An ExxonMobil Corp. oil pipeline spill late last month in Mayflower, AR, has left many questions in its wake for regulators and state officials about what the accident may mean for future pipeline projects from, including the controversial Keystone XL project that would carry oilsands crude to Gulf Coast markets.

ExxonMobil noted that the cleanup was continuing. “Much of the free-standing oil has been recovered,” the company said Thursday. There were nearly 600 people responding to the incident.

The cause of the spill on March 29 remained under investigation, but ExxonMobil has reiterated that the spill did not involve tar sands oil. An industry source noted that may be splitting hairs because there is only a technical difference in heavy crude and tar sands-produced oil. They are both heavy in viscosity and require help to be more transportable.

Arkansas Attorney General Dustin McDaniel has opened an investigation into the spill and directed the company to “preserve and maintain” all documents. McDaniel said he is investigating the cause and impact of the rupture of ExxonMobil’s 850-mile Pegasus pipeline, which at times has carried tar sands oil from Canada, although not at the time of the spill.

ExxonMobil said the Pipeline Hazardous Materials Safety Administration also has weighed in, citing the company with a corrective action order. ExxonMobil’s website describes the Pegasus line as “a 20-inch [diameter] pipeline, which originates in Patoka, IL, and carries crude oil to the Texas Gulf Coast.”

The company said nearby Lake Conway, AK, remained oil-free and Mayflower’s water supply has not been impacted. The number of residents filing claims had climbed to 140 last week as continuous air quality monitoring continues by ExxonMobil, the U.S. Environmental Protection Agency and Arkansas’s Department of Health.

The spill hardened opposition in the high-stakes business of finding markets for Western Canada’s oilsands crude. A coalition of environmental groups in California called for an investigation last week of large-scale shipments of heavy Canadian crude being transported through the West regularly en route to Southern California refineries.

Representatives from the Natural Resources Defense Council and Communities for a Better Environment urged closer scrutiny of potential air quality fallout from the processing of the Canadian crude in California.

They called attention to the Los Angeles Harbor community of Wilmington, where three refineries reside: Valero Energy Corp., Phillips 66 Co., and Tesoro Corp. Valero CEO Joe Gorder told shareholders recently that his company plans to import an added 30,000 b/d of Canadian crude to its Wilmington refinery, the Los Angeles Times reported Wednesday.

An engineer with the South Coast Air Quality Management District, which regulates refineries, told the Times that while officials cannot force refineries to divulge the source of their supplies, the district has the “most stringent rules” in the nation for refinery operations. The same strict rules apply regardless of where the crude comes from.

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