3M Co. must pay for polluting Minnesota’s water, according to the Minnesota attorney general’s office.

The state sued 3M on Thursday in Hennepin County for allowing PFCs — perfluorochemicals — to leach into groundwater in Washington County over several decades. The company also allegedly discharged PFCs into the Mississippi River.

The lawsuit does not ask for specific damages. But potential damages would be in the tens of millions of dollars, which would make the case one of the largest environmental suits in Minnesota history.

Attorney General Lori Swanson said her office agreed with 3M in May to try to negotiate an out-of-court settlement. Those negotiations failed, and the deadline to reach agreement was Thursday.

“3M polluted and damaged our waters with these chemicals,” Swanson said. “The lawsuit asks the company to make right the problems caused by its contamination of our waters.”

The suit takes a novel approach by claiming the PFCs hurt the environment — but not people.

That makes it different from a landmark 2009 lawsuit. That suit filed by a group of Washington County residents alleged the PFCs harmed people who drank the water.

In that trial, lawyers pointed out that mega-doses of PFCs have been shown to cause cancer, birth defects and thyroid problems in mice. But a judge ruled, in effect, that the traces of PFCs were so small that no one was hurt.

The trial ended in a jury decision that supported 3M.

The new lawsuit takes a different tack. It says the state is a steward of natural resources and can sue when those resources are damaged.

“We take natural resources very seriously in Minnesota,” Swanson said.

To illustrate the environmental damage, the suit says the state Health Department issued warnings about eating too many PFC-contaminated fish caught in certain areas.

The suit said that in 2008, the state Pollution Control Agency listed certain areas of the Mississippi River and Lake Elmo as “impaired” under the Clean Water Act.

The chemicals were discovered in the groundwater consumed by about 67,000 people in Washington County.

Officials capped private wells, forcing homeowners to rely on bottled water or filters. In some cases, city water mains were extended to homes whose well-water supply was cut off.

“If someone says you can’t use your well at home, that is harmful to you,” Swanson said. “Or someone saying, ‘Eat the fish at your own peril.’ ”

Besides damage to the environment, there has been financial damage.

When state officials found that PFCs were leaking from a landfill in Lake Elmo, they decided in 2009 to remove all the garbage, install plastic liners and put the garbage back.

The work cost the state about $13 million; 3M voluntarily contributed an additional $8 million.

The troublesome chemicals were first developed in the 1940s. 3M used them to make such products as Scotchgard stain repellent, Teflon and fire-extinguishing foam.

For decades, 3M legally disposed of the waste products of the manufacturing process in four landfills in Washington County. It stopped disposing the wastes in landfills in the 1970s.

But in the 1990s, scientists found trace amounts of PFCs in animals and humans around the world. The chemicals can accumulate in people’s bodies and take years to eliminate.

3M stopped making PFCs in 2002, although other companies still manufacture them.

In 2004, officials found traces of PFCs in water in Oakdale and Lake Elmo. Soon, they discovered a 15-mile swath of PFC-tainted groundwater stretching from Oakdale to Hastings.

3M has been removing PFCs from groundwater for several years. By 2007, it had spent about $56 million on various cleanup efforts, such as pumping the groundwater out, filtering it and returning it to the ground. The company has since stopped giving estimates of its cleanup costs.

It has removed about 1,600 truckloads of contaminated soil from sites in Woodbury and Cottage Grove, and will start removing soil from the Oakdale site in January.

The issue of harm to humans remains controversial.

The suit says “numerous studies have shown that PFCs pose serious risks to human health and the environment.”

But 3M has said — a claim bolstered in court — that while mega-doses of the chemicals cause health problems in mice, the traces in Washington County water are harmless.

“3M is surprised by those kinds of claims,” said company spokesman Bill Nelson. He said claims of personal injury are not part of this suit.

The lawsuit also cited a study of 3M employees exposed to PFCs. “A recent study of 3M employees showed a positive association between some types of PFC exposure and prostate cancer, cerebrovascular disease and diabetes,” said the study, cited in the lawsuit.

Nelson pooh-poohed that report. Like many others, he said, it shows a “statistical association” but doesn’t show PFCs caused any illness. If 3M employees got sick, he said, it could be for any number of reasons — such as obesity or smoking.

“The entire body of evidence, as much as we know, shows there is no adverse health effect by levels we see in the environment,” Nelson said.

Swanson said it will be many months before the case reaches court.

Bob Shaw can be reached at 651-228-5433.