The two largest spills in Keystone's history in 2017 and 2019 were among six accidents that met the Department of Transportation's Pipeline and Hazardous Materials Safety Administration's criteria for accidents "impacting people or the environment", the report said.
The GAO said in the report that the two accidents spilled 6,592 barrels of oil at Amherst, South Dakota in November 2017 and 4,515 barrels at Edinburg, North Dakota in October 2019.
"In its thorough review of the pipeline’s history and construction, GAO found that preventable construction issues contributed to the current Keystone pipeline’s spills more frequently than the industry-wide trends. … half of Keystone’s accidents were caused by material failure of the pipe or weld", the report said.
Four of the pipeline’s largest spills were caused by its construction, manufacturing of the pipe, or involved original design issues, the report added.