All wells shall be protected by an RPZ except when used for __________________.

An RPZ, or Reduced Pressure Zone backflow preventer, is a critical safety device used to protect the water supply from contamination due to backflow. It is designed to prevent any potential pollutants or dangerous substances from entering the potable water system in the event of a drop in water pressure.

The correct answer pertains to the specific use scenario of wells in residential settings. When wells are used for single-family domestic settings, the potential for hazardous cross-connections is significantly lower than in industrial or commercial applications where a variety of hazardous materials may be present. In such cases, the requirement for an RPZ may be relaxed as long as additional precautions are taken to ensure the well is maintained properly and remains free from contaminants.

Other uses of wells, such as agricultural or industrial, often involve equipment and practices that may introduce contaminants, thereby necessitating a stricter requirement for backflow prevention through the use of RPZs. This ensures that any potential risk to the public water supply is minimized effectively.