NASHVILLE, Tenn. — A federal utility’s top executive estimated Thursday that it would cost $900 million and take 24 years to comply with a court order to move coal ash from unlined and leaking pits and ponds to a lined landfill on the site of a Tennessee power plant.

Tennessee Valley Authority CEO Bill Johnson said in an interview Thursday that a less-preferred option, moving the ash offsite from the Gallatin Fossil Plant, would cost $2 billion. TVA had mentioned the $2 billion figure during a federal trial in February over pollution claims at the plant about 40 miles from Nashville.

The Southern Environmental Law Center has said the estimates are far too long and high.

Johnson said the timeline might be reduced, but not dramatically.

“There’s a lot of material, and it just takes time to do this,” Johnson said. “We have one benefit in our estimating, which is, we’ve done a lot of this kind of work.”

In August, a federal judge ordered the ash excavated and removed, saying it’s leaking pollutants into the Cumberland River in violation of the Clean Water Act. But the judge said there was scant evidence of any harm caused by the pollution.

TVA is appealing the order, hoping it can instead cap the unlined ash ponds over 12 years at a $200 million cost. The TVA contends capping the coal ash would be environmentally friendly, since it wouldn’t risk a spill while crews excavate and move the polluted material.

Environmentalists and the district judge say capping risks further pollution, since the trial already determined that the current pits and ponds are leaking.

A related case brought by Tennessee environmental officials against TVA over the Gallatin facility’s pollution is ongoing in federal court.

TVA powers 9 million customers in parts of seven Southern states. This week, TVA reported a net income of $685 million for the budget year that ended Sept. 30, down from last year’s $1.2 billion, largely due to an additional $500 million spent shoring up its retirement system.