The problem with pollution from last year\'s Enbridge oil spill in southern Michigan isn\'t about what is seen but what isn\'t seen, EPA officials said.More than a year after an oil spill near Marshall, Mich., the U.S. Environmental Protection Agency said it was still working on remediation efforts in Talmadge Creek and the Kalamazoo River.Line 6B of the Lakehead oil pipeline ruptured in July 2010 near Marshall, Mich. The EPA said it estimated more than 23,000 barrels of heavy oil from Alberta tar sands spilled from the pipeline.The nature of oil from tar sand deposits causes some of it to sink to the bottom of the river, where it has soaked about 6 inches of sediment. Mark Durno, an emergency response official with the EPA, told an audience at a meeting in Marshall that much of the remaining oil pollution wasn\'t immediately apparent. \"The river looks great,\" he was quoted by West Michigan\'s WZZM-13 as saying. \"The problem isn\'t what you see. It\'s what you don\'t see.\"EPA officials said there was about 2,000 barrels of the original spill lingering at the bottom of the Kalamazoo River.\"There is still submerged oil spread along 35 miles of the river impacted by the spill,\" said EPA regional administrator Susan Hedman.The EPA said it has incurred $29.1 million in cleanup costs, which Enbridge will be required to reimburse.