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A pipeline spill detected last week in North Dakota has reportedly released over 170,000 gallons of oil in Billings County.

The North Dakota Department of Health told the Duluth News Tribune that a pipeline operated by Belle Fourche Pipeline released 130,200 gallons of oil into a tributary of the Little Missouri River and an additional 46,200 gallons onto a hillside.

The state health department said the current spill figure is a rough estimate and is equivalent to about 4,200 barrels.

The North Dakota Department of Health (NDDoH) said the spill was discovered on December 5 about 16 miles northwest of Belfield.

The spill impacted the Ash Coulee Creek but NDDoH told the Duluth News Tribune that the oil has not reached the Little Missouri River.

A NDDoH official told the paper that cleanup crews had recovered 878 barrels as of Sunday night.

A spokeswoman for True Companies, the firm that owns Belle Fourche, told CNBC that the leak was contained within hours of being discovered and the line has been shutdown.

Owens told the AP that electronic monitoring equipment failed to the detect the leak.

The cause of the monitoring equipment failure has not been determined yet.

Owens added that the leak may have been caused when a hillside that the line is buried in “sloughed.”

“That is our No.1 theory but nothing is definitive” Owen told the AP.

An investigation into the spill is ongoing.

According to the Duluth News Tribune, the NDDoH, North Dakota Industrial Commission, Environmental Protection Agency and Pipeline and Hazardous Materials Safety Administration are investigating the incident.

The spill occurred about 150 miles away from Cannonball, North Dakota where protesters have gathered to demonstrate against the Dakota Access Pipeline.

The pipeline has drawn opposition from Native American and environmental groups who say they are concerned about potential water impacts tied to the line.

Earlier this month, the U.S. Department of the Army declined to approve an easement that would allow the Dakota Access line to cross under Lake Oahe in North Dakota.

Army Assistant Secretary for Civil Works Jo-Ellen Darcy said when the decision was announced that alternative routes for the pipeline will likely be explored through an  Environmental Impact Statement with full public input and analysis.

Energy Transfer Partners, the operator of the Dakota Access line, called the decision the latest in a “series of overt and transparent political actions” undertaken by the Obama administration.

The company added that it remains “fully committed” to completing the line and said it expects to finish construction “without any additional rerouting in and around Lake Oahe.”

The decision followed a lawsuit filed earlier this year by the Standing Rock Sioux Tribe that claimed the U.S. Army Corps of Engineers violated historic preservation and environmental laws when it approved the project.

Although a federal judge ruled in September that construction could continue the Army Corps continued to halt construction on a stretch of pipeline at Lake Oahe pending a review.