The term cross-connection refers to a condition where the public water system is exposed to contamination through a service connection to a private plumbing system or other source. Such contamination could occur if a private pump were to force water back into the public water system. This example is called back-pressure.
Another condition, called back-siphonage, may occur when a water main breaks, or even under fire-fighting demands. In these situations, normal water pressure in the mains may turn to suction that can draw water from a private system into the public system. In this event, even a simple garden hose that may be filling a swimming pool or in a backyard pond can become a source of potential contamination.