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Update: On Thursday, April 7, TransCanada said it had found an estimated 16,800 gallons of oil in the Keystone I pipeline right of way. That figure is 90 times higher than the company's original estimate of 187 gallons. Company spokesperson Mark Cooper said in a written statement that the oil came from a "small leak" in the pipeline. The line was shut down from April 2 through April 9 and went back into operation on April 10.

An oil spill that surfaced in South Dakota over the weekend of April 2 prompted Canadian pipeline company TransCanada to shut down its Keystone I pipeline, a predecessor to the controversial Keystone XL project.

TransCanada had still not confirmed the leak as of Tuesday, April 5, calling it a "potential incident." According to Chris Nelson, chairman of the South Dakota Public Utilities Commission, the leak was first reported by a passerby. TransCanada reported to the U.S. Coast Guard on April 2 that 187 gallons of oil had leaked, Nelson said. The line was expected to remain closed all week.

The leak is the most recent of dozens reported since the pipeline, which moves about 500,000 gallons of oil per day from the tar sands of Alberta to refineries in the U.S., was commissioned in 2010.

According to Nelson, the leak was not revealed by the company's own leak detection systems. Environmentalists familiar with pipeline leaks said the equipment's failure to detect it is cause for concern.

"It's another piece of evidence in the inherent risk of some of these systems and our oil transportation infrastructure," said Anthony Swift, the Canada program director for the Natural Resources Defense Council. "Contrary to industry talking points, the reality is pipeline systems do fail."

Keystone I was commissioned in 2010 with a number of advanced leak detection technologies. In its first year, the pipeline leaked 35 times in the and . Most were minor leaks; one was major spilling more than 21,000 gallons of oil in North Dakota.

This leak comes as TransCanada seeks to build the Energy East Pipeline, which would carry 1.1 million barrels of crude oil per day 2,800 miles from Alberta and Saskatchewan to refineries in eastern Canada.

"Just last month hearings began in the province of Quebec on TransCanada's proposed Energy East Pipeline and TransCanada was making a lot of very big claims about how in near minutes they would detect any leak and be able to shut down the pipeline in event of a spill," said Keith Stewart, who leads the energy campaign for Greenpeace Canada.

TransCanada that its Energy East Pipeline would employ leak detection technology that "can detect events immediately" allowing "the line to be shut down and valves surrounding the area to be closed within minutes, limiting the impact of a potential spill."

"Given that the Keystone Pipeline is less than a decade old it doesn't give us a lot of confidence in their claims of how good the technology is to detect spills and thus minimize them," Stewart said.

An investigation by InsideClimate News in 2012 found leak detection systems used by pipeline companies detected only 5 percent of pipeline spills in the U.S.

Leak detection experts said the current leak is likely too small to easily detect.

"I know the public would love to have a leak detection system that is 100 percent reliable but it's an extremely difficult challenge," said Richard Kuprewicz, president of Accufacts, Inc., a consulting firm that provides expertise on pipelines to government agencies and industry. Kuprewicz has worked with TransCanada in the past, but is not currently.

Kuprewicz said companies typically rely on a combination of sensors inside pipelines that measure temperature, pressure, flow rates and other hydraulic data as well as external sensors that can detect fluid outside the pipeline. Internal sensors are unlikely to detect small leaks and external sensors are prohibitively expensive to use everywhere along a pipeline's path, Kuprewicz said.

The spill was most likely tar sands crude, also known as diluted bitumen, Swift said.

"Keystone I transports almost exclusively diluted bitumen," he said. "While we don't know the classification of the oil spilled, it was almost certainly tar sands."

Spills of diluted bitumen or "dilbit" are more difficult to clean up than conventional crude oil and pose a significant environmental and safety hazard. An released more than a million gallons of dilbit, mostly in the Kalamazoo River. The massive spill displaced 150 families, forced a , and cost pipeline operator Enbridge at least $1.2 billion to clean up.

A 2015 study by the National Academies of Science found dilbit behaves like conventional oil in the first few days following a spill but then quickly degrades into that it defies standard spill responses.

TransCanada did not respond to an InsideClimate News request about the type of oil in the pipeline at the time of the leak and Nelson, whose agency is not responsible for overseeing the cleanup, said he did not know.

"The lack of transparency when it comes to what is moving in these pipelines is a problem," Swift said. "Often spill responders don't know the characteristics of the crude oil that they are dealing with in a spill and not all crudes behaves the same way."

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