BELFIELD, N.D.-An oil pipeline spill that contaminated a tributary of the Little Missouri River last December is now estimated to be three times larger than originally thought, making it one of the most significant pipeline spills in North Dakota history.
Belle Fourche Pipeline Co. reports about 12,615 barrels, or 529,830 gallons, of oil spilled as a result of a pipeline leak the company now believes started on Dec. 1 and was discovered by a landowner on Dec. 5, said spokeswoman Wendy Owen.
The spill contaminated a hillside and Ash Coulee Creek about 16 miles northwest of Belfield in southwest North Dakota.
An earlier estimate put the spill at 4,200 barrels, or 176,400 gallons, but was revised after the company was able to pinpoint when the spill started and review metering data, said Owen.
Cleanup crews continue to be on site, where traces of benzene have been detected throughout the creek, which flows into the Little Missouri River, said Bill Suess, spill investigation program manager for the North Dakota Department of Health.
An oil sheen has been detected in the creek but did not appear to reach the river, Suess said. Health officials continue to collect water samples from the creek and the river as they oversee cleanup efforts. The segment of pipeline was isolated and shut down.
The company estimates that 4,000 barrels, or 168,000 gallons, have been recovered through skimming operations.
Contractors also have performed 1,200 burns on top of the creek to recover oil, ranging from a few minutes to all-day operations, Suess said.
"We continue to work on the recovery and the cleanup. We will be there until this is finished," Owen said.
Crews are focusing efforts on cleaning the 5½ miles of creek, which runs through privately owned land and U.S. Forest Service grazing land.
"They want to get that back into operation and healthy for cattle prior to the grazing season," Suess said.
The spill did not affect human drinking water sources. The landowner reported losses of cattle he attributes to the spill, but he did not agree to medical testing to confirm the cause of death, Suess said.
The cause of the spill is still under investigation. The pipeline leak occurred in a hillside that is slumping, which is the company's leading theory of what caused the break, Owen said.
Weather conditions and the instability of the rugged Badlands terrain have slowed cleanup efforts and prevented crews from doing much work in the hillside.
Some oil continues to discharge from the hillside into the tributary of Ash Coulee Creek, Suess said. Crews hope to do more analysis of the hillside in the next month, Suess said.
"At the very worst case, we think there's about 20 percent of the volume remaining in the soil," Owen said.
The health department has issued a notice of violation for the spill, but has not yet proposed a fine.
The North Dakota Oil and Gas Division is investigating the cause and working with the health department and Attorney General’s Office to draft a letter to the company requiring that all evidence related to the spill and pipeline failure be preserved, said Kevin Connors, pipeline program supervisor.
In the past, regulators have had a difficult time accessing information about the root cause of major spills.
"We wanted to make sure that all evidence was retained for our investigations," Connors said.
The federal Pipeline and Hazardous Materials Safety Administration issued a corrective action order in late December. The Environmental Protection Agency also is monitoring cleanup.
The revised spill estimate makes the Belle Fourche Pipeline spill one of the largest in North Dakota's record of tracking oil spills.
The largest oil pipeline spill on record is the 2013 Tesoro Logistics spill discovered in a farmer's field near Tioga involving an estimated 20,600 barrels, or 865,200 gallons. That spill contaminated groundwater and cleanup efforts are ongoing.
The Belle Fourche spill is likely the largest oil pipeline spill that contaminated a water body in North Dakota, Suess said. The state also has had larger pipeline spills involving produced water, a waste byproduct of oil production.
Belle Fourche is part of True Companies of Wyoming, which also was responsible for the January 2015 Bridger Pipeline spill that involved 30,000 gallons of oil in the Yellowstone River, contaminating the drinking supply for the city of Glendive, Mont.