What tool is essential for properly labeling Non-Potable Water systems?

The use of a purple valve box is essential for properly labeling Non-Potable Water systems because purple is the recognized color for indicating non-potable water across many regulations and standards, including those related to water conservation and irrigation practices. By standardizing the color of the valve box to purple, it helps ensure that the public and maintenance personnel can easily identify systems that carry non-potable water, thus reducing the risk of cross-contamination with potable water supplies. This color-coding practice is part of broader efforts to promote safety in water use and management. Other options, although they might be colors used for different types of water or services, do not carry the same recognition or regulatory importance in the context of non-potable water systems.