What does the lower check valve prevent in a water supply system?

The lower check valve plays a crucial role in maintaining the integrity of a water supply system by preventing backflow. Specifically, it ensures that water from a private supply does not flow back into the public supply system. This is essential because backflow can introduce contaminants into the public water supply, posing health risks to the community. By effectively blocking this reverse flow, the lower check valve helps maintain the safety and quality of drinking water.

In the context of other potential answers, while preventing water loss, managing fire hazards, and controlling pressure variation are important aspects of water supply management, they do not specifically address the core function of the lower check valve. Its primary purpose is to act as a safeguard for public water systems against contamination through backflow, making the option related to backflow the most accurate choice.