The City of Chicago is suing US Steel after the company dumped toxic metal into Lake Michigan in October.

US Steel leaked nearly 60 pounds of chromium, 89% of the amount permitted over 24 hours, into a Lake Michigan tributary on 25 October after a wastewater treatment system malfunctioned at the company's northwest Indiana plant. According to WGN9, it is the second time this has happened in the past six months.

Chicago Mayor Rahm Emanuel claims US Steel tried to keep the leak a secret by requesting "confidential treatment" while reporting it to the Indiana Department of Environmental Management.

The company notified the Environmental Protection Agency (EPA) three weeks after the incident, the agency said.

Emanuel alleges the company had repeatedly violated the Clean Water Act and said the City of Chicago will file a notice of intent to sue US Steel on Monday (20 November) morning, The Chicago Tribune reported.

"Not only was the discharge way in excess of what is permitted. It happened twice," the mayor said. "In every area they're in violation. Now you have to ask, 'What is the company thinking about,' and then more importantly, 'What is the Trump EPA doing?'"

The lawsuit is part of an effort to protect Lake Michigan for residents, Emanuel said.

"Lake Michigan is our most precious natural resource; we have a responsibility to preserve it, we have a responsibility to protect it and we must pursue and punish those who pollute it," he said.

"It's unacceptable and it's not an accident that US Steel did not report the incident to the EPA, because they think that, in fact, there is nobody there that they are responsible or accountable to," Emanuel added. "Chicago will fight against that mindset."

The mayor noted that despite the toxic chemical spill, drinking water in Chicago remains safe.

In a press release on Sunday (19 November), US Steel said the spill "did not pose any danger to water supply or human health". The company said it was committed to complying with environmental standards and safeguarding the environment.