INDIANAPOLIS | The state's coal-fired power plants would have roughly two decades to reduce toxic mercury emissions by two-thirds under preliminary rules approved Wednesday.

The Indiana Air Pollution Control Board voted 7-4 to advance the regulatory plan after more than five hours of contentious testimony from environmental groups and electric industry officials.

Executives from Merrillville-based NiSource and the parent company of State Line Energy in Hammond urged the panel to move ahead with the proposed rules, which meet minimum standards set by the U.S. Environmental Protection Agency.

The Michigan City-based Save the Dunes Council, and other environmental advocates argued Indiana should follow Illinois and other states that have adopted more aggressive plans to curb release of the toxin.

"Strong protective mercury regulation is indeed affordable," said Conni Clay, resource specialist for Save the Dunes. "Currently, nine states have finalized rules with stronger mercury controls, including the coal states of Illinois and Pennsylvania."

The Hoosier Environmental Council asked the panel to consider its plan to reduce mercury emissions 90 percent by 2010.

But installing smokestack scrubbers and other controls necessary to meet that timetable would cost the industry $207 million to $373 million a year through 2010, said Stan Pinegar, vice president of the Indiana Energy Association. The cost would be passed on to ratepayers, he said, and the state economy also could take a hit.

"Low electric rates are one of the top advantages Indiana enjoys," Pinegar said.

Robert Asplund, projects manager for Dominion, the Virginia energy firm that owns the State Line plant in Hammond, said limitations in current monitoring technology would make it impossible to even tell whether facilities were meeting a reduction target as high as 90 percent.

The hearing had a subplot, with at least one environmental group charging that Pollution Control Board member Jeffrey Quyle, an appointee of Gov. Mitch Daniels, should stay out of the debate because of a potential conflict of interest.

Quyle, a Morgan County commissioner tabbed to fill the local government seat on the board, holds an economic development post with Bloomington-based Hoosier Energy. Quyle said state ethics rules did not prevent him from weighing in on the mercury regulations, but he ultimately decided to abstain from voting.