Rep. Ed Markey (D-Mass.) sent a letter to ExxonMobil Tuesday seeking clarification about the "troubling and apparently conflicting information" given by the company regarding its March 29 pipeline rupture that dumped at least 210,000 gallons of Canadian oil in an Arkansas neighborhood.

The letter, addressed to ExxonMobil CEO Rex Tillerson, asks the company to address conflicting reports about when the pipeline began leaking and when and how the company first became aware of the spill. Police reports obtained by InsideClimate News suggest that Exxon may not have known about the spill until contacted by Arkansas officials. But according to what Exxon reported to the federal government, the company may have known about a pressure drop in the line hours before it sent responders to the scene of the rupture.

Markey's letter also asks whether the oil that spilled from its Pegasus pipeline was diluted bitumen, or dilbit, from Canada's tar sands region, and whether it was exempt from paying into the Oil Spill Liability Trust Fund, a federal cleanup fund that does not tax dilbit.

In an April 5 letter to the U.S. Environmental Protection Agency, an Exxon lawyer said the line was carrying dilbit, contradicting public assertions by company officials that the spilled oil was simply "heavy oil."

"In order to respond most effectively to the spill, it is critical to have a clear understanding of the make-up of the spilled oil," Markey wrote. He cited the ongoing cleanup of the 2010 dilbit spill in Michigan's Kalamazoo River, where workers are dredging submerged oil from the riverbed. "Because of its unique properties, tar sands oil had to be cleaned up up at every level of the river—the top, the middle, and the bottom, where it sunk into the sediment."

Exxon's website says the company is not using the Oil Spill Liability Trust Fund for cleanup, yet the fund "is currently being drawn down to help fund clean-up efforts in Mayflower," Markey wrote. "It is very important that ExxonMobil reimburse the Fund for these costs in a timely fashion."

Markey concluded his letter with a list of 19 questions about Exxon's cleanup plans, emergency response protocols and the chemistry of the oil that spilled in Arkansas.

The letter to Rex Tillerson, CEO of ExxonMobil, from Rep. Markey:

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