Long Island Sound has been making a comeback. Slow and hard-won progress has begun to reverse years of degradation. The recovery is fragile, and there still is a long way to go, but the incredible sight on Christmas Day of a pod of more than 100 dolphins frolicking in the Sound between Port Jefferson and Bridgeport, Connecticut, is evidence that things are getting better.

But now the federal Army Corps of Engineers threatens to make progress more difficult. It has finalized a plan to allow the continued dumping of 53 million cubic yards of dredge spoils from Connecticut into the Sound over the next 30 years. This comes after protests against the preliminary plan last summer by two New York State agencies and a galaxy of state and federal elected officials and environmentalists. It’s a bad plan and it must be stopped.

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Yes, Connecticut’s harbors and rivers must be dredged regularly for marine traffic. But some of that muck, which has been dumped at four sites in the Sound, is toxic. The rest consumes oxygen, blocks sunlight and kills organisms on the Sound’s floor. When dumping permits were set to expire 10 years ago, then-Gov. George Pataki blocked a similar plan and New York, Connecticut, the Army Corps and the Environmental Protection Agency agreed the Corps should devise a plan with alternatives to dumping. Alternatives exist: Toxic materials can be sent to hazardous-waste landfills, and other dredged material can be used to restore wetlands and cap landfills, as happened in Jamaica Bay and Staten Island. The Corps says those options are too expensive. But so is the price tag of degrading the body of water that is an economic generator for the region and one of its principal playgrounds.

New York’s Department of State, which governs the coast, says this proposal does not meet the goal to reduce or eliminate dumping, and it continues to negotiate with the Corps. If talks fail to produce a solution, the department must stop the plan — on its own or by asking the state attorney general to sue the Army Corps. Whatever is takes.