There was a public meeting Wednesday night regarding the closing of Stevens Pond last summer due to bacteria.
The problem, experts said, had to do with bacteria enjoying too much food. Blue-green algae thrives on phosphorus, and high levels of phosphorus in the pond allowed the algae to grow and bacteria levels to skyrocket.
Last summer was unusually dry and warm, which also may have added to the problem.
"It's really the result of the right conditions coming together," said Keith Gazaille, senior biologist with Aquatic Control Technology.
So what can the town do about it?
Gazaille and fellow ACT biologist Erika Haug presented several options for dealing with the algae. The company surveyed the vegetation of the area and collected water samples last fall.
ACT recommends low-dose aluminum treatment to reduce phosphorus and shift the phosphorus/nitrogen ratio; monitoring algae density; copper algaecide treatment; and continued water and watershed assessment. In the long-term, they recommend looking into watershed management -- structuring slopes and banks to carry more healthy nutrients to pond.
The price tag?
"For next year's entire management program, you're talking under $25,000," Gazaille said.
That money would likely come from the Division of Public Works budget.
Alternative methods include emitting ultrasonic frequencies to disrupt algae and adding barley straw that breaks down and releases substances that kill algae. But the ultrasonic route is expensive and there's not much data about its effectiveness, and barley straw makes more sense for ponds that are smaller than Stevens.
Another possible treatment is aeration -- pumping air through the water to circulate water temperatures and reduce nutrient build-up. But the water isn't deep enough for that to have much impact, and it would likely cost hundreds of thousands of dollars.
"The hope is that, with a reactionary plan in place, even if it does get closed it's not going to be closed for the whole summer," Gazaille said.
Why not flush the pond with water from Lake Cochichewick?
DPW Director Bruce Thibodeau doesn't think flushing the pond is a good idea. Flushing water out of the lake sufficient to clean Stevens Pond could bring lake water levels to a dangerously low level, and that's where the town gets its drinking water.
Blake Martin of Weston & Sampson Engineers said the same, presenting a chart of how various levels of water in the lake correlate with levels of water in the pond and how that has been impacted at different times.
"What we saw is that the volume of water required to flush during the dry summer months is so great that it would drop you into some severe, severe conditions," Martin said. "So in essence, using the lake to flush Stevens Pond isn't really a viable option."
Thibodeau said as long as he's been with the DPW the town has never flushed the pond using the lake.
Stevens Pond stores between 45 million and 60 million gallons a day and covers about 25 acres.
Selectmen will meet to discuss the proposals with Town Manager Andrew Maylor. If approved, the project will have to go through permitting.