Name a situation that would require a backflow preventer to be installed underground.

A backflow preventer is essential in applications where there is a potential for contaminants to enter the potable water supply. In the case of a lawn irrigation system connected to the main water supply, there is a significant risk of backflow, especially during events such as sudden drops in water pressure or when the system is exposed to chemicals, fertilizers, or other contaminants that may be present in irrigation water.

Installing a backflow preventer underground is necessary because it protects the drinking water system from any potential backflow that could occur from the irrigation system. This placement helps maintain the integrity of the system, ensuring that the backflow preventer is not impacted by external factors such as freezing weather, ground erosion, or accidental damage that may occur if it were placed above ground.

In other contexts, like water fountains or swimming pools, the risk of contamination does exist, but the specific requirements and installation practices for backflow prevention might differ. Furthermore, the lack of a basement does not inherently necessitate an underground backflow preventer, as the need primarily revolves around the connected systems' potential cross-connection hazards.