The U.S. Environmental Protection Agency is proposing to add Windham’s Keddy Mill to a national list of hazardous sites considered the highest priority for cleanup.

The EPA made a formal proposal Wednesday to add the contaminated seven-acre site on Depot Street to the National Priorities List for its Superfund program, which investigates and cleans up sites that pose a high risk to public health and the environment.

The Keddy Mill site contains an estimated 55,000 tons of material contaminated with polychlorinated biphenyls, or PCBs, from oils used in turning machines and contained in metal filings that were buried on the site.

PCBs have been shown to cause cancer and have other adverse health effects in animals and potentially in humans, according to the EPA.

The mill, which closed about 40 years ago, used to make steel flanges and, before that, paper.

Condominiums had been planned for the site, but the environmental concerns halted the project.

Windham Economic Development Director Tom Bartell said the EPA’s proposal to add the Keddy Mill to the National Priorities List will trigger a public comment period, following which the agency will decide whether to put it on the list.

Bartell said he expects that decision to be made in April.

Once a site is added to the list, he said, the EPA begins an extensive environmental review and, at the same time, a process to determine the parties responsible for the contamination, in the hopes of forcing them to pay for the cleanup.

Plans for the cleanup are put in place in order to be ready when money becomes available, either through the legal process or from the federal government, Bartell said.

The cleanup of the Keddy Mill would cost millions of dollars. If the site ends up in the Superfund program, Bartell said, it still could be 10 years before the cleanup occurs.

“In some ways, we’re not happy it ended up this way, but it is the best way and the only way this property will eventually be cleaned up and turned into a useful property again,” he said.

Also on Wednesday, the EPA announced that it added the former Collins & Aikman Plant in Farmington, N.H., to the National Priorities List.

Ground water contamination on the 123-acre site of an old rubber company is threatening nearby drinking water wells and wetlands and has already shut down one public water supply, the EPA said.

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