(Corrects to fix name of Natural Resources Defense Council in paragraph 13 (not National Resources Defense Council)
* Exxon gives no timeline for pipeline restart
* Company yet to excavate area around ruptured pipe
* Pipeline was built in late 1940s
By Suzi Parker and Kristen Hays
MAYFLOWER, Ark./HOUSTON April 1 (Reuters) - Exxon Mobil continued efforts on Monday to clean up thousands of barrels of heavy Canadian crude oil spilled from a near 65-year-old pipeline in Arkansas, as a debate raged about the safety of transporting rising volumes of the fuel into the United States.
The Pegasus pipeline, which ruptured in a housing development near the town of Mayflower on Friday, remained shut and a company spokesman declined to speculate about when it would be fixed and restarted. The line can carry more than 90,000 barrels of crude per day from Patoka, Illinois to Nederland, Texas.
Exxon had yet to excavate the area around the Pegasus pipeline breach on Monday, a critical step in assessing damage and determining how and why it leaked.
Twenty-two homes in the affected area were evacuated after the spill poured oil across lawns and down residential streets. The smell of oil permeated the town on Monday.
The spill has stoked a discussion about the environmental dangers of using aging pipelines to transport heavy crude from Canada, including tar sands, as a boom in oil and gas production in the United States increases volumes moving across the continent.
The Exxon pipeline was carrying Canadian Wabasca Heavy crude at the time of the leak, a bitumen oil from the massive Pelican Lake field in northern Alberta. It needs to be blended with lighter oils or natural gas liquids to flow through pipelines.
Exxon did not yet have a specific figure of how much oil was released when the 20-inch line ruptured on Friday. The company said Sunday that 12,000 barrels of oil and water had been recovered.
An oil spill of more than 1,000 barrels in Wisconsin last summer kept an Enbridge Inc pipeline shuttered for around 11 days.
Exxon spokesman Charles Engelmann said the ruptured section of the Pegasus pipeline was installed in the late 1940s, but had no information on when it last underwent maintenance.
To prevent and track corrosion buildup, pipelines are periodically "pigged," or cleaned with a device that moves through the line to remove buildup of hydrocarbons, dirt, and other substances. Often the device is outfitted with sensors that point out areas of corrosion or wear-and-tear that need repair.
The Pipeline and Hazardous Materials Safety Administration (PHMSA) said in a recent report that more than half of the nation's pipelines were built in the 1950s and 1960s in response to higher energy demand after World War II.
Some, like Pegasus, were built earlier.
"An influx of tar sands on the U.S. pipeline network poses greater risks to pipeline integrity, challenges for leak detection systems and significantly increased impacts to sensitive water resources," environmental group the Natural Resources Defense Council said in an emailed note on Monday.
Exxon said that trucks had been brought in to assist with the cleanup. Images from local media showed crude oil snaking along a suburban street and spewed across lawns.
The U.S. Environmental Protection Agency and PHMSA, as well as state and local responders, were also present.
Fifteen vacuum trucks were on the scene for cleanup on Sunday, and 33 storage tanks were deployed to temporarily store the oil. (Writing by Edward McAllister in New York, additional reporting by Scott Haggett in Calgary; Editing by Gerald E. McCormick, David Gregorio and Nick Zieminski)