The city of Varennes is investigating after a serious chemical leak at one of its pools Tuesday that sent 20 young children and two adults to hospital.

Most of the children were released Tuesday night but two children spent the night in the hospital before being released in good condition Wednesday.

The wading pool where it happened, which is annexed to Le Carrefour high school in Varennes,has been drained, while it and the bigger pool next to it are closed indefinitely.

Varennes Mayor Martin Damphousse said the fumes took everyone by surprise

“We heard that the smelling was not good and the colour of the water changed very fast so that's why all the children went out,” he said.

The water colour changed to brown and green.

The source may have been a cleaning product mixed with chlorine used by a maintenance worker

“Yesterday at 2:30 the change of the filter was made and the problem started there,” explained Damphousse, who, along with firefighters and employees with pool management company Recregestion visited the pool Wednesday to determine the cause of the leak.

“Is it possible it was human error, mechanical error? Are those all the different things being looked at? All those answers are good. We don't know the right one. We'll wait for the investigation,” said the mayor.

Recregestion hired subcontractor Sonic-Vortisand to perform the maintenance on the pool.

Both companies are cooperating with the investigation.

The wading pool and larger pool have separate filtration systems, but city manager Sebastien Roy said both pools are closed as a precaution.

“We don’t want to take any risks,” he said. “The problem that occurred in the small pool could occur on the large pool.”

Swimmer Nicole Campeau said she was hoping to do some laps, but now she hopes the pool will be safe again.

“I came for my workout and I hope that it will be regulated, and that it will be okay,” she said.

The pool will remain closed at least for a few days or until officials determine what occurred and make steps to ensure it doesn't happen again.