Electronic monitoring equipment failed to detect a rupture in the Belle Fourche pipeline that spewed more than 176,000 gallons of crude oil into a North Dakota creek, the pipeline's operator said Monday.

“It's not yet clear why the monitoring equipment didn't detect the leak”, Wendy Owen, a spokeswoman for Casper, Wyoming-based True Cos., which operates the Belle Fourche Pipeline, said.

Bill Suess, an environmental scientist with the North Dakota Health Department, reported that a landowner discovered the spill near Belfield on Dec. 5, and that the spill migrated almost 6 miles from the spill site along Ash Coulee Creek, fouling an unknown amount of private and U.S. Forest Service land along the waterway. The creek feeds into the Little Missouri River, but Seuss said it appears no oil got that far and that no drinking water sources were threatened. The creek was free-flowing when the spill occurred but has since frozen over. Suess said about 37,000 gallons of oil had been recovered as of Monday.

Owen said the pipeline was shut down immediately after the leak was discovered. The pipeline is buried on a hill near Ash Coulee creek, and the "hillside sloughed," which may have ruptured the line, she said.

"That is our No.1 theory but nothing is definitive," Owen said. "We have several working theories and the investigation is ongoing."

Owen said the pipeline was built in the 1980s and is used to gather oil from nearby oil wells to a collection point.

Federal pipeline safety regulators initiated 19 enforcement activities against the three True pipeline companies since 2004. Those resulted in $537,500 in proposed penalties, of which the company paid $397,200, according to Department of Transportation records.

The potential for a pipeline leak that might taint drinking water is at the core of the disputed four-state, $3.8 billion Dakota Access Pipeline, where thousands of people have been protesting its construction in southern North Dakota. That pipeline would cross the Missouri River. Dallas-based pipeline company Energy Transfer Partners says the Dakota Access pipeline would include safeguards such as leak detection equipment and that workers monitoring the pipeline remotely in Texas could close valves within three minutes if a breach is detected.

Source: ABC News