SCE&G will clean up all of the coal ash that has polluted groundwater and leaked arsenic into the Wateree River from its power plant upstream from Congaree National Park in Lower Richland County, according to an accord announced Monday.

Environmentalists agreed to settle their lawsuit with the Midlands utility in exchange for binding agreements to clean out the Wateree power plant’s coal waste ponds in the next eight years.

By the end of 2020, the company will remove all of the ash, about 2.4 million tons. It will start by getting rid of 240,000 tons during the next three years, according to the agreement.

The company has struck several deals with state regulators to address coal ash and arsenic discharges at the Wateree power station. But environmentalists sued earlier this year, saying a 2011 agreement between SCE&G and the Department of Health and Environmental Control was not binding. A 2001 agreement also has been criticized as weak, because it allowed the company to continue polluting groundwater from ash ponds.

Now, with Monday’s deal, the cleanup at the Wateree power station will be binding, said Frank Holleman, an attorney with the Southern Environmental Law Center.

That’s significant because the SCE&G plant is on the banks of an important South Carolina river, the Wateree, and just a few miles upstream from Congaree National Park, said Holleman and Rick Gaskins, the riverkeeper for the Catawba-Wateree basin.

“This is an historic agreement,’’ Holleman said. “SCE&G has bound itself to remove its coal ash from the impoundments near the Wateree River, has accelerated its schedule for removal, and has committed itself to improved handling and storage of coal ash.”

The January lawsuit against SCE&G was brought by the Catawba Riverkeeper Foundation. The law center handled the case for the foundation.

SCE&G officials said the settlement shows how government, industry and environmental groups can work out differences. A company official said SCE&G has been moving to clean up the coal ash, and Monday’s agreement underscores that commitment.

“From our voluntary agreement with DHEC a year ago to our affirmation of that agreement in our settlement with the Catawba Riverkeeper today, SCE&G continues to demonstrate its commitment to efficiently and effectively decommission wet ash storage facilities at all of our coal-fired stations,’’ said Jim Landreth, vice president for SCE&G Fossil and Hydro Operations.

Leaks and spills from coal ash waste ponds have become an issue of increasing concern across the country since a massive December 2008 spill in Tennessee. Today, the federal government is trying to decide whether to regulate coal ash as a hazardous waste.

Coal ash, the waste material from burning coal at power plants, has historically been dumped in man-made ponds, many of which were not lined. The ash contains arsenic and other toxins that, in some cases, leaked into groundwater or spilled into rivers and across the landscape.

SCE&G’s site contains about 2.4 million tons of coal ash in one waste pond, but the banks of the Wateree River are eroding toward the pond, the environmentalists’ suit said. The suit said a break in the waste pond could result in catastrophic pollution to the Wateree River.

“This agreement and SCE&G’s commitments are a major step forward for the protection of the Catawba-Wateree basin,’’ Gaskins said. “Coal ash will be removed from unlined impoundments near the banks of the river and the ash pond will be closed. SCE&G’s binding commitments are in the best interests of the Wateree, the Congaree National Park, and SCE&G.”

Read more in Tuesday’s edition of The State.