A crude oil spill in western North Dakota in December is now believed to be about three times bigger than originally estimated, pipeline owner True Cos. Inc. said March 24, making it the largest crude leak to affect water in the state in over a decade.

The Belle Fourche crude oil pipeline spilled an estimated 12,615 barrels (bbl) of oil, more than the December estimate of 4,200 bbl, spokeswoman Wendy Owen said in a phone call.

The spill is the second-largest crude spill in the state in more than 15 years, behind a 20,600-bbl leak by a Tesoro Logistics LP (NYSE: TLLP) pipeline in 2013, according to data from the Pipeline and Hazardous Materials Safety Administration.

Around 80% of the cleanup is complete, Owen said, noting the incident occurred following ground movement. Oil from the pipeline leaked into the Ash Coulee Creek and on a hillside.

The pipeline operator has collected around 3,900 bbl of oil from the creek by skimming and vacuuming, Owen said. No oil moved further down the creek, which feeds into the Little Missouri River and eventually flows into the Missouri River, a major source of drinking water, she said.

The North Dakota Department of Health has not yet completed a subsurface investigation on the hillside affected by the leak to confirm how much oil remains, agency program manager Bill Seuss said by phone March 24.

The spill was not originally detected by monitoring equipment, which True Cos. has said was likely due to its intermittent flow.

While Belle Fourche's impacted line remains shut for cleanup operations, another line is moving crude oil from its origin and receipt points.

The incident led to U.S. pipeline regulators ordering the company to improve leak detections, along with other actions.