The Door County Board voted 15-4 Tuesday in favor of a two-year draw down of the Forestville Millpond that supporters say will improve long-term water quality near the dam and opponents say will damage the Ahnapee River downstream in Kewaunee County.

A 2017-18 study revealed high concentrations of total phosphorus in the pond’s accumulated sediment, along with elevated levels of oil and grease and traces of heavy metals. About 25 percent of upstream water samples exceeded the established stream threshold for total phosphorus concentrations, and two of three samples taken in the pond itself exceeded thresholds.

As long ago as 1996 a one-year draw down of the pond’s water levels was seen as a way to “freeze out” undesirable fish and plant species, create a “hard pan” of dried sediments that would not be re-suspended when the water levels were restored, and improve the aesthetics of the pond, all at a relatively low cost.

The downside of a draw down would be potential odors associated with plant material decay, disruption and/or destruction of aquatic habitats, and potential negative impacts downstream during the draw down and refill, the 1996 study said.

At that time the county chose to do nothing, but water quality issues persisted and the problem was revisited starting in the fall of 2016.

Opponents of the draw down again turned out in force Tuesday to argue against the proposal, but the county board went ahead with the recommendation after about two hours of discussion.

The plan is to draw down the pond’s water levels at about 3 inches per day starting in early November.