Backflow is the undesirable reversal of the flow of water or mixtures of water and other substances back into the potable water supply. This can occur when there is a sudden drop in water pressure in the supply line or when there is an increase in pressure downstream of the supply line, causing the water to flow in the opposite direction. Backflow can contaminate the clean water supply with substances from sources such as irrigation systems, fire sprinkler systems, or even chemicals stored near plumbing fixtures.