Unless you have been living under a rock, you probably know that a week ago Friday an ExxonMobile pipeline known as Pegasus ruptured in Arkansas, spilling what at this writing is estimated at 84,000 gallons of "crude oil" into a housing subdivision, causing the evacuation of 22 homes and of course doing considerable environmental damage in the process.

The quotes are around the words crude oil because this was not your common or garden variety of crude. Although referred to as "Wabasca Heavy," the substance spilled was actually "dilbit," or diluted bitumen: bitumen from the tar sands region of Canada, diluted with other toxic hydrocarbons to make it thin enough to flow through a pipeline. Unlike normal oil, bitumen, diluted or otherwise, does not float on water; it sinks, and the potential for increased contamination of both waterways and groundwater compared to ordinary oil is obvious.

On March 18 a Chevron pipeline in Utah ruptured, leaking more than 25,000 gallons of diesel fuel into a wetlands area about 50 miles from Salt Lake City. And shortly before the Pegasus pipeline incident, a train carrying tar sands oil derailed in Minnesota, spilling 30,000 gallons of bitumen into the environment. Meanwhile, cleanup is still under way from a 2011 ExxonMobile pipeline rupture that dumped an estimated 63,000 gallons of oil into the Yellowstone River in Montana.

Extending further back, two Chevron pipeline spills, one in June 2010 and the other in January 2011, spilled 33,000 and 21,000 gallons of oil, respectively, into Red Butte Creek near Salt Lake City. This information, widely available on the internet, demonstrates some of the perils of transporting oil long distances by pipeline or, in the case of tar sands bitumen, even by rail. Bitumen in particular is considered by many experts to be much dirtier and harder to clean up than ordinary crude, which is itself far from easy to extract from the environment after a spill.

All of these incidents set a troubling context for the proposed Keystone XL Pipeline intended to carry tar sands bitumen from Canada to the Gulf of Mexico. With a capacity 10 times Pegasus' 90,000 barrels per day, any rupture in Keystone would likely be even more catastrophic than what we have seen so far. A coalition of environmentalists and Native tribes on both the U.S. and Canadian sides of the border are challenging the Keystone XL pipeline, both in the courts and through grassroots activism.

But it's not just that tar sands bitumen is hazardous to the environment when spilled. Blogger George Dvorsky points out that bitumen is a very low-grade form of petroleum, and difficult to extract.

"It's done via an astoundingly environmentally unfriendly process involving surface mining and subsurface production; large areas of boreal forest are cleared to make way," he said. "Production requires huge quantities of water which [is] taken from local rivers, and is subsequently turned into toxic waste. It's also a major contributor to greenhouse gas emissions. Nearly all the input energy required to fuel the process comes from fossil fuels. Tar sands production is also highly inefficient; it takes one joule of energy to produce four to six joules of crude oil energy. Traditional oil production returns something closer to a 1:15 ratio."

It is that phenomenal return rate which has made the techno-industrial boom of the last hundred years possible. If we are now pinning our hopes on energy sources only a third as efficient, with major adverse effects on the environment, that's a problem. And no matter where you stand on oil pipelines, the recent string of damaging spills is surely worth taking into account as we consider whether to run a new mega-pipeline across the country.

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