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When the French explorer Joseph Nicollet (for whom Nicollet Island was named) visited what is now Minneapolis in 1836, he measured the temperature of Coldwater Spring, south of Minnehaha Park, as 46 degrees Fahrenheit. Summer or winter, it retained this temperature, which is based on our latitude north of the equator. Groundwater averages out the large seasonal fluctuations in air temperature that we experience.
As the DNR hydrologist most directly involved with mapping and measuring thousands of springs for the Minnesota Spring Inventory, I’ve been focused on the temperature of springs and groundwater for years. A large subterranean spring in a cave under downtown Minneapolis presented the highest groundwater temperature I’ve ever measured in the state, a whopping 66 degrees Fahrenheit, which is 20 degrees Fahrenheit above Nicollet’s baseline data. And it has public health implications that residents should be aware of, especially as we are now heading into the winter season, with its water main breaks.
This warming creates a “microbial soup” scenario when sewage leaks occur, since warmer temperatures correlate with increasing threat from waterborne pathogens, which can be sucked into ruptured water mains through “back siphonage.” Minneapolis has issued many such boil-water alerts after water main breaks over the years, based on concerns about back siphonage. Failure to heed such alerts can lead to gastrointestinal illnesses, usually involving diarrhea.
Jackson, Miss., provides an instructive comparison. An outdated water-treatment plant and aging infrastructure have prompted numerous boil-water alerts in recent years, some of them citywide and lasting for months. Water turbidity (cloudiness), associated with bacterial contamination, is usually cited as the reason for the precaution. While it’s unknown whether Jackson has artificially elevated groundwater temperature, its expected natural groundwater temperature, correlating with its latitude, is 63 degrees Fahrenheit. In other words, the groundwater under downtown Minneapolis is warmer than that of Jackson, in the southern United States. While our infrastructure is in better shape, the potential for contamination, when breaks do occur, is similar.
Another concern should be among those drinking “raw water” from the natural springs of Minneapolis, especially those in the Mississippi River gorge. Most of these springs already have a mildly elevated temperature, and historically have been responsible for typhoid cases. In my mapping journeys along the river, I found empty 1-gallon plastic jugs at more of these springs than you might expect.
Most of this warming can be attributed to heat conducted downward by buildings and pavements. This subsurface urban heat island would exist regardless of climate change, which, however, contributes to the total temperature rise. This thermal anomaly is the strongest signal of anthropogenic groundwater warming measured in Minnesota, and the most striking groundwater phenomenon I’ve seen in my years as a hydrologist. While first described in my 2009 book “Subterranean Twin Cities,” the data has now been made available in the peer-reviewed scientific literature.