"We did experience a moderate malfunction that did not impact New York City's water supply in any way," DEP spokesman Chris Gillbride said. "We detected higher-than-normal fluoride levels in a contained chamber. It did not enter New York City's water supply system."
Opening the hydrants, Gillbride explained, maintained water pressure in city aqueducts. "If the pressure in an aqueduct is high, nothing can enter it," he said.
The DEP asked fire houses to keep water flowing from two hydrants in areas that would not be disruptive or interrupt traffic, a fire department spokesman said. There are 218 fire houses citywide, according to the FDNY website.