Environmental Protection Agency officials say small dams erected on a bank overnight appear to be containing an oily muck that's seeping into a Denver-area creek. Officials are trying to keep it from reaching the South Platte River, which is a major source of drinking water, wildlife habitat and agricultural water for Colorado and the Midwest.The oil seep is located near Interstate 270 and Vasquez Boulevard, about one mile west of the Suncor Energy Refinery in Commerce City.Suncor Energy emergency response crews found the seep Monday afternoon after responding to a complaint from a resident, who called around 11:30 a.m. to report a visible sheen on Sand Creek.The slick stretched about 200 yards on Monday, said John Gallagher, vice president of refining for Suncor."We believe, from what we've seen, it's some form of hydrocarbon. We don't know the source of the material, and we don't know what the material is exactly," Gallagher said. EPA spokeswoman Karen Edson said Wednesday that the dams seem to be containing most of the substance. Workers are vacuuming the heaving muck out of the isolated area created by the dams but a sheen remains on the creek. State health department spokesman Warren Smith says the toxic spill is believed to be coming from Suncor's refinery. Tests are under way to identify the chemicals and the source.Suncor produces crude oil and refines it for the production of gasoline, diesel, jet fuel, and asphalt.