U.S. Steel reported the latest spill in a letter that asked Indiana regulators to keep it secret. Law students at the University of Chicago discovered the letter last month while researching pollution problems at factories along the southwest shore of Lake Michigan for Surfrider, a nonprofit group that represents Great Lakes surfers.

"It is shocking and mind-boggling," Mark Templeton, director of the Abrams Environmental Law Clinic at U. of C., said about the new documents. "Once U.S. Steel observed that the illegal chromium discharge was blue and full of solids, did they not run the test because they didn't want to know the results?"

Templeton is preparing a lawsuit that will accuse U.S. Steel of repeatedly violating the federal Clean Water Act since 2011. Chicago Mayor Rahm Emanuel announced last month that the city is drafting its own lawsuit, citing the steel mill's close proximity to a Lake Michigan drinking water intake off 68th Street.

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