HALIFAX -- People living in Halifax were startled awake at 1:15 Thursday morning by the loud wailing tone of a provincial emergency alert coming from their cell phone.

"I thought there was a gunman on the loose at first, at 1:15 in the morning," says area resident Bryne McDonell.

But, it turns out the alert was warning of the problem with the water at Grand Lake, a large lake within the municipality.

"Halifax Regional Municipality is issuing a drinking water alert for Grand Lake, Halifax, Nova Scotia," it read. "All residents who have water feeds from Grand Lake are to cease using the water immediately."

Receiving the early morning alert meant McDonnell knew to keep his three-year-old Golden Doodle, Chelsea, out of the water on their daily walk in Oakfield Provincial Park, which borders the large body of water.

He's glad the alert was issued.

"It's worth it to keep everybody in the know, and safe," he says.

But for many residents, the alert led to some early morning questions, about whether their drinking water supply was safe.

"There was confusion this morning because of the text of the alert that went out," says District 15 Coun. Paul Russell.

"People saw that alert was coming in, and they weren't sure where their water was coming from, they knew it was related to Grand Lake," he adds.

He's glad the alert system was used, but the initial confusion kept him busy addressing constituents' concerns.

Officials with the city of Halifax and Halifax Water issued notices later Thursday morning, assuring residents their municipal water supply is not affected.

The decision to use the provincial emergency alert system was triggered after Halifax Regional Fire responded to a 911 call in the Oakfield area.

"Last night at approximately eight o'clock, Halifax Regional Fire and Emergency responded to call to investigate an unknown substance that had washed up on the shore of Grand Lake," says the department's Deputy Chief of operations, Peter Andrews.

Andrews says the call came from a resident whose dogs died after coming into contact with the lake. That resident was also taken to hospital and has since been released.

After Halifax Fire crews arrived on the scene and began assessing the situation, the department contacted municipal emergency officials. They, in turn, contacted Nova Scotia EMO to request the Alert Ready message be sent.

"Given the severity of this particular hazard," says Andrews, "and the darkness coming on, we thought that the provincial system would give us the ability to reach more folks."

The city does have its own alert system – called hfxAlert – but Andrews says officials decided to use the provincial system because the province is the lead agency on water quality and environmental concerns.

In hindsight, Andrews says the wording of the alert could have been clearer.

"To clarify that it was really about if you were drawing and drinking water from the lake, and not actually impacting the domestic water supply."

District 1 Coun. Cathy Deagle-Gammon agrees sending an alert was the right move.

"It was a safety first," she says. "I absolutely respect the request of HRM staff to the province to use the emergency alert, I think it was the best decision at that moment."

Nova Scotia's Emergency Management Office says it's the first time the Alert Ready system has been used for a water quality issue.