WASHINGTON (AP) – One of the nation’s largest power generators has agreed to end a years-long federal lawsuit by paying $4.6 billion to reduce pollution that has eaten away at Northeast mountain ranges and national landmarks, The Associated Press has learned.

The settlement requires American Electric Power, based in Columbus, Ohio, to reduce chemical emissions that cause acid rain by at least 69 percent over the next decade.

It also fines AEP an additional $15 million in civil penalties and another $60 million in cleanup and mitigation costs to help heal parkland and waterways that have been hurt by the pollution.

Details of the agreement were provided by two people familiar with its terms who spoke on condition of anonymity Monday because it had not yet been filed in federal court. Spokesmen for AEP declined comment Monday.

The Justice Department and the Environmental Protection Agency also declined comment.

The settlement marks one of the largest government fines in an environmental case. By contrast, Exxon Mobil Corp. estimates it has paid $3.5 billion in cleanup costs, government settlements, fines and compensation for the 1989 Exxon Valdez oil spill. The company is fighting an additional $2.5 billion in punitive fines.

Eight states, a dozen environmental groups and the EPA brought the lawsuit against AEP in 1999, accusing the energy company of rebuilding coal-fired power plants without installing pollution controls as required under the Clean Air Act.

Environmentalists blame acid rain caused by coal-fired power plants for plaguing the Northeast over the last quarter-century, including damage that has eaten away at the Statue of Liberty and the Adirondacks mountain range in upstate New York. Smog and acid rain have been linked to sulfates and nitrates that are products of coal-fired plants.

AEP has more than 5 million customers in 11 states. It has agreed to clean up 46 coal-fired operations in 16 of the plants in its eastern system – a group likely to include at least nine plants in Ohio, Indiana, Virginia and West Virginia.

The settlement will be filed on the day a six-week trial in federal court in Columbus, Ohio, was expected to begin in front of U.S. District Judge Edmund Sargus. AEP has maintained that the work done in at least some of its plants was routine maintenance that didn’t fall under federal requirements for pollution controls.

The settlement requires AEP to:

-Spend $4.6 billion on so-called scrubbers and other pollution controls to reduce emissions of nitrogen oxide and sulfur dioxide, which cause acid rain and smog.

-Cut nitrogen oxide emissions by 69 percent by 2016, and reduce sulfur dioxide emissions by 79 percent by 2018.

-Pay civil fines of $15 million.

-Pay $60 million in mitigation measures. The money includes $21 million to reduce emissions from barges and trucks in the Ohio River Valley; $24 million for projects to conserve energy and produce alternative energy; and $3 million for the Chesapeake Bay, $2 million for Shenandoah National Park and $10 million to acquire ecologically sensitive lands in Appalachia.

Environmentalists said they still plan to keep an eye on how quickly AEP starts cleaning up its plants – or whether it drags its feet in complying with the settlement.

The eight states that brought the lawsuit, along with the EPA and environmentalists, are Connecticut, Maryland, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island and Vermont.