Which of the following scenarios requires the installation of a backflow prevention device?

The necessity for a backflow prevention device is particularly relevant in scenarios where there is a risk of non-potable water mixing with the potable water supply, posing a health hazard. In the case of water used for irrigation, the potential for contaminants—such as fertilizers, pesticides, and other chemicals in the soil—entering the public water system through the irrigation system is significant.

When irrigation systems are directly connected to the potable water supply without a backflow prevention device, any drop in water pressure can create a backflow situation, allowing contaminated water to siphon back into the drinking water lines. Therefore, installing a backflow prevention device is crucial in these irrigation systems to ensure the safety and integrity of the public water supply.

Other scenarios, like a standard residential water supply and garden hose connected to a faucet, typically don't pose the same level of risk for backflow contamination compared to irrigation systems. While connections to fire hydrants can also require specific backflow prevention measures, especially for non-emergency use, the focus in this instance is on irrigation systems as they are more directly associated with environmental contaminants.