TC told the Pipeline and Hazardous Materials Safety Administration (PHMSA) about the reported leak and conducted aerial observation and water analysis that found no evidence of a spill before restarting the line, PHMSA said.

A spokesperson for Calgary, Alberta-based TC Energy said a landowner in South Dakota's Marshall County noted a sheen in water from his well, one mile from the pipeline corridor.

"This was not an operational incident. The pipeline was shut down out of an abundance of caution," the TC spokesperson said.

The shutdown of the 622,000 barrel-per-day (bpd) pipeline interrupted the flow of Canadian oil from Alberta to Nebraska, where the line splits, with one arm running east to the Midwest and the other running south to the U.S. crude storage hub in Cushing, Oklahoma.

The outage took down the entire Keystone system for five hours, according to real-time pipeline monitoring by consultancy Wood Mackenzie.

Wood Mackenzie said as of Friday morning, Keystone flows had rebounded to their pre-shutdown level of more than 600,000 bpd.

The pipeline has been dogged by problems, most recently a major spill in rural Kansas in 2022.

TC shares eased slightly in New York.

(Reporting by Rod Nickel in Winnipeg, Manitoba; Editing by Nia Williams and Marguerita Choy)

By Rod Nickel