A cross connection is any physical arrangement whereby a public water supply is connected, directly or indirectly, with any secondary water supply system, sewer, drain, conduit, pool, storage reservoir, plumbing fixture, or other device which contains or may contain any water, contaminated liquid, or other waste of unknown or unsafe quality that could impart a contaminantor pollutant to the drinking water as a result of backflow caused by back pressure or backsiphonage.Backflow is generally defined as the flow of any foreign liquid, gas, or other substance into the distributing pipelines of a potable supply of water from any source or sources other than the intended one. For backflow to occur, two conditions must be present:
1.A link must exist between the potable and the nonpotable system.
2.The resultant flow produced by the differential pressure must be toward the potable system. If both systems are at pressures greater than atmospheric (positive pressure), backflow due to back pressure can occur. A pump, elevated tank, or boiler can create a back pressure that is greater than the pressure in the potable system. If the potable system is at a pressure less than atmospheric (negative pressure), the atmospheric pressure on the foreign liquid will force it toward the partial vacuum and backsiphonage occurs. A more explicit term for backflow when sub-atmospheric pressure exists, backsiphonage is the backing up, or siphoning, of a foreign liquid into a potable water system.