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.

The spill, which began on Tuesday night and has been contained, occurred in a low-gradient drainage area near the small town of Edinburg in northeast North Dakota , less than 50 miles from the Canadian border, according to Karl Rockeman, the director of the state Department of Environmental Quality’s division of water quality.

“It is one of the larger spills in the state,” he said in an email on Thursday.

There are no residences near the site and the wetland is not a source of drinking water, he said.

Emergency crews have begun cleaning up the leaked oil, which would fill about half of an Olympic-size swimming pool, with vacuum trucks, backhoes and other equipment, according to its operator, TC Energy.