Demonstrators protest the Keystone XL Pipeline in 2011. (AP Photo/Evan Vucci.)

When the Exxon Pegasus pipeline ruptured Friday in Mayflower, Arkansas, tens of thousands of gallons of diluted bitumen were sent forth into a residential neighborhood, and 22 homes had to be evacuated. Since this is the same sort of oil that would be carried by the Keystone XL pipeline, were it to be built, the Arkansas spill is appropriately spurring a conversation about safety. Is Keystone also going to lead to more spills? (There have been twelve on the completed portions of the pipeline already.) And how dangerous is this stuff spilling all over the ground?

But this conversation must start with a simple fact: There are too many known unknowns about diluted bitumen. We don't know exactly what's in it, and the government hasn't fully studied how safe it is to transport.

Bitumen is a form of petroleum that occurs in a solid, or semi-solid, state: It can be sludgy or even be brittle, like rocks. That’s what is buried deep in the Canadian oil sands. In order to transport this bitumen through thousands of miles of pipelines so that it can be refined, it has to first be diluted, so it flows like a liquid.

That diluent is usually a natural gas liquid—but we don’t know for certain what it is. The industry considers its diluent formulas proprietary information and won’t share it with regulators.

When the State Department released its first Environmental Impact Statement on Keystone XL, the EPA gave it an “inadequate” rating in part because it didn’t have any specific information on diluents. “We believe an analysis of potential diluents is important to establish the potential health and environmental impacts of any spilled oil, and responder/worker safety, and to develop response strategies,” the EPA said at the time.

Yet, the second and final Enviromental Impact Statement of Keystone XL released recently by the State Department still had no specific information on dilbit diluents, and evaded the question with some generalities:

The exact composition of the dilbit is not publicly available because the particular type of bitumen and diluents blend produced is variable and is typically a trade secret. The bitumen­ diluent mixture with bitumen from the oil sands is generally similar to heavy sour crude. […]

Although reported information on dilbit releases is scarce in the literature, once diluents and bitumen are mixed together to form dilbit, they behave as a conventional crude oil. Therefore, this assessment has focused on the impact of crude oil in general, but when applicable, evaluated the specific characteristics (i.e. viscosity) of dilbit. The degree of impact can vary depending on the cause, size, type, volume, location, season, environmental conditions, and the timing and degree of response actions.

Researchers and regulators know roughly what’s in dilbit—just not enough. “I think what they don’t know are what the specific chemicals are in any pipeline or any batch, because companies could use different chemicals at different times, depending on what’s cheapest at hand at any one moment,” Carl Weimer, the executive director of the Pipeline Safety Trust, told The Nation.

The composition of those chemicals could greatly influence how the dilbit behaves once spilled—whether it sinks or floats in water, for example. In an emergency clean-up situation, that’s really good information to have.

And what we do know about the danger of tar sands oil is already disturbing. The State Department’s report does focus on the damaging properties of benzene, and notes that “[b]ecause the diluted bitumen crude oils have a significant amount of lighter hydrocarbons added, they tend to have higher benzene concentrations than many other heavy oils.” Benzene, the report states, “was determined to dominate toxicity associated with potential crude oil spills.” Benzene is an unusually toxic chemical that was found in the air after the Enbridge tar sands spill of 2011, and can cause a wide range of severe physical damage to animals, plants and humans.

So the government doesn’t really know what’s in the dilbit once it spills—but it also knows shockingly little about how to prevent these accidents to begin with. Many conservation groups contend, with ample evidence, that tar sands oil leads to more spills because it is “highly corrosive, acidic and potentially unstable."

The chief regulator of pipelines is the Pipeline and Hazardous Materials Safety Administration. In 2011 Cynthia Quarterman, the agency’s director, appeared before Congress and admitted under questioning from Representative Henry Waxman that her agency had no idea whether dilbit is more dangerous to transport than normal crudes, and had not even studied the question:

REP. WAXMAN: Ms. Quarterman, when PHMSA adopted its basic safety requirements such as establishing maximum operating pressures or setting integrity management requirements, were many US pipelines transporting diluted bitumen? And were any of your regulations developed with the properties of diluted bitumen in mind?

MS. QUARTERMAN: When the integrity management program requirements were first put in place on the hazardous liquid side, I think it was 2000, 2002, there were pipelines in existence that transport diluted bitumen. I don't believe any study was done at that time of the characteristics of the crude.

REP. WAXMAN: Were your regulations developed with the properties of diluted bitumen in mind?

MS. QUARTERMAN: I don't believe it was a part of the equation, no.

REP. WAXMAN: Have you received [sic] your regulations to assess whether they adequately address any risks specific to diluted bitumen?

MS. QUARTERMAN: We have not done so.

Since that exchange, Congress passed the Pipeline Safety Act, which was panned by many environmentalists as too weak. The bill didn’t update any regulations to include dilbit transportation, but did mandate that PHMSA study the problem and act, if needed.

The National Academy of Sciences is currently working on that study, and will release its findings this summer. If they find that diluted bitumen is abnormally dangerous to transport, as many other independent researchers have, they will recommend new regulations.

But for now—and for the next several months, and probably years—the government is flying blind about how to transport diluted bitumen. It also doesn’t even know what’s in the stuff once it spills.

That’s crucial context when considering whether to build Keystone XL, which would carry millions of barrels of dilbit across the entire country.

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