When the Keystone Pipeline burst last week, half of an Olympic-sized swimming pool’s worth of a particularly dirty fossil fuel spilled into wetlands in North Dakota. And the thick liquid, known as tar sands oil, will be nearly impossible to clean up.

The Keystone pipeline system, an addition to which has been the subject of environmental protests for years, leaked about 383,000 gallons of crude oil in North Dakota, covering an estimated half-acre of wetland, state environmental regulators said.

Nebraska regulators on Monday allowed the Keystone XL oil pipeline to  clear its final major hurdle, granting a victory to President Trump and  Republicans who have for years pressed for the project. But the pipeline company will not be allowed to build along its preferred route, the regulators announced, opening up new questions about how the project will proceed.

About 5,000 barrels of oil, or about 210,000 gallons, gushed out of the Keystone Pipeline on Thursday in South Dakota, blackening a grassy field in the remote northeast part of the state and sending cleanup crews and  emergency workers scrambling to the site. “This is not a little spill from any perspective,” said Kim McIntosh, an environmental scientist with the South Dakota Department of Environment and Natural Resources.