When bringing community water service to a home with a private well, what is the most positive method of preventing a cross connection between the two systems?

• A. Residential dual check valve
• B. Reduced pressure zone backflow preventer
• C. Complete isolation between the two systems using an air gap
• D. Air gap device

Answer: (C)

The strongest way to prevent a cross-connection between a private well and a community water service is to completely separate the two systems with a true air gap. By creating a physical separation between the well and the public supply, there is no hydraulic path for water to flow from one system into the other, even if pressure changes occur. An air gap is inherently fail-safe because it relies on a lack of connection rather than on mechanical parts that can wear out or fail.

Backflow-prevention devices, while effective when properly installed and maintained, depend on correct installation, ongoing testing, and reliable operation. They can fail or be bypassed, which keeps a potential cross-connection risk present. Therefore, the most positive protection is unconditional isolation using an air gap.