A Nuns' Island spa is being given two weeks to correct a plumbing error that has been spewing raw sewage into a once-idyllic basin bordering a residential neighbourhood.

When the Strom Spa was built on the island – a suburb in Montreal's Verdun borough – in 2009, its wastewater piping was connected to the municipal rainwater sewer instead of the sewer that channels domestic waste to a treatment facility.

Since then, fetid wastewater from the spa has been gathering in a retention basin meant for overflow rainwater, and a thick green algae now coats the surface.

Residents who live near the Ruisseau aux Hérons basin have been complaining to Verdun’s borough council for the past few years about the stench.

Pierre L’Heureux, a longtime Nuns' Island resident who represents Verdun on Montreal city council, told CBC News that the wastewater and natural biological processes in the basin are producing a nasty smelling brew.

“Combined, they create an unbearable stench,” he said.

The problem was traced to the spa’s piping three weeks ago, and borough authorities have now given the spa's owners until mid-September to fix the problem.

In a news release issued Tuesday, Verdun Mayor Jean-François Parenteau pledged $1.5 million in borough funding for restoring the basin to a healthier state.

The spa’s owner Guillaume Lemoine said he will take immediate action to correct the piping issue.

He said the borough only brought the issue to his attention three weeks ago.

Lemoine said the spa will be closed until the problem is fixed.