Investigators say they’ve determined what prompted December’s oil spill in northeastern Kansas.  The Keystone oil pipeline’s massive spill in northern Kansas was likely caused by a faulty welding job, the company that owns the pipeline said Thursday.

Federal regulators in December ordered Canada-based TC Energy to investigate the cause of the spill in Washington County, which dumped oil onto adjacent farmland and into Mill Creek.

The company said in a release Thursday the investigation is ongoing. But so far, an independent analysis revealed the pipe burst was caused in part by bending stress on the pipe and a faulty welding job completed at the facility where the pipe was fabricated.

“Although welding inspection and testing were conducted within applicable codes and standards, the weld flaw led to a crack that propagated over time as a result of bending stress fatigue, eventually leading to an instantaneous rupture,” the company said.

The company estimated the cost of “remediation, investigation and shared learning” would be $480 million.

The Keystone pipeline, which carries oil from Canada to Illinois and Texas, spilled near Washington, a small town along the Kansas-Nebraska border. The spill, which took place in December, was first believed to be about 14,000 barrels – or 588,000 gallons. The company revised that estimate to 12,937 barrels, the largest in the pipeline’s history.

TC Energy has paid just over $300,000 fines for more than 20 previous spills. That’s 0.2% of the more than $111 million in property damage resulting from those spills.

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Story by Allison Kite via Kansas Reflector