UW-Madison lake scientist Hilary Dugan stands in front of a very large pile of road salt.


“I think about Ice Nine on a regular basis.”

Coming in the middle of a conversation about salt pollution, this surprising detour into literary fiction by University of Wisconsin–Madison lake scientist Hilary Dugan is a little confusing at first.

Ice Nine was a fictional water molecule in Kurt Vonnegut’s Cat’s Cradle — a fabulist meditation on technology and religion. A mad scientist creates a version of water that freezes at room temperature. Plus it’s infectious. One drop of Ice Nine in the ocean and the whole thing seizes up.

Dugan was reading Vonnegut when she first learned about lake stratification, the way lakes settle into layers of temperatures, warm on top and cold on the bottom, and Ice Nine just stuck. “The physics of water structure lake habitat in such a unique way that … is totally unexpected,” says Dugan. “Water is this crazy molecule. Its properties make the planet what it is. If ice sank then the oceans would freeze and we wouldn’t have life.”

If you ever have the opportunity to swim in the middle of a lake in summer, take it. “You have to have actually been in a lake to experience stratification,” says Dugan. After basking in the easy surface warmth, execute a dive towards the bottom. You’ll find the water cools quickly, dramatically. The depth of the warm zone depends on how large and deep the lake is, and on how warm and windy it’s been.

Sun warms the top few feet of the water column, but that’s not the only reason it stays warm. Warm water doesn’t mix with colder, deeper water because temperature dramatically affects density. A steep boundary zone — called a thermocline — forms as a result of these density differences. All you need is a swimsuit to immerse yourself in this phenomenon of physics organizing nature.

You knew this already, mostly, because you know that ice floats. And this little detail facilitates not just cocktails and hockey but life as we know it. Unless, of course — spoiler alert for a novel that turned 60 last year — Ice Nine escapes, devastating the planet.

“That was really when it all connected,” says Dugan.

Hilary Dugan gesticulates while talking about salt pollution
Hilary Dugan gets animated about salt pollution. (Photo courtesy Wisconsin Academy of Sciences, Arts & Letters)


Dugan’s scientific journey began in the Arctic, where the rapid transition from winter to summer happens in June. “Nowhere is changing as fast as the Arctic when it comes to losing ice,” she says. In many places in this warming world, winter is the fastest warming season. Wisconsin has lost snowpack and almost a month of lake ice in the last century. Dugan wondered: what happens to ecosystems as they lose ice? “Maybe it’s not important, but we don’t actually know,” she says. “I’m generally fascinated by how humans are changing lakes,” says Dugan.

You’ve heard the words “cooler by the lake”? That’s because lakes buffer, the climate by storing energy. We tend to think of a frozen lake as bitter and forbidding, but for the plants and animals underneath the ice it’s an oasis even if the wind chill drops to negative 40. “Organisms that live in lakes really just don’t experience the extremes that terrestrial organisms do,” says Dugan. 

Lakes cycle with the season, and it’s the changing temperatures that, literally, stir the drink. Across much of the temperate north lakes mix twice a year, in spring and fall. During this turnover, oxygen from the surface is delivered to the bottom, and nutrients cycle back to the surface. This mixing is a critical part of the rhythm of life, but doesn’t happen continuously because of the thermocline. The density of water at different temperatures simply won’t let it happen. 

Salt has the potential to change this dynamic, with uncertain consequences.

Throwing salt on pavement began in the mid 1940s and continues to rise. In 2017, the U.S. and Canada used 23 million metric tons of salt. “I have always lived in areas that have used egregious amounts of road salt. It’s something that I’ve witnessed every winter of my life,” says Dugan. With salt applied by the millions of tons, she figured something had to be happening to lakes.

Dugan and 14 colleagues pulled the curtain back with the first large-scale analysis of chloride trends in freshwater lakes. Data came from 371 North American lakes — including an important subset of 284 lakes in the North American Lakes Region (Connecticut, Maine, Massachusetts, Michigan, Minnesota, New Hampshire, New York, Rhode Island, Vermont, Wisconsin, and the province of Ontario). This area has lots of lakes and uses a lot of road salt, and long-term salinization was already evident in many lakes.

Taking into account different landscapes and climate patterns, she calculated that you could predict the risk of salt pollution just by measuring the impervious land cover — pavement and buildings. As little as 1% of impervious land cover surrounding a lake increases the long-term risk of salinization.

Already 27% of the large lakes in the U.S. are above this threshold. Across a 17-state area that encompasses almost 50,000 lakes, Dugan and her team estimate that some 2000 lakes larger than 9.8 acres (4 hectares) are vulnerable.

graph of salt pollution in the Madison lakes
Chloride levels have been rising in Madison lakes for more than 60 years.


Salt’s radical impact on flavor and food preservation hint at the power of salt’s chemistry, and posts in the coming weeks will explore how this stresses the fine balance of aquatic life. But because of salt’s profound impact on water density, it can also influence lake stratification. And changes in how water temperature and density interact could fundamentally change how lakes operate.

Dugan lives and works between two major urban lakes in Madison, Wisconsin. Lake Mendota has a larger and more rural watershed. One of the most studied lakes in the world, it flows into the smaller, shallower, and almost wholly urban Lake Monona. With salinity readings dating back to 1940, we know that just 80 years ago these waters contained virtually no salt. Levels have steadily increased since then. 

In 2019 Dugan and Robert Ladwig — now a professor at Aarhus University in Denmark — deployed a new generation of sensors that allowed them to be under the ice over winter and “in” the lakes during the short and sometimes dangerous period of ice-off. Ladwig and Dugan figured that over the course of the winter salt would wash off roads and sidewalks and into storm sewers, sinking to the bottom of the lake. This briny addition would make the bottom water even denser. In theory it would delay the spring turnover.

Mixing happens soon after ice out. Temperatures at the bottom and top of the lakes are close enough to break down the density barrier. Wind helps with the actual mixing.

As expected, the sensors showed salinity rising at the bottom of both lakes, with much larger increases in Lake Monona. Mixing usually happens when lake temperature equalizes top-to-bottom. That happened on March 20th, 2020, but salinity readings showed that full mixing still hadn’t happened. That deep reservoir of salty water didn’t mix for another 20 days.

Hilary Dugan and a colleague sampling on a frozen lake in northern Wisconsin.
Hilary Dugan and a colleague sampling on a frozen lake in northern Wisconsin. (Image courtesy Katie Thoresen/WXPR.)


Humans notice when the seasons are out of kilter. We say that fall is late, or summer is early.

For many plants and animals, their welfare and very survival can hinge on seasonal timing, called phenology. This choreography is subtly different in every lake, but the basic progression goes like this:

During mixing, nutrients trapped in the bottom layer combine with oxygenated water from the surface. Algae bloom, drawing on the nutrients released from the bottom. Zooplankton start to flourish when there is algae to eat. Fish, in turn, eat zooplankton. As summer passes, the lake stratifies. Fish are confined to different layers depending on their temperature tolerances. Dead organisms sink, their nutrients trapped at the bottom. Oxygen gets used up.

In autumn, falling temperatures allow the lake to mix again, sharing nutrients and oxygen throughout the water column before the winter freeze.

Predicting how changes in phenology will play out is hard — there are too many moving pieces. It’s easier to measure the benefits when phenology doesn’t change. For example, Wisconsin Department of Natural Resources research led by Zachary Feiner show that fish have their best years when the spring is close to the historical average. “Everything’s just like timed out perfectly, because all of the organisms have evolved to that timing,” Dugan says.

Big swings in climate throw off the timing. Salt pollution has the potential to further confuse things.

A map of the lower 48 states of the U.S. Colored dots show a projected risk of salt pollution to lakes. The region surrounding the Great Lakes, from Minnesota to New York is most at risk.
Predicting a salty future for lakes in the U.S. (Image courtesy Hilary Dugan.)


“The solution to pollution is dilution” is old-school environmental management. This antiquated maxim sounds like it comes from the same collection of 1950s civics documentaries that promoted “duck and cover” as a precaution against nuclear annihilation. (Dugan recommends the book Pollution is Colonialism for a deeper critique of this mindset.)

But if you live in snow country, you’ve seen it in action as you watch truck after municipal truck scatter salt. “Then it’s just gone,” Dugan laments. “It dissolves and we forget about it. We literally just can’t see what we’re doing to lakes and rivers.”

“It is difficult to imagine a salt-free future for global freshwaters,” Dugan wrote last year with colleague Shelley Arnott of Queen’s University in Kingston, Ontario. “To turn off the tap of salt requires a societal shift towards recognizing and prioritizing freshwater quality and eliminating our dilution mindset.”

While dilution is a flawed way to think about water quality, the fact that rain keeps falling and water keeps flowing downhill does drive some hope for reversing salinization. Dugan, working in part with Chris Solomon of the Cary Instute for Ecosystem Studies, looked at salt pollution inflows and water outflows from lake systems. They computed that with more careful management of salt use the salinization of lakes can be reversed. Indeed, in most lakes, the salt pollution threshold of 230 milligrams per liter set by the Environmental Protection Agency won’t be reached.

Those thresholds are probably too high — we’ll cover that in the next two newsletter installments — but what’s more exciting is the advances in salt management. Across municipal and regional governments, various pilot programs are now working to stem salt pollution. There’s a lot of low hanging fruit, and rising salt prices are pushing better management.

Dugan worries that we don’t fully understand the potential trajectory of salt pollution. And despite some local reductions, salt use overall is still increasing. One outstanding question: How much salt is stored in the landscape?

And there will be backlash. This year Madison, Wisconsin implemented a salt reduction plan. When a difficult weather pattern turned a wet snowfall into a thick shell on ice city roads, controversy erupted. A former mayor blasted the current administration, while one letter to the editor complained that the city was putting the lake water quality above human lives.

On a Zoom presentation for Wisconsin’s Salt Wise program, Dugan responded: “I would like to say for the record that we are trying to think about water quality *for* people’s lives. Freshwater quality is incredibly important, especially when it comes to things like drinking water.”

“If we acknowledged that these were pollutants, it would go a long way,” adds Dugan. “At this point we have enough evidence to show that it’s a universal problem in these areas where we apply road salt. If freshwater is near a high density of roads, it’s being impacted.”


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Environmental change…

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