A barge of dredged material—the infamous “black mayonnaise” from the Gowanus Canal—sank right back into the water in a “mishap” that’s now under investigation, the federal Environmental Protection Agency revealed on Tuesday. Now, the agency faces the messy prospect of removing this vessel without spreading its contaminants into a new environment, the Gowanus Bay, where the ship sank.

Cashman Dredging, the contractor hauling out the muck from one of the most polluted bodies of water in the country, has set up equipment to pump the water from the sunken boat into an empty ship during low tide, according to the EPA’s original statement.

On Wednesday, the federal agency added that the barge was filled with 850 tons of toxic gunk on January 22nd and then moored in the Gowanus Bay. On Monday, its officials were alerted that the barge was partially submerged on mudflats.

Their initial review indicates that “the bulk of the sediment in the hopper container within the barge remains in place." The EPA is confident the incident didn't result in a public health hazard, but it is reviewing field operations. Dredging of the canal remains temporarily paused.

The 1.8-mile-long Gowanus Canal was named a Superfund site in 2010. The first phase of its $1.5 billion cleanup began late last year with the removal of sediments filled with heavy metals, sewage, and toxins from waste discharged from gas plants and paper mills in the mid-1800s. Monitoring is in place to track air quality at nearby dredging sites.

The EPA is now investigating the cause of the incident as well as whether contaminated sediment seeped back into the water. The cleanup process could be challenging, too. Normal dredging itself can accidentally spread toxins from the dirty sediments as they’re lifted from the water.

The agency now faces the tricky obstacle of lifting a whole barge covered in contaminated sediments, noted Alexander van Geen, a Lamont Research Professor at Columbia University in the geochemistry division. Cleaning up PCBs from the Hudson River and other bodies of water has occasionally re-released small portions of the chemicals back into the water, according to EPA documents.

If the sunken barge can be removed without shaking up too much toxic sediment, it wouldn’t be devastating for the cleanup. “But this is something that shouldn't have happened of course,” van Geen said.

EPA officials said Cashman was able to pump water out and refloat the vessel on Tuesday. It found a "small hole" that is believed to have caused the incident. The hole was patched up and the barge relocated to a staging site at Smith and Huntington streets.

Councilmember Brad Lander tweeted it was “very distressing” to find out about the sunken barge.

A senior attorney for Riverkeeper, a non-profit environmental organization, agreed.

“We are thankful no one was hurt,” the senior attorney, Michael Dulong, said in a statement to Gothamist. “It's bad enough for toxic contaminants to lay in the bed of the Gowanus underneath a century of sediment.”

The hundreds of tons of toxic debris were newly exposed to the surface of the water, “which could harm the fragile local ecosystem,” Dulong said.

Eric McClure, a member of the Gowanus Canal Community Advisory Group, said it was concerning that the barge wasn’t able to stay afloat. The Brooklyn Paper reported the barge ship sank in an area off the shore of Sunset Park. McClure added, based on an update from the EPA at a community meeting on Tuesday, the shallow water should make excavation more accessible.

“The CAG is hopeful we get a complete picture of how and why this happened quickly and that the situation gets resolved and that certainly something like this doesn’t happen again,” McClure told Gothamist/WNYC.

This article was updated with additional information from the EPA.