Backflow prevention devices are now required on all commercial and irrigation connections. Once installed, these devices must be tested annually. The Backflow Prevention Education Council of Colorado provides a listing of these testers by region.
Cross connections occur when the public drinking water supply is physically connected to contaminated sources (e.g., a hose submerged in a bucket or lying on the ground). Cross connections provide a pathway for the backflow of polluted or contaminated water into the drinking water system.
Backflow can also occur when the pressure in the public distribution system drops. Normally this pressure is high enough to prevent backflow; but certain events--such as main breaks, flushing, or firefighting--can lower the pressure enough to allow the water to flow backwards.