Middle Pond advisory lifted after tests show toxic algae bloom dissipated

SOUTHWICK — Despite a new algae bloom discovered on Monday on the Congamond Lakes Middle Pond, the town’s Board of Health director lifted the advisory Thursday afternoon for people to avoid using the waters of after tests showed the levels of a toxic algae bloom were now safe for humans and pets.

“Based on the [Department of Public Health’s] recommendation, it looks like the test results were so low, even though there was the more recent bloom that just popped up the other day, they still thought it would be acceptable to lift,” Health Director Tom Hibert said Thursday.

On Friday, Sept. 26, a photo of an algae bloom was posted by a member of the Facebook group Congamond Neighbors that set off a chain of events that led to an advisory being issued by Southwick’s Board of Health for people and pets to avoid the Middle Pond of Congamond Lakes.

After the water was tested by Lake Management Committee member Eric Mueller that day and found that the bright green slick of slime had levels of bacteria considered too high for exposure to humans and pets, the Board of Health issued the advisory.

The advisory was that people not use the pond for swimming, operating personal watercraft or boats or eating fish caught in it.

There was another bloom reported on the North Pond on Tuesday, Sept. 30, but it couldn’t be found after a search by Hibert, a member of the Conservation Commission, and the Police Department.

On Monday, Oct. 6, Hibert said there was another report of a bloom on the Suffield side of the Middle Pond.

When he went to check it out, the bloom had already started dissipating and by Tuesday the telltale green algae specks were completely gone.

“The water was crystal clear,” Hibert said while pointing to a photo he took of the pond’s shallow bottom along the shore that was free of algae.

However, he did alert the DPH, which had the water tested and found the low bacteria levels, which prompted the lifting of the advisory Thursday.

“The result came back yesterday and they were good,” Hibert said.

Hibert said Mueller believed the bacteria that caused the bloom on Monday and the one on Sept. 26 were kicked off by the unusually warm weather for this time of year in shallow water.

Mueller, he said, believes the bacteria hangs out on the pond’s bottom and when in shallow water, the sun’s heat activates it and it rises to the surface and begins multiplying.

As for the reason it dissipates so quickly after being detected, Hibert said Mueller believes the aluminum sulfate treatment that was used to treat the ponds in 2020 changes the water’s chemistry just enough to keep the bacteria from thriving.

The $800,000 cost for alum treatment was approved by Town Meeting in 2020 after the Community Preservation Committee requested the allocation.

With the cooler temperatures, Hibert said he thinks it unlikely another bloom will appear anytime soon.

As a precaution, Hibert said he would be surveying the pond on Thursday just to make sure no other blooms had appeared.

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