An April 2011 failure on the Rainbow pipeline in Alberta resulted in the release of about 28,000 barrels of crude oil.
The Alberta Energy Resources Conservation Board said Plains faces "high-risk enforcement actions" for its failure to respond adequately to the spill, one of the largest since the 1970s.
An assessment of the incident indicated stress on a crack in a weld on the pipeline contributed to its failure. Plains committed to pulling and inspecting all sections of the pipeline containing the type of welds in question, the ERCB said.
The regulator said Plains didn't have a sufficient leak detection system in place at the time of the spill nor did it test its emergency response plan. The ECRB added that the company's efforts to keep the public informed of the incident were "substandard."
Parent company Plains All American Pipeline was scrutinized in 2006 for a 7,500-barrel leak at a section of the same pipeline in Edmonton. Investigators blamed stress and corrosion for that leak. The pipeline was built in 1966.
The ECRB said the spill was cleaned and the pipeline was restarted without incident Aug. 30, 2011. There was no public response from Plains.