Why are below-grade storage tanks included in fire service mains systems?

Below-grade storage tanks are included in fire service mains systems primarily to provide additional water supply for firefighting operations. The inclusion of these tanks ensures that there is readily available, reliable water supply in times of emergency when quick response is crucial. Since they are typically located underground, these tanks are less susceptible to contamination and can maintain water pressure effectively, making them essential for fire protection systems.

In firefighting contexts, having an additional water supply source can mean the difference between effectively extinguishing a fire and experiencing severe damage. This is particularly important in areas where municipal water supply may be insufficient or where fire hydrants may not provide adequate flow rates during critical moments.

The other choices do not align with the primary purpose of below-grade tanks in this specific context. While they may serve other roles in different systems (such as stormwater management or irrigation), their central function within fire service main systems is to enhance the water supply available for firefighting efforts.