Grand Lake toxins caused by blue-green algae bloom, province concludes

After eliminating other possible contaminants, the provincial government has concluded that a blue-green algae bloom is the cause of toxins in the water at Grand Lake, near Halifax.

Elizabeth Kennedy, the provincial director of the water branch, says final test results came back Wednesday that eliminated dozens of petroleum products, as well as pesticides, as the source.

Testing has confirmed the presence of blue-green algae consistent with levels dangerous to dogs, according to media release from the province. Pesticides and other contaminants were below detectable levels, the release said.

Kennedy said there's no way to treat the bloom.

"Once that's in the lake, we can't ever guarantee that it's safe from those risks," she said. "People would have to consider what water they're going to be drinking."

The toxins were first detected a week ago after two dogs died and a person was sent to hospital.

Kennedy says it's likely the lake will see more blooms, especially following hot weather.

"Across Nova Scotia, we're seeing blue–green algae …  more frequently," she said. "With climate change, we anticipate that this is going to be a growing concern."

The province is now organizing a virtual public meeting to inform people in the area about the next steps.

Julie Towers, deputy minister for the provincial Department of Environment and Climate Change, said no one should be using lake water as their water source. She said it isn't known how many homes will be affected.

She said her department estimates there are between 200 and 250 homes without registered wells, but that doesn't necessarily mean the homeowners depend on above-ground water.

"That's one of the reasons why we've been trying gradually to encourage people to go to a non–surface water supply. Use a municipal drinking water supply. It's extensively monitored, regularly tested."

The province will not ban recreational use on Grand Lake, but anyone who wants to swim or boat there will need to be cautious.

"They should be very careful about their contact with the water," said Towers. "Think of it as a risk. Everyone's going to have to make their choices about what level of risk they're comfortable with."

All tests for nearby Fish Lake came back negative, but Towers said there's a possibility the bloom could shift, or a new one could develop.

Grand Lake, meanwhile, will not be tested again. Kennedy said samples don't give an accurate picture of the conditions in the entire lake.

She uses the example of the last week, where numerous samples from the lake came back negative for blue–green algae.

"That just demonstrates that it's really about the concentration and the location that you're taking that sample from that can give you a result."

She said algae blooms can also appear in as little as 24 hours, so the conditions could change shortly after the water is tested.

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