(Adds details on refineries served by pipelines, activist comment, impact on prices and stocks)
By Rod Nickel
Feb 8 (Reuters) - Enbridge Inc said on Friday that it has begun a restart plan for its Platte pipeline, as authorities investigated the source of an oil leak near St. Louis, Missouri, that shut down parts of two major pipelines this week.
The leak amounted to 43 barrels of crude oil on land, according to Missouri's Department of Natural Resources. The spill and shutdown of portions of Enbridge's Platte pipeline and the bigger Keystone pipeline owned by TransCanada Corp raised fresh concerns about pipeline safety, and about the already constricted flow of Canadian oil to U.S. refineries.
Canadian pipelines are congested because of expanding production in recent years, forcing the Alberta provincial government to order production cuts starting last month. Canadian heavy oil has attracted greater demand following U.S. sanctions against Venezuela's state oil company.
Enbridge expects the full Platte pipeline to resume normal operations by Saturday morning and is "highly confident" that Platte is not the source of the leak, spokesman Devin Hotzel said.
TransCanada spokespersons did not immediately respond but have said they would comment once they had updates. The company has previously said the leak occurred on its right-of-way, but has not confirmed that Keystone was the source.
The spill in rural St. Charles County, Missouri, on Wednesday led TransCanada to shut Keystone between Steele City, Nebraska, and Patoka, Illinois. That pipeline runs parallel to Enbridge's Platte line and feeds refineries owned by BP Plc , Marathon Petroleum Corp and one jointly by Phillips 66 and Cenovus Energy.
Wood River Refinery is currently undergoing planned maintenance, said Phillips 66 spokeswoman Melissa Ory.
A spokesperson for Marathon declined to comment and BP did not immediately respond.
The 590,000 barrels-per-day Keystone pipeline is a critical artery taking Canadian crude from northern Alberta to U.S. refineries.
The discount on Canadian heavy crude compared with U.S. light oil grew to as much as $11.50 per barrel on Friday from the previous day's settlement of $10.25, according to Net Energy Exchange.
TransCanada and Enbridge shares were down 2 percent and 3 percent respectively in Toronto.
The spilled oil was contained to an area of approximately 4,000 square feet, Missouri's Natural Resources Department said.
But the leak happened near the Mississippi River, at a spot upstream of the intake for St. Louis's drinking water supply, said John Hickey, director of the Sierra Club's Missouri chapter. The immediate concern is that leaked oil could contaminate farm wells, he said. (Reporting by Rod Nickel in Winnipeg, Manitoba; additional reporting by Devika Krishna Kumar in New York Editing by Chizu Nomiyama and Dan Grebler)