fire hydrants which feature additional structure for reducing the potential that those with ill intent can foul municipal water supplies by introducing toxins or other materials into fire hydrants . various embodiments include valves which close off portions of the hydrant otherwise available for receipt of toxic or other materials when the fire hydrant nozzle cap is unscrewed and open . such valves preferably open , manually or automatically , when water is flowing from the water supply through the nozzle , a condition which renders it difficult at best to introduce toxins through the hydrant into a water supply , but close off portions of the barrel when the nozzle is open and water is not flowing through the nozzle .