Over the next eight years, SCE&G will dig up and remove more than 2 million tons of coal sludge that has contaminated groundwater in lower Richland County and leaked arsenic into the Wateree River near Congaree National Park.

Environmental groups and the utility have struck an accord that requires SCE&G to clean out all of the coal ash now resting in waste ponds adjacent to the company’s 42-year-old Wateree power station, a coal-burning plant with a legacy of air and water pollution.

The agreement, announced Monday, settles a lawsuit by the Catawba Riverkeeper Foundation.

SCE&G has in the past struck several deals with state regulators to address pollution leaking from its ash ponds, but the settlement for the first time includes binding requirements that SCE&G get rid of the 2.4 million tons of coal ash by late 2020. The ash will be removed a year ahead of a nonbinding date agreed upon last year by SCE&G and the S.C. Department of Health and Environmental Control. The settlement also includes an interim date to remove 240,000 tons of coal ash by 2015.

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 with plastic or some other synthetic material.

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 lower-Richland plant is among those that have leaked arsenic — an ancient poison and suspected carcinogen — into both groundwater and the Wateree River upstream from the national park.

Environmentalists beamed about the accord Monday, saying it could influence other power companies to clean up coal ash ponds that threaten the environment.

“This is a model for utilities in the region,” said Frank Holleman, an attorney with the Southern Environmental Law Center, which handled the suit.

Most notable is Santee Cooper, the state-owned utility that at one point had more coal ash ponds than any other power company in South Carolina. The company had about half the approximately 20 coal-ash ponds in the state, as of 2010.

Like SCE&G, Santee Cooper also has been sued by environmental groups seeking to close coal ash lagoons at its Grainger power station in Conway, but it is continuing to fight the legal action. The Grainger lagoons are prone to leak and are in a low-lying area that is perilously close to the Waccamaw River, environmentalists say.

Environmentalists also said they hoped Duke Energy would heed SCE&G’s plan and close four “high-hazard” ash ponds on the Catawba River in North Carolina. The Catawba River drains into Lake Wateree just above the Wateree River near Camden.

“This agreement and SCE&G’s commitments are a major step forward for the protection of the Catawba-Wateree basin,’’ said Rick Gaskins, the riverkeeper for the basin, which covers both Carolinas.

The Catawba River Keeper Foundation filed the lawsuit against SCE&G in January after Holleman’s organization uncovered what it said was a quiet agreement between DHEC and the power company to address the discharges and move toward a cleanup. But the agreement did not force a cleanup and allowed SCE&G to get out of the deal at any time.

Last winter’s lawsuit said the banks of the Wateree River were eroding toward SCE&G’s ponds and that a break in a pond wall could result in catastrophic pollution to the Wateree River. The utility’s coal-fired power plant is about three miles upstream from Congaree National Park. Congaree is the only national park in South Carolina.

Last week’s settlement, however, does not require SCE&G to clean up groundwater already polluted with arsenic at levels many times the safe-drinking water standard in a part of Richland County where people drink from wells. The suit had asked a judge to order SCE&G to clean up the groundwater.

Although the contamination is not known to have escaped the site, lower Richland farmer Heath Hill said he’d like to see more done. Hill’s lawsuit against a landfill at the SCE&G power plant uncovered evidence of arsenic draining into the Wateree River.

“To me, they should be made to clean it up,’’ Hill said of the power company. “They are the ones who put that stuff there. We know who is doing this.”

Robert Yanity, a spokesman for SCE&G, said contamination in the groundwater should dissipate as the source of the pollution – coal ash – is removed. According to the agreement, SCE&G also must excavate and remove at least 2 feet of polluted soil below the bottom of the main ash waste lagoon.

Ash dug up from the waste ponds will be disposed of in the landfill Hill opposes. Ash produced in the future will also go into the landfill or recycled for use in pavement or other materials, the power company says.

Yanity and company officials 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.

The power company, which as recently as two years ago still had five ash ponds, says it is moving to close all of its coal ash lagoons. SCE&G closed the one at its McMeekin station on Lake Murray about 10 years ago and, in 2011, shut down a pond at its Urquhart station near Aiken, Yanity said. The company’s Canadys station near Walterboro also will switch from waste lagoons to a landfill for disposal of coal ash, he said.

SCE&G’s closing of coal ash ponds comes as the agency also is scaling back the use of coal-fired power plants in the wake of concerns about air pollution.

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. That spill sent a wave of polluted coal waste into the Emory River, releasing heavy metals into the waterway.

In addition to toxic effects on people who drink polluted water, past studies have shown that exposure by wildlife to coal ash can cause deformities. Tadpoles found living in old coal ash basins at the Savannah River Site near Aiken had deformed mouths from exposure to selenium, researchers at SRS found in the mid-1990s. Fish also were identified with abnormally curved spines at SRS.

In the past 45 years, leaks and spills from coal lagoons have caused some $2.3 billion in damage to fish and wildlife, according to a recent report by researchers with the U.S. Fish and Wildlife Service and the U.S. Forest Service. Today, the federal government is trying to decide whether to regulate coal ash as a hazardous waste.