Backsiphonage and Back Pressure
There is a submerged inlet in the second floor bathtub. An automobile breaks a fire hydrant nearby causing a rush of water and negative pressure in the service line to the house, sucking in dirty water out of the bathtub. Individuals who may be using water at that moment are susceptible to the contaminated tub water. The hot and cold-water inlets to the bathtub should have an air gap and be above the rim of the tub to prevent the backsiphonage of water into the water distribution system.
The supply line to the pressurized boiler at the fertilizer plant is leaking. The boilers pressure exceeds that of the distribution system, which by force allows chemicals in the boiler to backflow into the water main supplying the fast food restaurant. The supply line to the fertilizer plant should have a backflow prevention assembly in place to isolate the boiler and prevent the backpressure of water from entering the distribution system.