The most reliable method to prevent flow from a contaminated source into a potable water system is by means of what?

The most reliable method to prevent flow from a contaminated source into a potable water system is through the implementation of an air gap. An air gap serves as a physical barrier that separates the potable water supply from any potential contaminants. It is created by maintaining a vertical distance between the outlet of a water supply (such as a faucet) and the flood level of the fixture or receptor that could potentially be contaminated. This vertical separation ensures that even if backflow were to occur, the contaminated water cannot physically enter the clean water system, thereby providing a fail-safe against pollution or contamination.

In situations where there is a risk of backflow, such as with heavy rain or water main breaks, the use of an air gap remains one of the simplest yet most effective methods to protect against contamination. While backflow preventers, double check valves, and pressure-reducing valves all serve important roles in preventing backflow and regulating pressure, they may not provide the same level of assurance as an air gap, which offers a direct physical safeguard against contamination from all types of sources.