NORTH ANDOVER — Residents who enjoy swimming at Stevens Pond hope that a dose of aluminum sulfate will ward off the blue/green algae that kept them out of the water last summer.
Public Works Director Bruce Thibodeau said he expects the chemical, commonly known simply as alum, will be sprayed into the pond by the end of May. Before the treatment can be administered, the Department of Public Works must have permission from the Conservation Commission and the state Department of Environmental Protection.
Thibodeau, as well as two biologists, appeared before the Conservation Commission last Wednesday night to seek approval for the treatment. The commission is expected to issue an order of conditions for the application at its next meeting May 8.
Keith Gazaille, a biologist with Aquatic Control Technology Inc., a Sutton firm that advises the town on water quality questions, explained that the alum will bind with phosphorus in the pond water. The resulting combination of alum and phosphorus will then sink to the bottom of the pond.
A high presence of phosphorus in the water, Gazaille said, will attract the blue/green algae, technically known as cyanobacteria, that made Stevens Pond unsafe for swimmers last summer.
Exposure to blue/green algae causes humans to break out with a rash. It can be fatal to dogs, who like to play in the water and drink it.
Gazaille said the alum will be added in the form of a liquid spray. Thibodeau said the quantity of the chemical will not cause any adverse effects on swimmers or the pond’s ecosystem.
Gazaille, who analyzed the pond along with his colleague Erika Haug, said the alum treatment was effective in driving blue/green algae from a pond in Pembroke on the South Shore.
Asked if he expects the treatment to be effective, Thibodeau said, “We’re hoping. You never know with Mother Nature.” If aluminum sulfate does not rout the cyanobacteria, the town’s next option is to try copper sulfate, a more powerful chemical, he said.
The high quantity of blue/green algae blooms forced the Board of Health to close Stevens Pond, the town’s most popular outdoor swimming place, at the beginning of last July. The town reimbursed the many residents who had bought season passes for the pond.