The Riverbend Steam Station west of Charlotte will close in April after devouring coal for 84 years, and the red-brick power plant and its tall smokestacks will come down.

Riverbend was among the first of the large-scale power plants that electrified, and transformed, the Piedmont. But part of its environmental legacy – two large basins that for decades have filled with slurries of water and coal ash – won’t be easily erased.

As Riverbend and other Duke Energy coal-fired plants near retirement, Duke and state regulators will have to decide what to do with millions of tons of leftover ash and the tainted groundwater under it.

After a century-plus history in the Carolinas, much of it based on coal, it will be the first time Duke has permanently closed an ash basin. How that happens is of particular concern at Riverbend because 69 acres of basins overlook Mountain Island Lake, Charlotte’s main water supply.

Tests of groundwater have found contamination near ash basins at all 14 of Duke’s North Carolina coal-fired power plants. Most is apparently naturally occurring iron and manganese.

Potential fingerprints of ash, such as thallium and selenium, also have been found at some plants. Some ash constituents can be toxic in high doses. Thallium, a metal, can hurt the liver and kidneys; the mineral selenium may cause cancer.

State regulators use the groundwater data to spot trends that would guide further water sampling or computer analyses. The findings will also help determine how to close the basins.

“The trick is in trying to decipher whether that impact is associated with … ash or background sources” not affected by the basins, said Jay Zimmerman, who heads the N.C. Division of Water Quality’s aquifer protection section. “I’m not aware of any site where we’ve seen an (ash-associated) element that shows up time and again.”

At Riverbend, iron and manganese in concentrations above state standards have been found in the groundwater. Boron, a naturally occurring compound, has been detected at concentrations within state standards at four wells and selenium, also within standards, at one well.

Not found in the groundwater was arsenic, said Andrew Pitner, the state’s regional aquifer-protection supervisor in Mooresville. The cancer-causing metal has been detected in Mountain Island Lake near a discharge point for water from Duke’s ash basins.

Duke says its basins have not hurt private wells near its plants.

“We don’t have any indications that anybody’s water is being impacted by groundwater contaminated by ash ponds,” said Dave Mitchell, director of air and waste programs in Duke’s environmental unit.

Riverbend closing years early

Duke, which now includes Progress Energy Carolinas, will close 15 of its 23 active ash basins in the Carolinas as it retires old power plants. Duke said Friday that Riverbend and the vintage-1926 Buck plant in Rowan County will retire in April, two years earlier than expected.

The methods used to close them, state regulators say, will depend on their individual characteristics and contamination risks.

Duke sees two basic approaches.

In the first, water would be pumped out of the basins, leaving dry, compacted ash. The dike impounding the basin would be removed and the ash capped to keep rainwater from soaking through it to groundwater.

Monitoring of groundwater would continue for decades, Mitchell said, until state regulators say it’s no longer required.

The second, more expensive way: Muck out the ash and haul it to a landfill. But there would be so much ash, Mitchell said, that no municipal landfill could take it. The company might have to look for a private partner to dispose of the ash.

Riverbend’s two basins alone hold 2.7 million tons of ash.

The cost of ash disposal is a major concern across the electric industry.

The Environmental Protection Agency is mulling federal standards for coal ash in the aftermath of a disastrous 2008 spill in Tennessee. A decision to regulate ash as hazardous waste could cost utilities $43 billion over 10 years to close their ash ponds, according to the Edison Electric Institute, a utilities trade association.

Duke expects capping Riverbend’s basins to be as effective as removing the ash. The metals in the ash would bind with soil particles, Mitchell said, the groundwater cleansing it over decades.

Zimmerman, the state official, said the ash would ideally be stored in a basin with a cap above it and a liner under it to keep groundwater out of it.

“Excavation would be an option, but a very expensive option,” he said.

“If they leave it in place, they would have to provide for continual monitoring … A cap has been discussed before, but given the size of some of these ponds, it would have to be of quite a large size.”

The state’s final determination, Zimmerman said, will be based on the groundwater data and the likelihood stored ash could lead to contamination later. It will be especially conservative with basins near drinking-water supplies, such as Riverbend, he said.

Riverkeeper favors removal

Catawba Riverkeeper Rick Gaskins, whose staff has investigated Riverbend’s impact on Mountain Island Lake, would rather the ash and any contaminated soil under it be hauled away.

“If you leave it in place, you’d be monitoring (groundwater) for years,” Gaskins said. “I don’t see how there would ever be an end to it.”

In a settlement with the Catawba Riverkeeper Foundation last year, S.C. Electric & Gas agreed to remove the ash and soil beneath the pond from its Wateree Steam Station near Columbia by 2020. SCE&G is closing all its ash ponds and replacing them with lined landfills to store dry ash.

The foundation is also part of a legal appeal filed in January that seeks an interpretation of state rules to force Duke to clean up contamination around its ash ponds.

Gaskins’ foundation says contaminants are also seeping from the Riverbend basin dikes into Mountain Island. (Duke says such seepage is normal). Capping the ponds, he said, would control water filtering through the ash but not stop contaminated groundwater from moving.

“They seem to be fighting (ash removal) more than you would expect,” he said. “The costs aren’t that different and they’re a regulated utility so they’ll be reimbursed. You have to wonder why they’re fighting this.”

One possible explanation, he said, is that excavating soil for a landfill could uncover contamination elsewhere on the site. Duke says it doesn’t have room for landfills at many of its power plants.

Once Riverbend stops burning coal, the site will be assessed for contamination from asbestos, PCBs and other toxic substances. The buildings would be torn down two or three years later, their sites covered with soil and planted.

Henderson: 704-358-5051 Twitter: @bhender