A "cross-connection" as defined by the environmental protection agency, is "any actual or potential connection between the public water supply and a source of contamination or pollution".

Legally, the "purveyor" of the water supply is responsible for the cleanliness and potability of the water supply, and for implementing and maintaining a cross-connection control program in order to prevent the contamination of the public water supply. The water purveyor is the public water department, up to and including the service connection from the public water main. From the outlet of the water meter or service connection including all piping downstream, to the furthest extent of the piping system inside the owners premises, the purveyor of the water supply is actually the property owner.

Typically a program which is designed to protect the public water system from contamination through the use of a "containment" device, or backflow preventer. This device is installed at the consumers service connection, usually immediately downstream of the water meter, but upstream of any branches of the owners piping system. This device separates all of the owners’ water supply piping from the public water main, thereby theoretically containing any contamination which may occur on site, and protecting the public water supply.

A program designed to isolate any cross-connections which exist within the owners’ premises, through the use of adequate backflow prevention devices, thereby isolating any hazard to that particular location, and protecting the remainder of the owner’s potable water system from contamination.

A backsiphonage condition can occur whenever there is a lowered pressure between the potable and non-potable supply piping. Such conditions typically occur during periods of high demand in the public water main, lowering the supply pressure. For instance during the demands imposed by fire fighting operations, or in the event of a water main break, which suddenly and significantly lowers the city water pressure below that of the non-potable system. This results in a partial vacuum being drawn on the non-potable system, and siphons the pollutants or contaminants into the potable water system through an unprotected cross connection, such as a hose bibb or hydronic system make-up connection. Backsiphonage may also occur when a high velocity stream of water passes by a small pipe outlet, such as a residential service tap, due to the "venturi" effect.

A backpressure condition occurs whenever an elevated pressure exists between the potable and non-potable source. These pressures can be imposed by the installation of pumps which increase pressures above the city water supply pressure, thereby forcing non-potable water in the opposite direction of normal flow into the potable water line. The installation of boilers or other equipment which heat water, causing thermal expansion and resulting inpressures in excess of the incoming water pressure, also can force non-potable water into the potable piping system.

Pollutants

A "pollutant" is any substance which may affect the color, taste or odor of the potable water, but which does not pose a direct threat to human health through exposure or consumption of the water. Pollutants may impose an objectionable odor or appearance to the water, but do not in themselves pose a health threat, and therefore, are considered to be a lesser hazard, when compared to contaminants.

A "contaminant" is any substance which, when introdued into the potable water system, constitutes a direct threat to life or health of a human, if the substance was ingested through consumption, or if the substance came in contact with the skin. A contaminant can therefore be a caustic chemical, a fluid containing bacteria or disease, or any other substance which could threaten human health. Therefore, contaminants compose the highest degree of hazard to the potable water system.