Which one of the following gases is least likely to collect in a below-ground vault?

Carbon monoxide is least likely to collect in a below-ground vault due to its properties. It is a colorless, odorless gas that is the result of incomplete combustion of carbon-containing fuels, and it can disperse quickly into the atmosphere. Unlike gases such as hydrogen sulfide or methane, which have specific accumulation behaviors in confined spaces, carbon monoxide does not have a tendency to settle or concentrate in one area. Instead, it tends to dissipate rapidly when ventilation is present, making it less likely to reach dangerous levels in a below-ground environment.

In contrast, hydrogen sulfide is heavier than air and can pool in low-lying areas, while methane, being lighter than air, can still accumulate in enclosed spaces but poses a different risk due to its flammability. Oxygen can also be present; however, its concentration does not lead to the same type of hazardous buildup as the others. Understanding the behavior of these gases in different environments is crucial for safety in wastewater operations.