BELFIELD, N.D. – Belle Fourche Pipeline Co. estimates that 130,200 gallons of oil spilled into a tributary of the Little Missouri River last week and another 46,200 gallons leaked into a hillside, the North Dakota Department of Health said Monday, Dec. 12.

The spill discovered by a landowner on Dec. 5 was not detected by monitoring equipment on the pipeline, which is owned by of True Companies of Wyoming.

The spill has contaminated 5.4 miles of Ash Coulee Creek but does not appear to have reached the Little Missouri River, said Bill Suess, spill investigation program manager for the health department.

The spill estimate of 4,200 barrels, or 176,400 gallons, is a “rough estimate” provided by the company, Suess said. Cleanup crews had recovered 983 barrels, or 41,286 gallons, of oil as of Monday, the company said.

At least two cows have been confirmed dead in the area of the oil spill, but the cause of death has not been verified by a veterinarian, Suess said. Crews assisted the landowner in moving cattle away from the creek and have trucked in water, he said.

It’s unknown how long the pipeline, which was built in the 1980s, had been leaking before the landowner discovered it.

The pipeline had pressure gauges and meters to monitor for leaks, said Kevin Connors, pipelines program supervisor for the North Dakota Oil and Gas Division.

The company also does aerial inspections of the pipeline about twice a month, Connors said.

Why the monitoring equipment didn’t detect the leak is part of an ongoing investigation, said True Companies spokeswoman Wendy Owen. The pipeline does not have a constant flow, which may have masked the leak from being detected, she said.

The 6-inch pipeline, which transports an average of 1,000 barrels of oil per day though rugged Badlands terrain, is buried in a hill that is slumping, Suess said. Crews have been unable to investigate further because the hillside is unstable, he said.

“Whether the slumping contributed to the break or the break contributed to the slumping, we don’t know yet,” Suess said.

The pipeline was shut down after the spill was discovered and has been capped at the spill location.

The company is working with SWAT Consulting, a spill response company based in Alberta with experience working in cold weather remediation, Suess said. An additional 60 workers were expected to arrive Monday to assist.

“The weather continues to still be the issue because the safety and security of the responders is our No. 1 priority,” Owen said.

The spill affected privately owned land as well as U.S. Forest Service land, Owen said.

“It’s very, very remote,” Owen said. “The terrain, the topography itself does make access points difficult.”

In addition to the state health department, the North Dakota Industrial Commission, the Environmental Protection Agency and the federal Pipeline and Hazardous Materials Safety Administration also are investigating.

Last year, the Laborers District Council of Minnesota and North Dakota opposed a pipeline proposed in North Dakota by True Companies, with union representatives citing the company’s track record of spills. The company also owns Bridger Pipeline, which had a pipeline spill in January 2015 that involved 30,000 gallons of oil in the Yellowstone River, contaminating the drinking supply for the city of Glendive, Mont.