Federal regulators have ordered Minnesota to impose more stringent limits on pollutants discharged into the state's lakes and rivers, an unusual step that could threaten state authority to enforce the nation's clean-water laws.

The order, the first of its kind for Minnesota, was issued by the U.S. Environmental Protection Agency (EPA) in response to a petition from a nonprofit environmental law firm that for years has accused the Minnesota Pollution Control Agency (MPCA) of lax enforcement in protecting the state's waters.

The petition focused primarily on municipal wastewater treatment and phosphorus, a damaging contaminant that causes noxious and sometimes toxic algal blooms in lakes and rivers. But some say it also could require the state to tighten up on a wide range of pollutants.

"The direction is pretty clear," said Kris Sigford, an attorney for the Minnesota Center for Environmental Advocacy, which asked the EPA to intervene. "Now we'll have limits that are designed to protect water quality going into permits for all dischargers."

MPCA officials said the agency is studying the order and has about 30 days to respond.

It has been discussing the matter with the EPA for some time and already has made changes in phosphorus regulation, said Wendy Turri, MPCA's municipal wastewater manager. "At this point I think we are doing what we need to do," she said.

The order, which was issued May 15, is the first time Minnesota has faced such an action since receiving authority to enforce federal pollution laws in 1974.

If it does not comply, federal law would allow the EPA to take over the job of writing all pollution discharge permits in the state, which now cover about 1,000 municipalities, businesses and other potential polluters.

But Minnesota is not alone. State regulation of pollutants — especially nutrients such as phosphorus and nitrogen — is a contentious issue nationwide, said Al Ettinger, a Chicago attorney who represents environmental groups on water quality issues. About a dozen states face similar petitions pending at the EPA, most filed by citizen groups that say state regulators are not doing enough to enforce the Clean Water Act.

The federal agency also has stepped in and ordered changes in Iowa, Illinois and Florida, he said.

In essence, the EPA is saying that "the citizens have a legitimate complaint, and if you don't fix it we will have to do something," Ettinger said.

Lake Winona

In Minnesota, the struggle started with a wastewater treatment plant in Alexandria that serves about 23,000 people and discharges water into Lake Winona, a small, shallow lake nearby. The lake has long had problems with phosphorus, and with carp that aggravate the problem by stirring up the pollutant from the bottom, where it collects in the muck. But Lake Winona also feeds into a chain of popular recreational lakes.

In 2007 Sigford's organization sued the MPCA, claiming that it allowed the wastewater treatment plant to discharge too much phosphorous. The plant should not be allowed to add phosphorus to a lake where levels are excessive, the law firm argued.

The case eventually went to the Minnesota Supreme Court, which ruled in the state's favor. Then in 2008, the law firm filed its petition with the federal government.

Now, according to the EPA's order, the state must write permits with numeric limits tied to specific water-quality standards. In essence, the Alexandria plant and others like it have to keep their discharge at levels designed to protect the lakes and rivers that receive it.

"This is potentially critical for the Mississippi watershed as a whole," said Trevor Russell, program manager for Friends of the Mississippi River.

Bruce Nelson, executive director of the Alexandria Lakes Area Sanitary District, said the plant removes as much phosphorus as possible. The nutrient comes from industrial customers, which have taken steps to reduce their discharges, and households, where it comes from consumer products and human waste.

"No matter what MPCA says, it was impaired for [phosphorus] 100 years ago," he said of Lake Winona. "We didn't cause it."

He also argued that the water quality in Lake Winona and the downstream lakes is far better than it used to be.

There are some treatment plants that, at great expense, have brought phosphorus levels down to barely detectable levels, he said. But he said he's not sure that investing the $20 million or $30 million in a new treatment process will necessarily fix the problem for Lake Winona, or be worth the expense.

Nonetheless, that's what the plant may have to do when it gets a new permit in a few weeks, he said, but it will have six or seven years to accomplish the task. It also has to devise a plan to reduce chloride, another significant contaminant that comes from water softeners most of the area's households use to treat the region's hard water.

"We've found the problem," Nelson said. "And it's us."

Josephine Marcotty • 612-673-7394