A peculiar sight greeted motorists on a busy freeway outside Detroit on Friday: a bright green liquid oozing from a retaining wall on Interstate 696.

Its appearance snarled traffic and prompted lane closings to allow for a hazardous materials cleanup that started on Friday and could last for several more days, Michigan environmental officials said.

The mystery substance is believed to be hexavalent chromium, said Jill A. Greenberg, a spokeswoman for the Michigan Department of Environment, Great Lakes, and Energy.

“It’s a serious situation,” Ms. Greenberg said on Sunday. “Clearly, this site needs to be cleaned up.”