Northern Indiana Public Service Company will be required to invest $600-million to control pollution as a result of a settlement with the U.S. Environmental Protection Agency.

According to the settlement, NIPSCO, based in Merrillville, violated the Clean Air Act when it failed to obtain permits before it altered four coal-fired power plants in the 1980s and 1990s. Moreover, the installed equipment may not have been state-of-the-art at the time it was installed. That is considered a violation of the act’s so-called “new source review” provisions.

NIPSCO has agreed to invest in pollution control technology at plants in Michigan City, Chesterton and Wheatfield. A fourth plant in Gary, called the Dean Mitchell Generating Station, will be permanently shuttered.  It’s been closed since 2002, although there have been efforts to restart it to produce more electricity.

The EPA says the settlement will improve the health of residents in Northwest Indiana and surrounding areas by substantially cutting emissions of sulfur dioxide, nitrogen oxides, particulate matter and carbon dioxide.

Apart from the pollution control investments, the settlement requires NIPSCO to spend $9.5 million on environmental mitigation projects and pay a civil penalty of $3.5 million.  

NIPSCO is the second largest electric distribution company in Indiana, supplying power to about one-third of the state.  A company spokesman said many of the settlement provisions are already completed or are in the works.

A federal judge must still approve the settlement.