More than 176,000 gallons of crude oil reportedly leaked into a North Dakota creek from the Belle Fourche pipeline, roughly 150 miles away from the Standing Rock protest camp.
A landowner near Belfield, North Dakota, found the spill on Dec. 5, and the pipeline was immediately shut down pending repairs, said Bill Suess, an environmental scientist working for the state's health department, reports CNBC.
"It's going to take some time" to clean up, Suess said, according to CNBC. "Obviously there will be some component of the cleanup that will go toward spring."
The leak was contained hours after officials learned of it. It is not yet known why electronic monitors did not detect it, though True Cos., the Wyoming-based company operating the Belle Fourche pipeline, believes the "hillside sloughed" and cracked the pipeline.
"That is our number one theory, but nothing is definitive," True Cos. spokeswoman Wendy Owen said. "We have several working theories and the investigation is ongoing."
Environmentalists, Native American tribes and activists across the country have joined with the Standing Rock Sioux to protest the Dakota Access pipeline, saying it will desecrate sacred land and pollute their drinking water from Lake Oahe, under which the pipeline would cross.
The Army announced on Dec. 4 that it would not approve a permit for the project to pass under the lake, which will delay -- but not necessarily cancel -- the project, as the Army looks into rerouting it, notes The Washington Post.
"Although we have had continuing discussion and exchanges of new information with the Standing Rock Sioux and Dakota Access, it's clear that there's more work to do," the Army's Assistant Secretary for Civil Works Jo-Ellen Darcy said in a Dec. 4 statement following the announcement. "The best way to complete that work responsibly and expeditiously is to explore alternate routes for the pipeline crossing."
President-elect Donald Trump, who previously owned shares in Energy Transfer Partners, the company responsible for the Dakota Access pipeline, has said he would approve the project.