A ruptured pipeline has spilled more than 176,000 gallons of crude oil into a hillside and a Little Missouri River tributary about 150 miles west of Cannon Ball, N.D., where thousands of activists have spent months fighting construction of the Dakota Access oil pipeline, state officials said Monday.

A segment of the Belle Fourche Pipeline near Belfield, N.D., began leaking earlier this month, contaminating nearly six miles of the Ash Coulee Creek before cleanup workers contained it, Bill Suess, an environmental scientist from the North Dakota Department of Health, told the Associated Press. An estimated 130,200 gallons of oil spilled into the creek, and another 46,200 gallons leaked onto a hillside, Suess told Forum News Service in a separate interview.

The spill dirtied private and federal land along the waterway, but no drinking water sources were affected, Suess said. About 37,000 gallons of oil have been recovered so far, he said, and a crew of about 60 workers were averaging 100 yards of cleanup per day, though snow and single-digit temperatures have complicated the response.

“It’s going to take some time,” Suess told the AP. “Obviously there will be some component of the cleanup that will go toward spring.”

The incident stirred up fears among opponents of the Dakota Access pipeline about what could happen if project developer Energy Transfer Partners builds a segment of pipe under a Missouri River reservoir that provides drinking water to the Standing Rock Indian Reservation.

The spill occurred about a two and a half-hour drive west from Cannon Ball, where members of the Standing Rock Sioux and other Native American tribes, as well as environmental activists from around the country, have camped out since April in an ongoing demonstration against the Dakota Access pipeline. Opponents of the $3.8 billion project, many of whom call themselves “water protectors,” argue that the pipeline will pollute Standing Rock’s water supplies and damage burial grounds and other sacred lands. Energy Transfer Partners has said that leak detection equipment and the pipeline’s thick steel walls would prevent a major accident.

Earlier this month, the Army Corps of Engineers denied Energy Transfer Partners an easement that would have allowed the company to drill under the reservoir. The 1,172-mile pipeline is nearly complete, but the Army Corps said it will weigh alternate routes.

The Belle Fourche Pipeline, owned by Wyoming-based True Companies, was immediately shut down after a local landowner reported the spill to regulators on Dec. 5, company spokeswoman Wendy Owen told the AP. Electronic monitoring equipment failed to detect the leak, according to Owen, who said the pipeline may have ruptured when the hillside slumped.

“That is our number one theory, but nothing is definitive,” she said. “We have several working theories and the investigation is ongoing.”

Built in the 1980s, the pipeline is six inches in diameter and transports about 1,000 barrels of oil daily, Suess told Forum News Service.

The Dakota Access pipeline is 30 inches in diameter and could transport more than 500,000 barrels of oil daily.

Shortly after the leak was discovered, a labor group in the region with members working on the Dakota Access pipeline criticized True Companies for what it called a track record of accidents.

“Our members take pride in their work, and we won’t just stand by and allow an irresponsible pipeline operator to harm North Dakota’s natural resources or damage reputation of our industry,” Evan Whiteford, a spokesman for the Laborers District Council of Minnesota and North Dakota, told Forum News Service. “We think it’s time for state officials to step in and force the True organization to clean up its act.”

True Companies has a history of oil spills in the region, reporting three dozen spills totaling 320,000 gallons of oil since 2006, according to the AP. Federal pipeline regulators have hit True Companies with 19 enforcement actions since 2004, resulting in nearly $400,000 in penalties, the AP reported.

The Poplar Pipeline, operated by a True Companies subsidiary, leaked about 30,000 gallons of crude oil into the Yellowstone River in eastern Montana in 2015, prompting a town to shut down its drinking water service to 6,000 residents, the Casper Star Tribune reported. Belle Fourche Pipeline Co., also part of the True Companies family, has reported 10 oil spills since 2011, totaling nearly 5,000 barrels and resulting in more than $2 million in property damage, Reuters reported.

A representative from True Companies did not immediately respond to a request for comment.

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