The controversial Dakota Access Pipeline, which Donald Trump pushed through upon taking the presidency, leaked about 200 miles south of where the well-reported Standing Rock protests were located.
A tank inside a pump station in the small rural town of Tulare, South Dakota, leaked about 84 gallons. The station caught it all inside a containment area made of gravel and a synthetic liner.
"They have the main line and the main pump, but they also have a surge tank, which they have pump a little off the main line into," said Brian Walsh, an environmental scientist with the Ground Water Quality Program of the South Dakota Department of Environment and Natural Resources. "And that's where the leak occurred — at the surge pump."
According to Walsh, the surge pump had a mechanical failure. A full report will be published once Walsh's department closes its investigation; it has reported that the pipeline company has done everything by the book thus far.
Any contaminated gravel was collected and stored for proper disposal, officials said.
The leak happened on April 4, but it wasn't noticed until Tuesday because of its small size. Local media first reported the spill Wednesday.
"These kinds of spills do occur," Rebecca Craven, program director at the Pipeline Safety Trust, told NBC News. "Sometimes they're contained on company property, and sometimes they do more damage as they escape off company property."
The public often does not know about those types of leaks because of their size. Companies do not have to report oil pipeline leaks to the federal government unless they're over five barrels, which is 210 gallons of oil, and if they clean them up quickly.
Opponents of the pipeline, who led a months-long protest against it, said the leak should not come as a shock to anyone. The seepage proves that there can be potential problems, they said.
"The Dakota Access pipeline has not yet started shipping the proposed half million barrels of oil per day and we are already seeing confirmed reports of oil spills from the pipeline. This is what we have said all along: oil pipelines leak and spill," Standing Rock Sioux Tribal Chairman Dave Archambault II said in a statement.
Craven agreed that these pipelines cannot guarantee a seamless transfer of oil.
"It's very hard to extrapolate from this to guess how many problems there will be with Dakota Access in the future," Craven said, noting an average of 420 significant pipeline leaks a year over the past five years. "But it's an indication that it's not perfect and it's really hard to build a pipeline that's perfect."
Energy Transfer Partners, the pipeline's owner, plans to start pumping up to 470,000 barrels between North Dakota and oil refineries in Illinois 1,170 miles away.
The nearby Standing Rock Tribe protested the route, claiming that a spill could occur and dirty the Missouri River — its main water supply. The protesters were joined by environmentalists, veterans and other demonstrators who camped out and blocked the pipeline's progress for months.
While the Obama administration eventually blocked its construction, newly minted President Trump pushed the project forward shortly after taking office.
"Our lawsuit challenging this dangerous project is ongoing and it's more important than ever for the court to step in and halt additional accidents before they happen — not just for the Standing Rock Sioux Tribe and our resources but for the 17 million people whose drinking water is at risk," Archambault said.
Energy Transfer Partners did not immediately respond to NBC News' requests for comment, but spokeswoman Vicki Granado told The Associated Press in a statement that the spilled oil "stayed in the containment area as designed."