A half dozen coal-fired power plants in Georgia face tougher federal rules that could force them to spend millions of dollars, or even shut down some, to reduce heavy metal pollutants in the state’s rivers and ground water.

Original permits allowing higher levels of pollutants are long past their initial expiration dates. But Georgia Power and Georgia’s environmental regulator said the permits have been “administratively extended” — by as much as 12 years in one case — and remain in force.

In a joint letter to Georgia’s Environmental Protection Division this week, the Sierra Club and other groups said the permits have expired and called on the state to replace them with new permits based on tougher water pollution limits that went into effect in January.

The groups said the permits for all of the state’s coal-fired plants are 4 to 12 years past original expiration dates. The permits are typically supposed to go through regulatory review and renewal every five years.

“These permits lag woefully behind what is necessary to protect rivers and drinking water in the state from mercury, arsenic, and other dangerous pollutants,” Sierra Club lawyer Zachary Fabish said in the letter to state regulators.

Regulators and utilities have until 2018 to get the new permits issued, in most cases.

The permits typically regulate waste water coming from systems that transport coal ash to storage ponds, and contaminated water from so-called “scrubbers” that remove sulfur from smokestacks.

“We agree that they need to be reviewed and renewed, and we are working on those permits,” said Kevin Chambers, a spokesman for Georgia EPD. He said the process will take the “next couple of years.”

Georgia Power spokesman Jacob Hawkins said the utility, which owns five of the plants, is in full compliance with the extended permits.

“Sierra Club is clearly attempting to present misleading and inflammatory information to further its agenda and try to stop the use of coal-fired generation,” he said.

Coal-fired power plants already face challenges on several fronts, including tighter federal rules on air pollution and storage lagoons that hold toxic coal ash, and financial pressures to close or switch coal plants to cheaper, cleaner-burning natural gas.

Earlier this week, Georgia Power announced plans to close 29 coal ash lagoons in the state, most within 10 years.

It will likely cost “huge sums of ratepayer money,” the environmental groups said, to upgrade the plants to meet the tougher pollution rules in order to get new permits.

The environmental groups called for new permits ahead of a 2018 deadline to meet federal pollution rules for waste water. They also called for a 60-day public comment period on proposed renewal permits, and for utilities to close plants that can’t be economically upgraded.

“Hardworking Georgians don’t deserve to see their money wasted on expensive, outdated and environmentally damaging energy sources,” said Ian Karra, of the environmental groups’ Beyond Coal campaign in Georgia.

Five of the plants are owned by Georgia Power: Wansley, near Carrollton; Hammond, near Rome; Scherer, near Macon; Bowen, near Eurhalee; and McIntosh, near Rincon. McIntosh has gone without a new permit since mid-2004 — long past the typical five-year duration of the permits, said the groups.

A sixth plant, near Cordele, is owned by Crisp County Power Commission. Its permit expired in 2010, according to the groups.