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What leaked in Mayflower?

Not tar oil, Exxon says.

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Some Wabasca heavy crude is produced from the Wabiskaw oil sands formation in northeastern Alberta, Canada and contains dense bitumen — tar. It is so dense that it to pump it through conventional pipelines it must be diluted with other liquids, and is referred to as "diluted bitumen."

ExxonMobil, however, says that while the Wabasca heavy crude that leaked in Mayflower comes from the same Athabaskan formation in Canada that includes the Wabsika oil sands, it does not contain tar.

ExxonMobil posted a story April 5 on its website, exxonmobilperspectives.com, called "Five Lies They're Telling You about the Mayflower Pipeline Spill." The company writes: "The crude that spilled is Wabasca heavy oil and it's from Alberta near the area where there is oil sands production. It's produced by conventional production methods — in other words by drilling a well into the ground through which the oil flows — and diluted by a light oil to help it flow through the pipeline."

The difference is important: It's more difficult to clean tar sands oil from streams and lakes than conventional oil. The heavier compounds in bitumen oils can separate from their lighter components and sink to the bottom, mixing with sediments.

Bituminous or not, Wabasca crude is dangerous. The chemicals in Wabasca heavy, according to Exxon, include the toxic chemicals benzene, cyclohexane, ethyl benzene, hydrogen sulfide, n-hexane, naphthalene, polynuclear aromatic hydrocarbons, sulfur, toluene and xylenes. It is extremely flammable; vapor accumulation can cause an explosion if ignited. Prolonged exposure may endanger health. It may cause cancer.

According to a Corrective Action Order issued to Exxon Pipeline Co. by the federal Environmental Protection Agency following the spill, a total of between 147,000 and 210,000 gallons of the stuff leaked into Mayflower's Northwoods Subdivision off Starlite Road when Exxon's 20-inch Pegasus pipeline burst the afternoon of March 29. The oil continued to flow until 3 a.m. March 30, after the valves, 18 miles apart, were shut.

The heavy crude flowed west along North Starlite Road into a ditch that followed the Union Pacific railroad line and then flowed east under Interstate 40 into a cove of Lake Conway.

The 858-mile Pegasus pipeline connects Patoka, Ill., with Nederland, Texas.

The density of Wabasca heavy could cause a whole new set of problems now that large amounts of it have reached the unnamed cove, which is south of state Highway 89. (The cove's culverts that allow water to enter the main body of Lake Conway have been blocked with plywood and gravel, according to Keith Stephens, a spokesman for the Game and Fish Commission, which owns the cove and lake.) Photos taken last weekend by activists who sneaked into an area near the cove that has been declared off-limits by ExxonMobil showed a johnboat floating on a veritable lake of heavy, black oil that stretched into a wetland. The EPA said Exxon has vacuumed up 16,329 barrels of oil and water from the site.

In July 2010, an Enbridge Co. pipeline break spilled 840,000 gallons of bitumen-heavy oil into the Kalamazoo River near Marshall, Mich. Three years later, Enbridge is still working to clean up that spill, with plans now calling for the dredging of large areas of the river to remove sediments contaminated with submerged oil.

At a press conference on April 6, Mark Weesner, ExxonMobil's on-scene coordinator in Mayflower, maintained he'd never heard the term "diluted bitumen."

Asked about whether ExxonMobil was monitoring the bottom of the cove to see if any petroleum has sunk, Weesner said there is a sampling program going on, with information being passed along to the Department of Health.

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