Story highlights "Any time oil gets into water ... we take it more seriously," North Dakota official says

Spill is 150 miles from Cannon Ball, where protesters have fought construction of the Dakota Access pipeline

(CNN) Activists who have demonstrated for months against the Dakota Access Pipeline may have some fuel to justify their protests.

A spill has occurred 150 miles from Cannon Ball, North Dakota, where protesters have fought construction of the Dakota Access Pipeline.

Officials estimate 4,200 barrels of crude oil have leaked.

State officials estimate 4,200 barrels of crude oil, or 176,000 gallons, have leaked from the Belle Fourche Pipeline in Billings County.

Of that amount, 130,000 gallons of oil has flowed into Ash Coulee Creek, while the rest leaked onto a hillside, said Bill Suess, spill investigation program manager at the North Dakota Department of Health. Built in the 1980s, the pipeline is 6 inches in diameter and transports about 1,000 barrels of oil daily, he said. The leak happened December 5.

"Any time it gets into water, we respond differently and we take it more seriously," Suess said. He said more than 100 people are working to clean up the spill. Investigators are still trying to determine the cause, he said.

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