PORTAGE, Ind. -- A spill at a U.S. Steel plant in northern Indiana that sent wastewater containing a potentially carcinogenic chemical into a Lake Michigan tributary was apparently caused by a pipe failure, the steelmaker said Wednesday.Tuesday's spill of an unknown amount of wastewater led to the closure of three beach areas at the scenic Indiana Dunes National Lakeshore and prompted a local water utility to stop drawing water from the lake out of "an abundance of caution," the U.S. Environmental Protection Agency said.U.S. Steel said a preliminary investigation shows an expansion joint failed Tuesday in a pipe at its Portage, Indiana, facility, allowing wastewater from an electroplating treatment process that contains hexavalent chromium to flow into the wrong wastewater treatment plant at the complex.That wastewater eventually flowed into the Burns Waterway, a lake tributary, at a point about 100 yards from Lake Michigan, the EPA said.The spilled wastewater came from a process used to treat steel after it has been electroplated, the steelmaker said in a statement. It said that wastewater is supposed to flow into a special treatment plant, but the pipe failure prevented that from happening.U.S. Steel has halted all production processes at the Portage facility as it works with the EPA, state and local officials to respond to the spill, including repairing the damaged pipe and removing the hexavalent chromium.The EPA, which is overseeing the response to the spill, said the spilled wastewater eventually entered the waterway about 100 yards from Lake Michigan from a drainage pipe.No hexavalent chromium has been detected in the lake following the release of an undetermined amount of toxic wastewater, but monitoring in the area continues, the federal agency said.The EPA has said that hexavalent chromium - a toxic byproduct of industrial processes - might be carcinogenic if ingested. The toxic heavy metal is used in a variety of industrial processes, including steelmaking and corrosion prevention, and as a pigment in dyes, paints and inks.Utility Indiana American Water said it has stopped taking water from Lake Michigan at its nearby Ogden Dunes water plant and is relying on reserves. The utility said Wednesday that preliminary water sampling by an independent laboratory under the oversight of the Environmental Protection Agency had found no hexavalent chromium in the water treated and stored by that plant.The National Park Service has temporarily closed the West Beach and Portage Lakefront beachfront areas and warns that people and pets must avoid contact with water there until further notice.