NEW YORK, Oct 11 (Reuters) - Initial investigations following a 20,600-barrel leak on Tesoro Logistics LP pipeline in North Dakota point to corrosion on the 20-year-old pipeline, state regulators said on Friday.
The six-inch pipeline was carrying crude oil from the Bakken shale play to the Stampede rail facility outside Columbus, North Dakota when a farmer discovered oil spouting from the pipeline on Sept. 29.
This is the largest oil spill in the state since it became a major U.S. producer. It is the biggest oil leak on U.S. land since March, when an Exxon Mobil pipeline spilled 5,000 to 7,000 barrels of heavy Canadian crude in Mayflower, Arkansas.
The release did not pose an immediate threat to groundwater sources or nearby rivers and lakes, the state Department of Health said on Thursday.
Initial concerns that the pipeline was punctured by nearby residents were dismissed and the cause appears to be corrosion on the pipeline, according to Brian Kalk, chairman of the state Public Service Commission.
"It started out as a small hole and got bigger," Kalk said.
The U.S. Department of Transportation Pipeline and Hazardous Materials Safety Administration is in charge of investigating the cause of the leak and oversees the pipeline's operation.
But the state commission approves the construction of pipelines and operators' compliance with initial designs submitted to the state, Kalk said.
PHMSA and Tesoro could not be immediately be reached for comment.
The pipeline, which runs 35 miles from Tioga to Black Slough in North Dakota, was built by BP Plc in 1993.
It is a part of Tesoro's "High Plains" pipeline system in North Dakota and Montana that gathers oil from the Bakken shale and delivers it to another Enbridge pipeline and Tesoro's 68,000 barrels-per-day Mandan refinery.
Tesoro bought the pipeline and the refinery from BP in 2001.