MONMOUTH — The water in Cochnewagon Lake is crystal clear — and it should stay that way for years to come, thanks to a recent alum treatment.

“It was getting so green, it looked like you could walk on it in spots,” said Joe Saunders, who is a Cochnewagon Lake resident and a member of the Cobbossee Watershed District board of directors representing Monmouth.

Each spring and early summer, the lake water is clear, and it remains so until August, when rapidly it becomes murky. With less oxygen, the visible depth of the water shrinks from 25 feet to 7 feet in a matter of weeks.  

It’s because of a dying algae bloom.

“If you came out in this lake in August or September (last year), you’d see the whole lake with a greenish tint,” said Wendy Dennis, limnologist for the Cobbossee Watershed District and the project manager for the alum treatment on Cochnewagon Lake.

“The (dying) algae sinks to the bottom, decomposes, and uses up oxygen as it decomposes,” she explained. “Then you have less oxygen in the lake.


“It’s a cycle you want to break,” she said.

A boat crosses Cochnewagon Lake in Monmouth, where an alum treatment aims to keep the lake clear from the harms of algae blooms. Photo courtesy of Ryan Burton, Cobbossee Watershed District

So why does algae concentrate here? Phosphorus — and plenty of it — a food source of the algae.

Phosphorus is in Cochnewagon Lake both because it washes into the water and it seeps out of lake bottom sediments into the water.

The alum treatment will prevent the algae from blooming this densely, or at all. “Alum” is a nickname for a chemical called aluminum-sulfate. The aluminum binds with the phosphorus, keeping it bound in the lake bottom sediment.

The watershed district contracted SOLitude Lake Management, based in Shrewsbury, Massachusetts, to treat 225 acres of Cochnewagon Lake.

Two chemicals were discharged into the lake — around 70,000 gallons of aluminum sulfate and 35,000 gallons of sodium aluminate.


Employees of Solitude Lake Management refill tanks with alum June 3 on Cochnewagon Lake in Monmouth. Kennebec Journal photo by Andy Molloy

“The two products balance the water’s pH level so there are no changes in water quality in the lake,” said Dominic Meringolo, Senior Environmental Engineer for SOLitude Lake Management.

“If they just put in the aluminum sulfate, the pH would get too low, it would get too acidic,” said Dennis. “The sodium aluminate offsets that.”

Daily, the chemicals are shipped in on tank trucks, which carry up to 5,000 gallons per load, and drained into holding tanks at the Monmouth Beach area. They’re then drawn onto a barge to be discharged into the water.

The chemicals are stored separately until being discharged into the water, at which time, Dennis said, “they form a precipitate, small solid particles that drop down through the water and settle on the bottom as a whitish floc that sits on top of the sediments.”

For 10 days, the barge discharged the chemicals into the lake for nine or 10 hours each day, refilling about 10 times.

“The barge has a boom on the back that’s lowered 8 feet into the water, and we drive back and forth,” Meringolo said.


Two or three people operate the barge, he said, which holds 1,200 gallons of product, or 6 tons of weight. One person drives and navigates the barge while the other one or two run the pumps and monitors the flow rates.

Following the barge in a monitoring boat is a team from the Cobbossee Watershed District using an underwater camera to make sure the flow settles evenly.

This team also starts and ends the day studying the fish and aquatic life.

“We do a patchwork pattern so we’re not treating a large area at one time,” Meringolo said, “and it allows the fish to swim away if they want to.”

A boat dispenses alum on Cochnewagon Lake in Monmouth to keep the lake clear from the harms of algae blooms. Photo courtesy of Ryan Burton, Cobbossee Watershed District

The lake is divided into nine sections that are at least 20 feet deep. “We bounce around so we’re not treating adjacent areas.”

During the treatments, there were no signs of stress to aquatic life, and the water was safe for humans to be in and fish from.


“Chemical settles quickly,” said Dennis. “There wouldn’t be time for it to be ingested and do harm.”

“The element in the chemical, aluminum, is naturally found in the environment,” she said.

“It’s one of the most abundant elements in the Earth’s crust,” Meringolo added.

This isn’t the first treatment the lake has had. The first was in 1986, and the district also conducted a treatment before that in 1978 on nearby Annabessacook Lake. Monmouth officials authorized the use of the aluminium treatment in 2016 to ward off the threat of algae. 

Since then, the underwater camera replaced a dive team to monitor the aquatic life, and the GPS navigation system replaced grids of buoys anchored by cemented milk jugs.

After the 1986 treatment on Cochnewagon Lake, the water remained clear for 20 years.


“It cleaned it up fantastically well before, so we’re expecting the same thing,” Saunders said.

Dennis and Saunders expect this treatment to last longer.

This time, a higher dose of aluminum is getting injected into the water, Dennis said, and CWD has worked around the lake’s watershed to prevent runoff into the lake.

“Vegetation is key to getting water to infiltrate into the ground,” Dennis said.

The watershed district has worked closely with Monmouth Public Works and owners of private roads to divert water into settling basins or into vegetated ground, she said.

The Cochnewagon Lake watershed encompasses 2,026 acres and includes around 100 shorefront homes.


Individuals can help this effort by keeping vegetation on their properties or directing drainage into vegetated areas where runoff can be absorbed into the ground.

They also can avoid using pesticides or other products that contain phosphorus.

“CWD arranged for the purchase of a sweeper truck that is shared between Monmouth and Winthrop to collect salts and other garbage in it to keep it from running into the lakes,” Saunders said.

“It all eventually comes to the lake.”

The cost of this treatment is $365,000, Dennis said, which covers the chemicals, contractor, equipment, CWD staff time to monitor the lake and conduct laboratory analysis.

The project is funded partially by a $175,000 grant from the U.S. Environmental Protection Agency under Section 319 of the federal Clean Water Act. The Maine Department of Environmental Protection administered the grant.


Monmouth matched that grant amount. In 2016, voters overwhelmingly approved raising $175,000 over three years.

“They’ve been overwhelmingly supportive,” she said.

The alum treatment was years in the making.

“The bloom reappeared in 2006,” Dennis said. “CWD monitored it to make sure it wasn’t an annual variation.”

Planning for the treatment began. That planning meant surveying the watershed, implementing management practices in the lake’s watershed to reduce pollution, designing the treatment, grant writing, acquiring permits from DEP and soliciting proposals for public notification.

Now CWD will monitor the lake intensely.


CWD was created by the Legislature in 1971 to protect and improve water quality. The watershed district has three year-round employees and one to two seasonal employees in the summer.

Eight towns are members of the district, with Monmouth and Winthrop having the largest amount of shorefront. Manchester, Readfield, Litchfield, Gardiner, Richmond and Wayne are also members.

All water in the Cobbossee Watershed eventually drains into Cobbosseecontee Stream, which empties into the Kennebec River in Gardiner.

“(The alum treatment) is beneficial for lakes downstream,” Dennis said. “Annabassacook will receive better quality of water from Cochnewagon, which feeds into it.”


Related Headlines

Comments are no longer available on this story

filed under: