Conservation groups filed a lawsuit Monday alleging that South Carolina government regulators illegally gave Santee Cooper a green light to build a new coal-fired power plant on the Great Pee Dee River that would pump 31 times more toxic mercury into the air than the legal limit.

The lawsuit against the state Department of Health and Environmental Control moves the Pee Dee plant debate to the courtroom.

Officials with DHEC declined to comment, but Laura Varn, vice president of corporate communications for Santee Cooper, said the utility had been expecting the suit. "We remain focused on meeting the growing energy needs of South Carolinians in a balanced, affordable, reliable and environmentally sensitive manner," she said.

The suit follows a split vote last month by DHEC's board to OK the plant's air pollution permit. Several DHEC board members, including those who sided with Santee Cooper, said they were troubled about the plant's mercury emissions.

The suit also could generate debate over carbon dioxide, the main cause of global warming. The groups contend that DHEC should regulate carbon dioxide, citing a 2007 U.S. Supreme Court case that said carbon dioxide was an air pollutant under the Clean Air Act.

In the past, Santee Cooper has argued that carbon dioxide isn't regulated now, so it wouldn't make sense for DHEC to base its decision on rules that don't exist. The conservation groups argue that given the Supreme Court's decision, DHEC should have required Santee Cooper to identify ways to reduce carbon emissions.

"This plant would add mercury pollution to an already contaminated region of the Pee Dee, but DHEC waived the maximum mercury controls required by law," said Blan Holman, an attorney for the Southern Environmental Law Center. "The tragedy of DHEC's decision is that we have far cheaper and cleaner options than a $2.5 billion coal plant, and those options would generate thousands more jobs than this dinosaur ever would." The groups argue that demand for power could be met with natural gas plants and conservation.

In the lawsuit, the Southern Environmental Law Center represents the Environmental Defense Fund, League of Women Voters of South Carolina, the Coastal Conservation League, Sierra Club and South Carolina Wildlife Federation.