Backsiphonage is caused by negative pressure from a vacuum (or partial vacuum) in the supply piping, just as drinking through a straw draws liquid from a glass. Backsiphonage can be created when there is stoppage in the water supply due to repairs or breaks in the County's water main; an increased demand at one location, such as fire fighting; or even undersized piping. Backsiphonage reverses normal flow in the system, and can pull contaminants into the drinking water.

Backpressure occurs when the downstream water pressure is greater than the water supply pressure. This can occur in any pressurized system such as chemical feed pumps, boilers, elevated tanks, or recirculating systems. For example, water in a boiler operating under a pressure of 15–20 psi would backflow into the potable water if its supply pressure fell below this level. Sometimes this pressure drop can be created just by flushing a toilet!