The 3M Co. won a landmark court case Wednesday when a jury ruled against claims of Washington County residents suing the company over chemicals in their groundwater.

For 3M, it was a triumphant end to a five-year case that once loomed as one of the largest environmental lawsuits in Minnesota history.

“Obviously, we are pleased with the verdict. It was supported by the evidence,” 3M spokesman Bill Nelson said.

The jury delivered the unanimous verdict with surprising speed — deliberating four hours to decide a case that involved five weeks in court, 35 witnesses, eight law firms and more than 300 exhibits.

It is not known if the verdict will be appealed. The plaintiffs and their lawyers could not be reached Wednesday afternoon.

Lawyers for four plaintiffs — Gary and Karen Paulson, Brad Krank and Bill Henry — argued that 3M negligently allowed chemicals to seep into groundwater, which damaged the value of their homes by as much as 25 percent.

But the nine-member jury sided with 3M. The company’s lawyers argued that from the 1950s to the 1970s, 3M disposed of chemical wastes in ways appropriate for the times. The company worked to clean up the chemicals as it learned more about them, lawyers said.

The verdict was a decisive blow to a case that had been deteriorating for years.

Originally, it had the potential to be enormous — with as many as 67,000 plaintiffs and hundreds of millions in damages.

Maplewood-based 3M began making perfluorochemicals (PFCs) in the late 1940s for household products, such as Teflon and Scotchgard stain repellant.

One PFC plant was the Chemolite Plant in Cottage Grove. For decades, sludge left over from the manufacturing process was disposed legally in four landfills in Washington County.

In the 1990s, 3M officials became alarmed at studies showing PFCs appearing in the bodies of animals and humans around the world. Megadoses of the chemicals given to laboratory rats caused health problems, including cancer, thyroid problems and birth defects.

Traces of PFCs were found in the drinking water of Lake Elmo and Oakdale in 2004.

Lawyers found PFCs accumulating in the bodies of local residents, and the case began. At one point, the number of residents joining the lawsuit ballooned to more than 1,000.

In 2005, the potential size of the suit became clear when a $300 million judgment was levied in an Ohio case involving the same chemicals. The DuPont Corp. had bought PFCs from 3M, and they were found in area groundwater.

The court ordered DuPont to clean the PFCs out of the water, but did not award damages for any harm to the health of water drinkers.

The lawsuit against 3M was given a boost in 2006 when related chemicals were found in a 15-mile swath of Washington County, including Woodbury and Cottage Grove. Plaintiffs’ attorney David Byrne called the PFCs “a recurring environmental nightmare.”

But 3M acted aggressively. It spent more than $50 million related to the PFCs, including paying for filters on an Oakdale city water plant and water-cleaning systems near landfills.

The company also paid to connect homes, including those of the plaintiffs, to city water in Oakdale and Lake Elmo.

Meanwhile, evidence grew to support 3M’s claim that the traces of PFCs were harmless.

State officials said in 2008 that, thanks to the cleanup efforts, PFC levels in water were well below limits considered safe.

A study by the Minnesota Department of Health found no increases in cancer or other ailments in the affected area — despite years of exposure to PFCs in drinking water.

It was calculated that a Woodbury resident would have to drink 500,000 glasses of the water at once to get a PFC dose at the level believed to be harmful in mice.

Legally, the case was unraveling well before it went to court in May.

In 2007, District Judge Mary Hannon decided that all people exposed to the polluted water could not sue 3M collectively in a class-action lawsuit. They would have to sue individually — a more cumbersome and expensive process.

In December 2008, she ruled that no arguments about the health effects of PFCs would be considered in the trial — as much as agreeing with 3M’s argument that traces of the chemicals were harmless.

The remaining issues in the lawsuit were damages to property values up to about $200,000, and whether 3M was negligent in handling the chemicals.

Those remnants of the lawsuit were put to rest Wednesday — unless the plaintiffs decide to appeal the case.