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A city barge accidentally dumped hundreds of gallons of sewer sludge into the East River near Astoria on Saturday after a hatch on the transport vessel mysteriously popped open.

The barge was emptying a load of biological waste into a treatment plant on Ward’s Island when the boat shifted to one side, causing a portal leading to a containment tank to fly open, according to Ian Michaels, a spokesman for the Department of Environmental Protection.

Kimberly Mullarkey, an Astoria resident, said she witnessed a black, oily liquid cascading over the side of a barge for roughly four hours before police and fire department responded to stop the flow.

DEP says that about 500 gallons were released, but witness accounts suggest it could have been more.

“It was overflowing constantly, you couldn’t even see the side of the boat,” Mullarkey said. “It was completely overflowing, like a bucket that gets filled to the rim and just spills over.”

The hatch was closed and the pumping was completed, Michaels said. The state Department of Environmental Conservation and the U.S. Coast Guard then were notified about the spill.

This spill of the sludge was the largest that Michaels could recall, but he said it was likely there had been other spills in the past. The DEP was still investigating what caused the hatch to open and release the sludge.

“Obviously, that’s not supposed to happen,” he said.

The sludge does not pose any significant risk to public health or the environment, especially due to the amount that was discharged, Michaels said. Although it might be a bad idea to swim in a pool of the substance, he said, it was not especially toxic.

“Sludge: it sounds like this terrible thing, but it’s really just leftover biological residue,” Michaels said. “It’s not like fuel oil that gets into a beach and stays there forever. It disappears into the environment very quickly.”

James Cervino, a marine biologist at Pace and Columbia universities, also said 500 gallons of sludge would not cause any serious damage to the environment.

But he was concerned that someone who was fishing near the spill could handle or consume an animal covered in the sludge, which could put them at risk for gastronomical infection.

People often fish off the shore near Astoria Park, directly across the river from the spill, residents said. Mullarkey said the park was full of people on Saturday, but she could not remember seeing anyone fishing.

Sludge is a biological byproduct left behind after bacteria break down organic matter in sewage water at the city’s water treatment plants. A fleet of tankers hauls millions of gallons of sludge between treatment plants each week to be dewatered and processed.