A day after signing a landmark decision assigning blame for the dramatic depletion of White Bear Lake, Judge Margaret Marrinan was making the rounds Thursday at the Ramsey County Courthouse, handing out doughnuts.

Having just turned 70, it was her last day on the bench. A peppery pioneer who more than 30 years ago walloped at the polls a judge who referred to female lawyers as “lawyerettes,” Marrinan was hours away from becoming just Peg.

Or perhaps even “Sweet Peg,” a sardonic nickname with roots in her hard-nosed approach as a Ramsey County prosecutor.

Between visits from fellow judges throughout the afternoon, she made it clear she was still buzzing with energy and feeling forced from office long before she was ready to go.

“This is the only elected position in the state where you have to leave at 70,” she said.

The White Bear Lake water case she ruled on Wednesday — in which she held that the state had failed to respond vigorously to halt the depletion of the lake and aquifer and that science suggested suburban wells were helping drain it — will echo for years. The long trial was fascinating but hard on her physically, she said.

“I couldn’t hold a pen in my hand when that case ended, from all the notes I took,” she said. More important, she added, the substance of the case gripped her. She wants to study geology in retirement.

“When you take a look around the nation at what is happening to our aquifers” — the underground water supplies — “my hair stood on end. This is 40,000-year-old water. How long will these things take to recharge?”

Over the years, Marrinan found herself engrossed by cases that she initially approached a bit reluctantly.

“I had an eight-week antitrust case that should have put me to sleep — price-fixing on concrete pipes and steel bridge girders,” she said. “And it was absolutely fascinating the way the lawyers tried the case.”

The same was true of a major 1998 case in which she threw a “legal wrench,” as was reported at the time, into the attempted sale and relocation of the Minnesota Twins.

“I don’t know the first thing about baseball and can’t stand it; it’s too slow,” she said. “But the issues were great.” She backed the effort by then-Attorney General Hubert Humphrey III to get information from Major League Baseball.

For all her lightness in recalling things, though, lawyers and judges stressed that as the seat of state government, Ramsey County District Court gets lots of demanding cases.

They also stressed, as Marrinan did herself, that it’s a pity so many experienced judges are being forced out at once.

“We have 16 judges with four or less years’ service, out of 29,” she said. “You don’t need to be here 30-odd years, but we have answers to a lot of things. You lose that collective wisdom.”

Ramsey County Chief Judge John Guthmann said Marrinan is legendary for her extensively plotted April Fool’s jokes. She once created, under the name of a more sober judge so that it would be believed, a note to colleagues warning that state court administrators were spying on them using all manner of surveillance devices.

But her proudest moment was extrajudicial: The time she sought to convince the Rev. John Malone, popular priest of a downtown St. Paul church, that he had been promoted to bishop. She told then-Archbishop John Roach the plan, “and he said, ‘I have a scrivener and parchment. We’ll do it up for you,’ ” she said. “And he sends it down in this big tube with return address as the Vatican. ... It got the pastor for a few seconds anyway.

“That one rippled for years.”

Marrinan comes from an era when women lawyers, much less judges, were a relative rarity. But her rise to the bench from the prosecutor’s office was aided by men, she said. She was encouraged to run for judge by Otis Godfrey, “a senior judge and a savvy person,” who helped her even though he told her no one had ever defeated an incumbent judge in Ramsey County.

“Those district judges were so embarrassed by this sexist judge that I got a lot of advice,” she said. “People all over the political spectrum told me, ‘This guy’s gotta go.’ I had 12 years of solid trial experience. And I won by 63, 64 percent.”

She herself made news over the years, not always in the best circumstances. She was one of the victims of John Patrick Murphy, a felon who terrorized judges and others who handled criminal cases against him, mostly in Ramsey County, slashing their tires and worse.

“He was the one defendant of all who did not accept the legal system,” Marrinan said.

Katie Crosby Lehmann, who tried the water case on behalf of the plaintiffs, said Marrinan inspired respect for the system.

“She was generous with her time,” Lehmann said. “Before she put on her robes, she’d come out to us as we were waiting in the hallway and talk to us. It’s a stressful environment, and she did her best to get to know us. She was also always very well prepared.”