By Marti Maguire

RALEIGH N.C. (Reuters) - North Carolina' state Senate on Wednesday unanimously approved a bill calling for the closure of Duke Energy's 33 coal ash ponds in the state within 15 years, legislation spurred by a massive spill at a retired power plant earlier this year.

Members of the chamber touted the measure as the nation’s toughest law regulating coal ash, a byproduct of coal-based power production that contains toxic materials such as arsenic and lead.

“This is a change that’s going to have North Carolina leading the country when it comes to getting rid of coal ash,” said Republican state Senator Tom Apodaca, who sponsored the bill.

Environmentalists raised doubts, however, that the provisions are strong enough to put a dent in the nation's coal ash problem.

The clean-up plan uses a tiered approach to dictate when ponds at 14 sites statewide must be closed. At the most dangerous sites, including four named in the bill, coal ash would have to be moved to lined landfills or reused as building material by 2019.

At sites deemed less dangerous, the cleanup requirements are less stringent and less urgent. Critics of the bill took issue with a provision that would allow the ash to be capped and left in place at sites deemed low risk.

“It is really leaving the polluting coal ash in unlined pits next to waterways,” said Frank Holleman, an attorney with the Southern Environmental Law Center.

California-based nonprofit Earthjustice, which tracks coal ash sites and is pushing for federal oversight, says more than 200 of the nation's 1,400 coal ash sites have contaminated nearby waterways.

At least 30,000 tons of coal ash were released into the Dan River when a pipe broke under the 27-acre (11-hectare) ash pond at a retired plant owned by Duke Energy in Eden, North Carolina, in a spill discovered on Feb. 2.

The Senate bill mandates water quality monitoring around all the state's coal ash pond sites and exploration of methods to reuse coal ash. The measure did not require Duke to pay for clean-up costs rather than passing them on to customers.

A Duke Energy spokesman called the timeline set by the Senate “incredibly aggressive,” cutting in half the years the company has proposed for clean-up.

Governor Pat McCrory, a Republican and former Duke executive, has voiced support for a House bill similar to the version passed by the Senate. The two chambers must agree on a final version before seeking McCrory’s signature.

(Editing by Colleen Jenkins; Editing by Steve Orlofsky)