Responders working for Enbridge Energy at the site of a 2010 oil spill in Michigan are going to be on site "for years," a spokesman said. Thursday marked the second anniversary of the costliest onshore oil pipeline release in U.S. history. Federal regulators are investigating the 2010 rupture of Line 6B, part of the Enbridge-operated Lakehead pipeline system. The rupture released about 20,000 barrels of oil into southern Michigan waters starting July 26, 2010. The U.S. Environmental Protection Agency warned residents last month that sheen may still be present in parts of the Kalamazoo River and nearby waterways. Some could be associated with residual oil, some may be natural or tied to recreational vehicles. Line 6B was transporting so-called tar sands oil at the time of the release. The nature of that type of crude oil causes it to sink and mix in with river sediment. Enbridge spokesman Jason Manshum told regional broadcaster WOOD-TV the company wasn't done with its cleanup operations. "We're going to be here for years, working with the (Michigan Department of Environmental Quality) in the water and the overbanks to ensure that this is the natural resource that it should be," he said. Some of the waterways affected by the spill were reopened in April. Enbridge aims to replace hundreds of miles of Line 6B under a $268 million plan that would upgrade the pipeline's safety features and increase its volume to 500,000 barrels of oil per day.