Which of the following is NOT a method for protecting a potable water system?

Fertilizer application is not a method for protecting a potable water system. Protecting drinking water from contamination involves using physical barriers, such as an air gap, or mechanical devices like a reduced pressure zone assembly. An air gap creates a physical separation between the potable water supply and any potential contaminants, which helps prevent backflow. A reduced pressure zone assembly is a device designed to maintain a lower pressure in the potable water system than in the irrigation or non-potable water system, effectively preventing cross-contamination.

Flushing the system is a standard procedure for maintaining water quality, particularly to remove stagnant water and contaminants, but it does not serve as a protection method in the same way that an air gap or a reduced pressure zone assembly does. In contrast, fertilizer application does not contribute to the protection of potable water; rather, it can pose a risk of introducing harmful contaminants into the water supply if not managed properly. This distinguishes it clearly from the other options, which are explicitly designed to safeguard drinking water systems from potential hazards.