AUGUSTA — The state agreed Wednesday to restore 21 acres on and around Sears Island, settling lingering complaints over a derailed development project that has cost taxpayers $15 million.

The agreement was part of a consent decree with the U.S. Environmental Protection Agency and two activist groups that protested the 1985 filling of 10 acres of wetlands for a proposed shipping terminal in Penobscot Bay.

The restoration is expected to cost Maine about $800,000.

“A private developer who disturbed these wetlands without a permit would face penalties under federal law,” said Dan Sosland, an attorney with the Conservation Law Foundation. “We are pleased that we played a central role in holding state government to the same standard.”

The state also agreed to acquire and preserve properties along the nearby Ducktrap River to restore and protect Atlantic salmon habitat.

State officials said the money would come from funds designated for the idled project.

Gov. Angus King suspended the cargo-port project in February, abruptly halting nearly two decades of development efforts. At the time, King said he was convinced the EPA’s scrutiny “was rigged.”

The original proposal to build a pier on filled land in a corner of the 940-acre island was put forth in 1978 and approved by regulators 10 years later. But a Sierra Club lawsuit forced officials to reassess the environmental impact.

Voters approved two bond issues for the project, and about $15 million was spent on preliminary work.

Suspending development plans in February, King said the state remained committed to developing a third port between Eastport and Portland.

Current options include a state purchase of Sears Island for future development or constructing a new port in the vicinity of Mack Point.

Maine Transportation Commissioner John Melrose called the settlement announced Wednesday “a fair and reasonable agreement, given the state’s desire to secure Sears Island for future marine transportation uses.”

The decree calls for restoration of 3.2 acres of wetlands at the terminal site, along with three-quarters of an acre on another part of the island.

About 17 acres of degraded pasture land in North Newcastle along Dyer Creek, a tributary of the Sheepscot River, will also be restored.

Federal officials said three contractors for the Maine Transportation Department who worked on the port project — T.Y. Lin International, Bridgecorp and Robert Wardwell & Sons — will pay a $10,000 cash penalty.

“This settlement will result in significant environmental benefits, and underscores this administration’s commitment to preserving the environment for all,” said Lois Schiffer, an assistant U.S. attorney general.

Melrose said the decree would not prevent the development of a new terminal at Sears Island. EPA officials said the state could renew planning for a port, but that a new federal permit would be required.

Melrose, acknowledging the state’s sharp differences with federal regulators in the past, said the consent decree was part of an attempt “to take the conflict out of our relationship.”