RALEIGH, N.C. — Over the last year, environmental groups have tried three times to use the federal Clean Water Act to force Duke Energy to clear out leaky coal ash dumps like the one that ruptured last week, spewing enough toxic sludge into a North Carolina river to fill 73 Olympic-sized pools.

Each time, they say, their efforts have been stymied — by the N.C. Department of Environment and Natural Resources.