Which condition could result in water contamination through cross connection?

The condition that could result in water contamination through a cross connection is turning off service to a multi-story building and allowing the water supply to drain. When water service is turned off, particularly in a multi-story structure, it creates a vacuum or negative pressure in the plumbing system. This situation can allow contaminated water from lower elevations or external sources to be drawn back into the clean water supply, especially if there are any cross connections present within the plumbing.

Cross connections are points in the plumbing system where potable (drinking) water can come into contact with a source of potential contamination. If the pressure drops significantly, it could lead to back siphonage, where the contaminated water is siphoned into the potable water lines, leading to contamination.

While keeping the service active or conducting regular maintenance on plumbing fixtures typically does not present a risk for contamination when these practices are done properly, the act of intentionally draining the system introduces a scenario where cross connections could pose a significant risk for water quality.