A photo of the spill, which was posted to the company’s Twitter account, showed a large, darkened area in a field. The Keystone Pipeline is part of a 2,687-mile system that carries crude oil from Alberta to several points in the United States, including Illinois and Oklahoma.
A reporter for The Aberdeen American News at the scene of the spill said on Twitter that the area was blocked off by emergency vehicles.
Opponents of Keystone XL, which is proposed to run about 1,100 miles and would become part of the Keystone system, quickly cited Thursday’s spill as evidence of the risks posed by such pipelines, and urged Nebraska regulators to take note.
“We’ve always said it’s not a question of whether a pipeline will spill, but when, and today TransCanada is making our case for us,” Kelly Martin of the Sierra Club said in a statement. “This is not the first time TransCanada’s pipeline has spilled toxic tar sands, and it won’t be the last.”
Keystone XL has the strong support of President Trump and most Republican politicians, but it has faced years of vocal opposition in Nebraska from some farmers and ranchers who worry that a spill could spoil their groundwater and decimate agricultural land.
“That’s our fear — that pipelines do leak,” said Jeanne Crumly, whose farm near Page, Neb., is along the proposed Keystone XL route, after being told about the South Dakota spill.
Ms. Crumly and about 90 other Nebraska landowners have not signed easements with TransCanada and have urged against issuing a permit for the project. Nebraska’s Public Service Commission plans to announce on Monday morning whether it will approve the permit, the last major regulatory hurdle before construction on Keystone XL could begin.
Thursday’s episode is one of several major pipeline spills in recent years. More than a million gallons leaked from a pipeline into the Kalamazoo River in Michigan in 2010, and 50,000 gallons of oil gushed into the Yellowstone River in Montana in 2015, contaminating drinking water there.
Oil pipelines have faced greater scrutiny since thousands of protesters gathered near the Standing Rock Sioux Reservation in North Dakota last year to protest the Dakota Access Pipeline. The site of Thursday’s spill was near the boundaries of the Lake Traverse Reservation, home of the Sisseton Wahpeton Oyate tribe.
Dave Flute, the tribal chairman of the Sisseton Wahpeton Oyate, said he was contacted early in the afternoon by emergency management services and told that there was a “substantial leak” in the pipeline.
“We are monitoring the situation as this leak is adjacent to our reservation,” Mr. Flute said in a statement. “We do not know the impact this has on our environment at this time but we are aware of the leak.”
Ms. McIntosh, the South Dakota environmental official, said that TransCanada employees and contractors were at the spill site and that soil cleanup workers were on the way. The state was overseeing the response.
Ms. McIntosh said that the leak was “a large release” of oil, but that “the location of this is not in a sensitive area.”
“They’ve got a response plan that they kicked in right away,” Ms. McIntosh said. “The area’s very rural, which is very positive. There’s no one nearby that is drinking any of the groundwater that may be impacted, so that’s less of an issue.”