"The National Park Service is very concerned that there have been several discharges from the plant during the last year and will continue to closely monitor the situation," said Rowe. "The park is maintaining close contact with IDEM and U.S. Steel as this investigation continues."

"We did speak with IDEM about a frothy substance in the area that we were made aware of, and they have told us they saw no evidence of a spill occurring and that there is no visible plume in the area around our Ogden Dunes intake," Joe Loughmiller, spokesman for the Indiana American Water Co., said, adding both of the company's Lake Michigan intakes/water treatment facilities are continuing to operate normally.

The plant has had a history of incidents raising concerns for the environment.

On April 11, 2017, nearly 300 pounds of hexavalent chromium -- or 584 times the daily maximum allowed under state permitting laws -- discharged into the Burns Waterway.

Johnson said the material spotted being discharged is not hexavalent chromium, which is green in color.