RALEIGH, N.C. -- Duke Energy told North Carolina lawmakers Tuesday that removing all of the company's coal ash away from the state's rivers and lakes would take decades and cost up to $10 billion, with its electricity customers likely footing nearly all the bill.

In a presentation to a state legislative committee, Duke's North Carolina president Paul Newton suggested the company needs flexibility to consider more cost-efficient options. The company's proposal is to remove the coal ash from unlined dumps at four of its power plants, but then leave much of what is stored at 10 other sites in place after covering it with plastic and soil.

Environmental groups are calling for new legislation requiring Duke to move all of its coal ash to lined landfills away from waterways following the massive Feb. 2 spill from a collapsed pipe in Eden that coated 70 miles of the Dan River in gray sludge.

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State officials say all of Duke's 33 unlined dumps at 14 coal-fired power plants scattered across the state are oozing out contaminants into groundwater. All told, Duke has more than 100 million tons of the ash, which contains potentially harmful chemicals including arsenic, lead, mercury and chromium.

Newton made no mention of that ongoing pollution as part of his presentation, which included photos of pine trees, waterfowl and deer living at some of the company's older ash dumps closed in the 1970s. He also emphasized recent testing that shows the high levels of toxic chemicals present in the Dan River immediately after the spill have dropped back to within state safety limits.

"We are doing, and will continue to do, what it takes to make this right," said Newton, who repeated his earlier public apologies for the spill.

Earlier this year, Brian Williams, a conservationist with the Dan River Basin Association, took a CBS News crew 20 miles downstream of the coal ash spill at a retired plant in Eden, N.C.

"It was just giant, gray sludge pouring into the river," Williams said. "It's down here on the bottom and it's mixing with the sediment and it's constantly leeching out the toxins that are in the coal ash."

Duke, the nation's largest electricity company, has spent more than $15 million to plug the pipe that collapsed at its Eden plant. It has also begun dredging out large deposits of ash found on the river bottom at three locations, the furthest more than 20 miles downstream in Danville, Va.

The company has said it will pay for the spill cleanup, but may ask state regulators to raise the rates it charges customers for any additional costs incurred as a result of new regulations or requirements at its other sites.

To help illustrate the potential challenge and expense of moving all the company's ash, Newton showed a graphic of what it would take to clear out the 22 million pounds stored at the Marshall Steam Station on Lake Norman, near Charlotte. If workers hauled away a dump truck full every three minutes for 12 hours a day, six days a week, Newton said it would take 30 years to remove all the ash from just that one facility.

Newton also disputed a recent finding by North Carolina environmental officials that Duke broke the law when it pumped out 61 million gallons of contaminated water from one of its dumps into the Cape Fear River, describing a large crack found in an earthen dam there as a "superficial gap" that the company quickly repaired. Newton contended that the pumping was allowed under the company's state-issued wastewater discharge permit.

Molly Diggins, the director the N.C. Sierra Club, was among those who urged lawmakers to pass a new law with firm requirements and deadlines that would make Duke deal with its coal ash pollution in a timely manner.

"Despite being a Fortune 500 company, with profits of $2.7 billion last year, Duke Energy has successfully been allowed to manage its wet coal ash waste as if the clock had stopped half a century ago," Diggins said. "Coal ash is a can that has been kicked down the road for far too long. The Dan River spill was a terrible disaster, but it's opened all our eyes to the reality that we need to deal with our state's coal ash problem now. "