A shallow waste lagoon at the edge of Lake Robinson sat unnoticed for decades as people swam, fished and water-skied on the placid recreational waterway near Hartsville.

But early this year, news began to dribble out that the 55-acre lagoon had leaked high levels of arsenic into groundwater near the lake. The pond also had been used as a low-level nuclear waste dumping site, area residents learned. And the waste pond contained far more polluted coal ash than ever reported.

Now, with community concerns rising, Duke Energy says it will clean up the mess made at the company’s H.B. Robinson power plant during five decades of producing electricity for eastern South Carolina.

Duke, which has owned the site for about three years, announced Thursday that it will dig out 4 million tons of waste ash from the pond, then truck it to a new landfill to be established nearby. The effort is part of a company commitment to protect groundwater flowing toward the popular lake, said Duke, which is headquartered in Charlotte.

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The company’s commitment means South Carolina is the only Southeastern state where every utility has agreed to dig out ash ponds that threaten groundwater and rivers, according to the Southern Environmental Law Center. The Robinson pond was the last power company lagoon in the state without an ash excavation plan.

“Protecting groundwater, public health and the environment is our guide when it comes to closing ash basins,” according to a Duke Energy news release that quotes John Elnitsky, senior vice president of ash basin strategy. “The science tells us that excavating ash from the site and relocating it to a new fully lined landfill is the most reasonable and prudent way to accomplish that.”

Company spokeswoman Erin Culbert said it’s too early to determine how much the cleanup will cost, but it could easily be “tens of millions” of dollars. She also said it could take five to 10 years to complete because Duke still needs to conduct more extensive studies and develop a design for a new landfill. It expects to apply to build the landfill in mid 2016.

Duke officials said they decided to dig up all the coal ash after determining that the volume of waste material continued to threaten groundwater. The company, which acquired the site from Progress Energy in 2012, notified the S.C. Department of Health and Environmental Control of its plans in a letter Thursday.

Culbert said the company has not decided if it will clean up groundwater already polluted by the ash basin. So far, Lake Robinson has not been affected by arsenic-polluted groundwater that is oozing toward the lake, nor have any private wells been discovered in the path of the pollution, company officials say.

But the threat remains. Recent research in North Carolina revealed that most of the private wells tested near Duke coal ash ponds showed pollution levels above state groundwater standards.

At Robinson, arsenic showed up at levels more than 100 times the safe drinking water standard after a new test well was sunk at the pond last year. Levels that high had never been discovered at the site before. Arsenic, contained in coal ash, is an ancient poison that can be lethal if consumed in high amounts.

Duke’s cleanup plans were the topic of a community meeting hosted Thursday night by the Southern Environmental Law Center and the Coastal Conservation League in Hartsville. The meeting, which drew about 75 people, produced questions about whether private wells were contaminated and how long the groundwater pollution had occurred, the league’s Nancy Cave said.

Among those who planned to attend the session was Hartsville resident Kent Segars. Segars’ family has owned land since Colonial times in an area near the Lake Robinson dam. A forester and self-described conservationist, he and his family depend on a clean environment for the trees he grows on his farm.

Segars, 60, said Duke’s plan to clean out the ash basin will better protect the landscape, the lake and Black Creek, which flows out of Lake Robinson through Hartsville and toward the Great Pee Dee River. The Pee Dee River drains into the Atlantic Ocean at Georgetown.

“It’s seems pretty crazy to me to have this kind of toxic material near the headwaters,” Segars said Thursday. “I’m glad to hear they are going to do the right thing. That’s what they should do.”

Law center attorney Frank Holleman said the Duke decision is big news.

“This is a victory for South Carolinians who have raised concerns about coal ash pollution at Robinson, and it appears Duke has decided to do the right thing,” he said. “It is well past time that this dangerous and polluting coal ash storage be moved to safe, dry, lined storage away from Lake Robinson and Black Creek.”

Like Segars, Lake Robinson residents Neil Montgomery and Jimmy Johnson said they had no idea the waste pond existed until The State newspaper reported on problems with the pond earlier this spring. Documents provided to the newspaper by the law center showed that contamination at the site was far worse than originally thought.

Johnson, 61, runs a campground across the lake from the Robinson power station, which includes an operating nuclear reactor and the closed coal site with the ash pond.

“It’s good news to get that stuff away from here,” Johnson said. “They needed to clean the mess up. But I think for a long time, people were more worried about the nuclear reactor than the coal ash.”

Montgomery, who moved to Darlington County from West Virginia about 25 years ago, said he was not initially concerned about pollution, but that changed as he learned more. Montgomery questioned why South Carolina environmental regulators allowed the contamination to go on for so long.

“At this point, I do have questions about where the inspectors were inspecting,” Montgomery, 75, said. “I’m not sure they were doing their job. I’m not sure if you can say anything about the power company. Did (regulators) actually go on the site and inspect and test or did they just make up a report from their office?”

Holleman said he has found that the S.C. Department of Health and Environmental Control has in the past year become aggressive about trying to resolve the problems at Duke’s coal ash ponds. Regulators in North Carolina have been heavily criticized for failing to regulate Duke ash ponds. The issue grew so intense that a criminal investigation was launched there.

In South Carolina, DHEC cited problems last year in the way Duke managed two aging ponds near the Saluda River between Greenville and Anderson. Ultimately, Duke agreed to dig out the ash basins at its Lee power plant near Williamston and put the ash in a lined landfill. Next week, the company is having an open house to explain to the community its cleanup plans.

Now, the company has agreed to dig up the Robinson pond, which once was thought to be the least of the company’s issues with ash pond cleanups in the Carolinas. Even the law center thought the Robinson site would entail little more than a minor cleanup until attorneys ran across documents last winter pointing to greater problems.

One of the most unusual was the discovery that low-level atomic waste from the Robinson nuclear plant had been dumped in the coal plant’s ash basin in the 1980s, with federal permission. DHEC spokeswoman Cassandra Harris said Thursday that nuclear material has apparently broken down since the disposal. The center also discovered that Duke had said last year that the basin contained 660,000 tons of coal ash. It later discovered some 4 million tons of coal ash were in the basin.

“DHEC does believe this action (by Duke) is a positive step,” Harris said in an email. “Removal of ash from the basin is a key step in cleanup of the groundwater.”