Regional officials said a chemical explosion at the Greenbrook water treatment plant in Kitchener posed no risk to residents.
A chlorine tank at 20 Greenbrook Drive exploded when it was filled with ammonia, leaving a strong chemical smell in the air.
"I was inside and I heard a very large boom," said Stephanie Althea, who lives just down the street from the plant. "I thought maybe there was a car crash because we are close to the highway."
Officials said no one was injured in the explosion.
Thomas Schmidt, the region's transportation and environmental services commissioner, said the plant was not active at the time.
There is no risk to the water supply or to people who are outdoors in the vicinity of the plant, he said. He added crews will continue to investigate the incident.
"We don't know exactly how much ammonia was filled into the chlorine tank and we just want to make sure the chemical reaction has stopped and isn't continuing," said Schmidt.