PRESQUE ISLE – Construction crews have unearthed a bit of local history while dredging up Mantle Lake to eradicate an algae problem.

The novelty wore thin when officials learned that the discovery nearly doubled the anticipated cost of the cleanup.

While working to remove an estimated 9,000 yards of phosphorus-rich soil from Mantle Lake, contractors found in the lake bed the remains of an old clay and wood dam built in 1887.

George Howe, cleanup project coordinator, said Tuesday that the dam acted as a sediment pool for more than a century, creating a vein of sediment about 10 feet deep. That blew the $63,000 project cost, based on sampling points that were about 3 to 5 feet deep, “right out of the water.”

Project officials believe the sediment removal could be more than double what they had anticipated. They immediately put together a funding plan, which was approved by the City Council on Monday night, to reach “substantial completion” of the project.

The Mantle Lake Cleanup Project began two years ago to remove lake algae which had an unpleasant smell, was depriving aquatic life of oxygen, and was preventing children from fishing, one of the lake’s proclaimed uses. Howe said kids couldn’t get their hooks through the algae and into the water.

Project organizers started with a stream survey and determined that phosphorus-rich soil, which was deposited into the lake by decades of erosion from nearby farms, was providing the ideal environment for algae to grow. The only way to get rid of the algae, Howe said, was to get rid of the sediment and implement erosion control.

That meant a need for project funding. Partners – including the city, the Presque Isle Rotary Club, civil engineering firm Wright Pierce, the state Department of Environmental Protection, and the Maine Outdoor Heritage Fund – worked together, raising about $70,700 for the sediment removal, though it cost only $63,000.

The lake was drained last fall, and dredging began a few weeks ago.

With the discovery of the old dam and more sediment, officials came up with a project funding plan that proposed the use of the remaining $7,000 first.

They also proposed that the city’s public works department assist with trucking and equipment to allow more dredging to be done. Also, the public works department and the solid waste department agreed to buy $2,700 and $2,000 of the soil, respectively, from their 2006 budget lines. Officials also proposed using $12,500 from the recreation and parks special revenue funds and reallocating $5,000 from the recreation and parks reserve account. All of this would provide an additional $29,400 in funding and allow for the removal of about 4,200 additional yards of soil.

After hearing the proposal and commenting that public support for the project has been tremendous, councilors approved the funding plan.

Howe said that if current estimates are correct, contractors and the city will get 90 percent of the project done.

“In our opinion, if we can get substantial completion, that should take care of the algae problem for many, many years,” Howe said.