Four environmental groups asked a North Carolina court Tuesday for a ruling aimed at forcing Duke Energy to clean up groundwater contamination around ash ponds at 14 coal-fired power plants.

The suit filed in Wake County Superior Court challenges the state Environmental Management Commission. The commission voted in December against environmental advocates’ request to reinterpret a state rule regulating groundwater cleanups.

The contamination detected at Duke’s plants – including at Riverbend and Allen west of Charlotte – was within a roughly 500-foot radius of the ash ponds. State regulators have interpreted the rules to say contamination within that so-called compliance boundary doesn’t violate state standards if the ponds were built before 1984.

That leaves Duke with “no obligation to stop the contamination or clean up the contamination,” said attorney D.J. Gerken of the Southern Environmental Law Center. The center represents Cape Fear River Watch, the Sierra Club, the Waterkeeper Alliance and the Western North Carolina Alliance.

The law center asked the environment commission last month to read the rules differently, forcing Duke to clean the groundwater.

The commission, instead, stood by its old interpretation. The groups asked the Wake County court Tuesday to overturn the decision.

State officials say they are still investigating the sources of the contamination at Duke’s plants. Coal ash contains heavy metals that can be toxic in high concentrations.

Duke said much of the contamination is of harmless iron and manganese, which occur naturally. There’s no evidence that contamination has flowed off Duke’s property, the company said.

The environment commission’s interpretation of the groundwater rule has been clear and consistent, Duke spokeswoman Erin Culbert said. “These organizations are really advocating for a rule change,” she said.

The law center counters that some of the contamination at the plants is unlikely to have occurred naturally.

At some power plants, it says, elements including thallium, selenium and arsenic have been detected downhill of ash ponds – the direction in which groundwater would flow – but not uphill of them.

The Charlotte-based Catawba Riverkeeper Foundation settled a lawsuit last August over coal ash storage at S.C. Electric & Gas’ Wateree power plant southeast of Columbia. SCE&G agreed to remove all coal ash from ponds at the plant and recycle or place it in lined landfills.

Duke Energy is converting to dry-ash handling at its plants, Culbert said, minimizing risks of water contamination. The ponds that were the focus of Tuesday’s court filing mix ash with water and hold the slurry in open lagoons.