oxygen, anaerobic conditions will develop as the oxygen is depleted.

In this scenario, low levels of dissolved oxygen coupled with high BOD contaminants create an environment where the natural process of decomposition leads to a rapid decline in available oxygen. This can result in anaerobic processes becoming predominant, as aerobic microorganisms are unable to thrive without sufficient oxygen. Therefore, the scenario accurately characterizes the conditions leading to anaerobiosis in a natural water system.

Having low BOD contaminants would not exert a significant oxygen demand, and high levels of dissolved oxygen would indicate robust aerobic processes, which are not conducive to the establishment of anaerobic conditions.