North Carolina on Tuesday ordered Duke Energy Corp to plug a leak of contaminated wastewater from a decommissioned power plant, which authorities in the state said might be leaking into a river that supplies drinking water.

The arsenic-laced discharge from a 36-inch stormwater pipe was the second this month from beneath a coal ash dump at the Eden plant.

In early February, thousands of tons of sludge spilled into the Dan River after a 48-inch pipe broke under the 27-acre ash pond, Duke said.

The company - which is mired in a long-running legal battle with the state over the storage of coal ash waste - said on Tuesday it would use a temporary system to cap the second discharge until it developed a permanent scheme.

Pipe water samples indicated elevated levels of arsenic, though the duration and volume of the discharge was not known, the Charlotte, N.C.-based firm said.

"Downstream public water supplies remain safe and the river meets all water quality standards," it said, adding it was continuously sampling and monitoring water in the Dan River.

The additional discharge was identified during an investigation by the state, the North Carolina Department of Environment and Natural Resources said on Tuesday.

"This second stormwater pipe ... may also be leaking water contaminated with coal ash pollutants into the Dan River," said Division of Water Resources director Tom Reeder.

The state regulatory agency was to hold a press conference at 12:30 p.m. (1730 GMT) on Wednesday.

U.S. 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 was built in the 1940s and retired in 2012. The ash pond stores the waste the plant produced.

(Editing by John Stonestreet)