By Marti Maguire

RALEIGH, North Carolina (Reuters) - North Carolina state officials defended their oversight of coal ash ponds on Wednesday, a day after a second leak was found to be threatening a river already tainted by toxic sludge from a spill earlier this month.

The state ordered Duke Energy Corp on Tuesday to plug the second leak of arsenic-laced wastewater into the Dan River from its decommissioned Eden power plant, this time through a 36-inch stormwater pipe.

The state's Department of Natural Resources (DNER) said the leak had been 90 percent contained.

The agency has come under fire for its handling of the state's coal ash ponds, which it concedes are contaminating groundwater as well as harming aquatic life. It filed suit against Duke last year, and then proposed a settlement, which is opposed by environmental groups.

DNER Secretary John Skvarla said the proposed settlement, which has been on hold since the spill, would prevent delays in cleanup that may be caused by a prolonged trial.

"This perception has been created that we are adversaries with the citizens' groups, when in fact we are on the same side of the table," Skvarla told a news conference.

"We all have the same outcome in mind ... to clean up the spill, protect the environment and protect the people of North Carolina."

Frank Holleman, senior attorney for the Southern Environmental Law Center, rejected Skvarla's comments. He said DENR's lawsuit was meant to derail litigation planned by his organization on behalf of several citizen groups.

"How can we be on the same side of the table if they don't even let us come into the room?" he asked.

Tuesday's spill came less than three weeks after thousands of tons of sludge spilled into the Dan River when a 48-inch pipe broke under the 27-acre ash pond, Duke said.

Federal prosecutors issued a second set of subpoenas to DNER on Tuesday as part of a widening probe into possible felony violations at North Carolina's 14 coal ash sites.

Prosecutors are investigating whether a crime was committed in the first spill, in which Duke said 30,000 to 39,000 tons of ash were released into the river.

The coal-fired plant in Eden was built in the 1940s and retired in 2012.

(Writing by Ian Simpson; editing by Gunna Dickson)