Another oil spill at a U.S. Steel plant in northwest Indiana sent an oily sheen onto a Lake Michigan tributary, prompting several lake closures.

The spill is the second detected in less than two weeks at the U.S. Steel Midwest plant in Portage. It was discovered Thursday morning on Burns Waterway, just outside the plant. By 8 p.m., it was no longer present on the tributary.

According to U.S. Steel spokeswoman Amanda Malkowski, an existing boom had contained the oil in an estimated 120 square foot area. She adds no oil was seen entering Lake Michigan.

Regardless, several precautions have since been put in place:

– U.S. Steel temporarily idled the plant 30 miles east of Chicago. It was back to normal operations by Thursday night.

– The Indiana Dunes National Park closed access to the lake on Thursday, off the neighboring Portage Lakefront and Riverwalk. Visitors can still go to the lakefront, but not into the water.

– Ogden Dunes also shut down access to Lake Michigan from its beach. Indiana American Water’s Ogden Dunes water treatment facility remained online.

The Indiana Department of Environmental Management is investigating.

In September, the U.S. Steel plant in Portage discharged iron-tainted wastewater into the Burns Waterway. The discharge led to the temporary closures of the Portage Riverwalk and Lakefront, as well as beaches at the Indiana Dunes National Park. Last week, U.S. Steel said that first spill was partially caused by a vendor, who failed to deliver sulfuric acid needed for wastewater treatment.