Burning coal in a power plant creates byproducts called fly ash and bottom ash. That ash contains a lot of bad stuff - mercury, lead, arsenic, to name a few.
While some plants ship the dry ash to landfills that accept hazardous materials, others mix the ash with water to make a slurry, which is moved into holding ponds.
Eventually, the water in those ponds is released into the nearest waterway.
The U.S. Environmental Protection Agency acknowledges that the slurries from coal-fired plants account for more than half the toxic pollutants dumped into U.S. waters by regulated industries.
Now, after being sued by two environmental groups over the issue, the agency is preparing to set the first-ever federal limits on toxic metals in wastewater from coal-fired power plants.
Depending on the EPA's final decision, the rule could affect either six of Michigan's power plants - or just one.
Under the most stringent interpretation of the proposed rule, six Michigan power plants would likely have to recirculate their wastewater and treat it. They include Trenton Channel Power Plant in Trenton, St. Clair in East China Township, Belle River in China Township, and Monroe Power Plant, all owned by DTE.
One other, owned by Consumers Energy, could also come under the new rules - JH Campbell in West Olive, along with a muncipal-owned plant, J.B. Sims Generating Station, owned by the city of Grand Haven.
Two others are on the EPA's list of potentially affected plants, but they are already slated to be shut down before the new rules would go into effect in 2017 - BC Cobb in Muskegon, owned by Consumers Energy, and Harbor Beach, owned by DTE.
But under the least stringent interpretation of the proposed rules, only one plant would have to make modifications. That's DTE's Monroe plant.
Spokesman Scott Simons says the changes could cost DTE $200 million -- and that will get passed on to customers.
"We do believe the recirculation of the water that's been mixed with bottom ash at existing plants is far more expensive than the environmental benefit, which is expected to be minimal," says Simons.
That's not the view of environmental groups. Hugh McDiarmid is with the Michigan Environmental Council.
"We have for decades willingly allowed coal-fired utilities to write off some of their costs," says McDiarmid. "We absorb the cost of people who get sick because of the pollution, we pay that through health care premiums, (and) the natural resources that are damaged that have to be cleaned up or mitigated, we pay from other funds."
The EPA will have a public comment period, but, due to sequestration, only one public hearing on the issue, in Washington, next month.
The EPA is under a court-ordered deadline to finalize its rule by late May of next year.