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Amid the sometimes-wild temperature swings in Minnesota, from hot summer days to frigid winter nights — conditions are a lot more stable just beneath our feet.

Anywhere in southern Minnesota, it’s long been understood that groundwater temperatures will be a consistent 45 or 46 degrees.

But a Minnesota geologist says he’s found an anomaly underneath Minneapolis. Greg Brick has measured water in caves and flowing from springs that’s more than 10 degrees higher than he expected. He’s concerned that it could cause problems for aquatic ecosystems and drinking water systems above ground — and he’s also concerned that there isn’t enough research to know exactly what those problems could be.

Brick previously worked as a geologist for the Minnesota Department of Natural Resources. During a project mapping springs, he started taking the temperature of groundwater beneath Minneapolis.

On a cool day late last year, he double-checked the numbers during a visit to Coldwater Spring, a creek flowing out of the rock near Minnehaha Regional Park.

It wasn’t 45 degrees. Instead, Brick found the water was flowing at 57 degrees Fahrenheit.

“In my years in private environmental consulting, in teaching geology and then in government work, that is certainly the most unusual groundwater-related phenomenon that I’ve encountered in Minnesota,” Brick said.

He first published those findings in his book “Subterranean Twin Cities” in 2009, and later in further peer-reviewed research. Since then, he’s confirmed the numbers over and over at various caves and springs. In May, a student of his checked the water in a cave underneath downtown Minneapolis — it came in at 62 degrees.

Researchers have reported similarly warm groundwater in big cities around the world — though Brick said he didn’t find much research on the issue in the U.S.

Those researchers say climate change is partially behind the high temperatures. But the rise in Minneapolis groundwater temperatures is much greater than that of its air temperatures. 

Brick said the warming is amplified in cities because of the urban heat island effect — the process of concrete or pavement transferring heat from the sun to the ground below.

“Downtown Minneapolis in the middle of summer — that street surface is heating up tremendously,” Brick said. “And then basements, you’ve got a boiler going down there, that’s warming up the groundwater that bathes the building.”

Warmer groundwater could have a wide range of ripple effects. Because groundwater supplies lakes and streams around the state, it could alter their temperatures, too.

John Nieber is a professor and groundwater researcher at the University of Minnesota.

“If the temperatures of groundwater are increasing, then what we expect to see is the temperatures of surface water are also going to be increasing. And this can be detrimental to different aquatic organisms,” Nieber said.

Trout, for example, rely on cold streams. And warmer water is also more susceptible to algae blooms, which can release toxins.

People rely on groundwater, too. Ground aquifers supply about 75 percent of Minnesota’s drinking water, according to the DNR.

As water warms up, it can sustain more bacteria and other organisms. Researchers who have studied groundwater warming in other cities say this could pose a problem for drinking water quality.

In Minneapolis, drinking water comes from the Mississippi River — not from groundwater. City water quality manager George Kraynick said the city keeps a close eye on the supply.

“We have a pretty robust network in place to account for any type of contamination that's on the river,” Kraynick said.

That includes hundreds of quality tests per day, and around-the-clock monitoring of water from the river. The city has the ability to shut off intake pipes coming from the river as soon as any contamination is detected, and to shut off pipes if there’s ever a water main break that could lead to contamination.  

Several state agencies also have a role in managing the state’s groundwater. The Department of Health monitors drinking water sources. A spokesperson for the Minnesota Pollution Control Agency said that agency also has staff dedicated to monitoring groundwater and any contaminants. 

“MPCA hydrologists are familiar with urban heat island effects on groundwater and the actual phenomenon of warming groundwaters, but scientists have not observed anything in relation to this issue,” the spokesperson said in a statement. 

Brick hasn’t observed the same temperature phenomenon under other cities, but he said he believes it could be happening in St. Paul — which has similar urban infrastructure, but no accessible springs to measure groundwater temperatures in the downtown area.

He’s concerned about any potential for increased contamination. Even with a carefully-monitored drinking water system, he said, accidents happen. Some people drink straight out of springs, even though health professionals caution against it. And underground pipes can break, potentially drawing in surrounding groundwater. 

“It’s not to create an alarm, but to raise awareness of this issue," he said. "It’s like a completely unconsidered factor in public health.”

Brick said he hopes to see more research on the issue — especially when it comes to drinking water quality and safety. 

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Estelle Timar-Wilcox

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Amid Minnesota’s frequent temperature swings, there’s one place that scientists expect to stay consistently cool all year: underground. But a geologist says groundwater beneath Minneapolis is warmer than it should be — and he’s concerned that could cause problems above the surface.

A man measures the temperature of groundwater

Estelle Timar-Wilcox | MPR News

Water emerges from underground at a spring

Warming groundwater raises questions about potential effects above ground in Minnesota

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