MARSHALL — The cleanup of the Midwest’s worst oil spill in Calhoun County last July 25 continues a year later, and officials from both Enbridge, the company that owns the pipeline, and the government agencies overseeing it say the spill has ‘unique challenges.’ “Capturing and cleaning up this heavy oil is a unique challenge,” said Susan Hedman, Environmental Protection Agency Region 5 director, during a press briefing last week. “No one at the EPA can remember dealing with this much submerged oil in a river.” The news comes on the one year anniversary of the spill.