The surface of Parkers Lake in Plymouth, on the west side of Minneapolis, was sheer ice one day in March when a crew of scientists walked out to a spot near its middle. The past few days had been warm and the sun had rapidly melted off the snow on top of the frozen lake. It was now a slick slab of cloudy white ice with big dark spots. The slick surface made for treacherous walking, providing little purchase for the boot, as the researchers pulled a sled full of equipment and carried other gear to a preselected point.

Parkers Lake is full of salt (chloride, specifically). In the last several decades, runoff from nearby roads has raised salt levels in the water significantly. Many lakes in Minnesota, primarily in the Twin Cities—where lots of people and traffic means more roads and more road salt used in the winter—are suffering similar pollution. 

Highway departments spread salt on roads before and after snow and ice storms to speed along the melting process. It has significant benefits for road safety, with a 1992 study from Marquette University finding a nearly 90 percent reduction in accidents within four hours of salt being applied to icy roads. But it also has complex effects on nearby waters.

The ways in which salt affects lakes are complex and interconnected, and this team from the Science Museum of Minnesota’s St. Croix Watershed Research Station was seeking to better understand some of salt’s ripple effects.

The crew moved efficiently through the steps of their protocols, a routine practiced and refined over the past two years as they criss-crossed the Twin Cities and the state of Minnesota to gather information from numerous lakes.

Spring was clearly on the way, as a bald eagle circled over the lake and a cardinal sang sweetly from shore. The sound of cars on County Road 6, which passes within less than a hundred feet of the lake’s north shore, also carried over the ice. That’s where the salt comes from.

“We need both safe roads and clean water,” said Mark Edlund, a senior scientist. “But all the salt that’s getting into lakes is really starting to mess things up.”

Even though the sun was strong and the season was changing, the ice was still thick. When the crew stopped to drill several holes through the ice to conduct their work, they found at least 16 inches. Working while wearing a single layer, it was hard to reconcile the comfortable atmosphere and the lake below, still locked in winter.

Ice, safety, water, traction—all come together when studying salt. Putting it on roads improves safety. It is also wreaking havoc on beloved lakes.

Hailey Sauer is one of the leaders of the study. She’s a postdoctoral researcher at the Research Station. Sauer says it’s important to put into perspective just how much salt is put on Minnesota’s roads each winter.

“Picture an African elephant standing about every 90 feet along busy highways,” she says. “That’s the average amount of salt applied each year.”

African elephants are the largest land animal on Earth, with mature males standing 10 to 11 feet high at the shoulder. They weigh six to seven tons. 

Anyone who has ever felt their car wheels break loose on a slick road has to appreciate any effort to reduce ice. Salt is also applied on parking lots, sidewalks, and any other surface where ice is unwanted. 

But when spring comes and ice and snow start to melt, the water flows into gutters and storm sewers, washing away most of the salt. It keeps going until it comes to a lake, where the chloride can accumulate over the years. It never breaks down into a less harmful substance, and it is rarely flushed through an outlet stream because salt makes water heavier and it settles to the bottom.

Too much salt is known to directly harm living things in lakes, with other studies showing increased chloride directly linked to declines in the diversity and abundance of some aquatic organisms. But salt’s impacts on lakes and their complex ecosystems are much broader than that. These scientists want to learn more about the chain reaction of effects it can cause, from water density to greenhouse gas emissions.

The crew started by drilling a dozen holes in a big circle around their gear, the auger chewing into almost a foot-and-a-half of ice before punching through to the liquid below, the water splashing up out of the hole with the last ice chips. 

Understanding the effects of salt on lakes requires a lot of information. The crew collected water, measurements, and mud. Everyone had jobs to do, and more lakes to visit that afternoon, so they didn’t waste time.

 Edlund was working to both gather data and teach the many skills of studying a lake to early-career researchers.

“This is an awesome day for doing this,” Edlund said. “The equipment isn’t freezing up and it’s comfortable, but there’s still lots of ice.”

During the open water months, the scientists had conducted intensive monitoring of several lakes, both in the Twin Cities area and the outstate city of Alexandria. The salt issue in Alexandria’s lakes comes partly from water softeners and wastewater treatment facilities that can’t yet remove all the chloride. 

In addition to taking measurements and water samples, the crew was here today to extract cores of the sediment from the lake bottom using some specialized equipment, advanced techniques, and cold, wet, muddy hands. 

Sediment cores can be used for several purposes, such as reconstructing the history of a lake by analyzing the layers of mud on the bottom. The cores being extracted today were short, only about four inches of the uppermost material. 

The tubes of mud would be brought back to the lab and put into a system with water also taken from the lake, replicating the natural environment. Gas would be bubbled through it to replicate different lake processes, allowing them to see how salt affected how much phosphorus entered the water from the sediments.

The work of taking cores through holes in the ice involved a lot of wrangling of tubes, buckets, clamps, water, and soft, black muck. After about an hour, the crew had gathered their mud and their data and packed up, heading back to the van at the parking lot, and driving to nearby Medicine Lake to do it all over again.

All the effort has paid dividends. Minnesotans now have access to rigorous science, based on local observations, to understand how road safety and clean water are connected in this part of the world.

“There are a lot of big questions about this issue,” Sauer says. “How are lakes changing across the region? Do lakes with more salt have different food webs? Does it alter when and how often lakes are mixing and stratifying?”

The last question is at the root of many of the others. Because salt makes water more dense, it disrupts how lakes separate into upper and lower layers during spring, summer, and fall. This cycle of stratification drives a lot about how a lake behaves throughout the year.

Many lakes separate into warmer upper and colder bottom layers in the spring and stay that way until fall, when the waters mix again. Some lakes repeat the cycle frequently throughout the seasons. 

Those aren’t the only ways lakes can be different and experience different effects from salt.

The day after Parkers and Medicine Lakes in Plymouth, the crew visited Brownie and Cedar Lakes in Minneapolis. Brownie is a small oval lake sitting at the bottom of a bowl, oriented southwest to northeast.  It is well sheltered from the wind by ridges that steeply rise 60 feet from the water. Eight lanes of traffic roar past on nearby I-394 day and night, summer and winter. It takes a lot of salt to keep those cars moving all year. 

After Brownie, the researchers lugged their gear out into the middle of Cedar, setting up the temporary lab. Zoe and Jackalyn drilled the circle of holes, and the data collection began.

“We collect sediment cores and water samples from each lake and analyze their biological and chemical properties,” says Erin Mittag, a postdoctoral fellow at the Research Station. “By looking at patterns across these numerous pieces of evidence, we can get a fairly accurate idea of what is going on in a lake today and what it was like in the past.”

Cedar is a popular lake with parks, beaches, fishing piers, and walking trails around it. The Minneapolis downtown skyline was visible to the east, with Bde Maka Ska and Lake of the Isles in between. Much larger and less sheltered than Brownie, the effect on Cedar was obvious, as the wind whipped across the open ice.

While salt is a powerful substance when it comes to affecting lakes, its impact can also be magnified simply by the surrounding topography. Sheltered Brownie Lake has a permanent layer of dense, salty water at its bottom that never mixes, while Cedar still mixes in the spring and fall each year. 

Like anything when it comes to water, the effects of salt on Minnesota’s lakes are not simple. Each one of the state’s 14,380 lakes is different. They are not just bodies of water, but complicated systems that depend on each one’s depth, size, water chemistry, climate, bedrock, history, surrounding landscape, and much more. 

“How certain amounts of salt are going to affect a lake depends much on the lake’s characteristics,” Sauer said.

Most of the salt that enters a lake in a year comes in a big surge in late winter, right when the landscape is melting and the lakes are losing their ice. The saltwater plunges to the deepest part of the lake, where it sits near the bottom.

What happens after that depends on the web of other forces.

Shallow lakes surrounded by flat country can mix more easily than deeper, more protected lakes. Wind blowing across the surface stirs up the water enough that the cold bottom and warm top combine. This mixing can bring up cold water, nutrients that have been pulled from sediment, and salt. Deeper lakes also offer more wind resistance, and will generally remain stratified for longer.

Some of the variables are easy to see and measure, like size and depth. Others are invisible to the naked eye.

The scientists gathered data from lakes large and small and deep and shallow, allowing them to compare and contrast and understand how different variables factor into the aquatic system, and what salt does no matter the unique qualities of a given lake. 

One question the scientists set out to answer was the ways salt is affecting food webs in lakes. The walleye or bass that dominate Minnesota’s recreational fishing industry grow big and strong by eating smaller fish and other creatures. Those organisms eat even smaller things, all the way down to tiny creatures called Daphnia. 

If Daphnia disappear, or are even diminished, the effects will be felt far up the food chain. Studying their sensitivity and resilience to change is possible thanks to two unique qualities of Daphnia. First, they produce egg-like cells that can remain viable a long time in sediment, and can be brought back to life later. Second, they have short lifespans and reproduce rapidly, creating numerous generations per year.

“Our team resurrected these eggs, allowed them to develop into adults, and then extracted their DNA,” says Mittag. “It’s like something from ‘Jurassic Park.’”

Living and reproducing fast means Daphnia can evolve quickly in changing conditions. Individuals with adaptations to saltier water will swiftly influence the larger gene pool. By comparing the DNA from older eggs to that of specimens caught alive in the water, the research team revealed that Daphnia in salty lakes has already rewritten its genetic code significantly because of chloride.

Adaptation over extinction. It’s the question every species must face at some point.

Salt affects lakes in a lot of ways, but it also changes how lakes affect the wider world. As complex natural systems, lakes have their own byproducts, like the gasses that bubble up from the bottom. Methane is one of the most common gases released by lakes, and it is a powerful contributor to climate change.

The world’s lakes are the third largest natural source of methane entering the atmosphere. Composed of carbon and hydrogen, methane causes a warming effect 84 times greater than the more common carbon dioxide (carbon and oxygen).

Zoe Plechaty, recent graduate of St. Olaf College and environmental research fellow at the Research Station, is analyzing the relationship between salt pollution and greenhouse gas emissions. Measuring the emission of greenhouse gasses from the surface of the lake, Plechaty found that one 75-acre lake from the study could release as much methane into the atmosphere in a day as 17 cows. Chloride intensifies the conditions for methane production.

“Increasing salt levels can increase greenhouse gas production, because stratified water creates better conditions for it,” Plechaty says. “But, chloride can also cause methane to stay in the bottom of the lake, giving it a chance to be converted into other gases with lower climate impacts.”

The study found that the trick to understanding salt’s effect on methane released from a lake comes down to the timing of when the water mixes. How much methane is released and how much is converted to carbon dioxide depends on whether the water mixes repeatedly through the season, or only once in the fall.

Plechaty also pointed out an irony to the findings, which seems to represent a lot of the contradictions of studying salt and water.

“Salt means more methane, which causes more global warming, which means less snow, which ultimately means less need for salt,” she said. It’s not a comforting thought, and not a solution to global warming, lake pollution, or road safety.

Salt is certainly not the only threat to Minnesota’s lakes. Nutrients from farm fields and urban runoff are also a major problem. Climate change and invasive species have serious effects. Whether it’s global warming causing longer ice-free seasons or zebra mussels filtering the water so it lets sunlight reach deeper under the surface, or if it’s the algae blooms that can create toxins, fed by fertilizer—the problems have complex causes and effects and are hard to disentagle.

Figuring out how chloride fits in is helping better understand how to prioritize efforts to reduce pollution, identifying the kinds of lakes that most urgently need protection. For example, the researchers have measured how salt coming into the Brownie-Cedar-Bde Maka Ska chain of lakes from I-394 declines as it gets farther from the source. They’ve improved understanding of how exposure to wind can overcome saltwater density, how filter feeders are evolving to survive in saltier water, and how salt can worsen the effects of excess nutrients.

The study has shown that many of those threats are currently bigger than the salt problem. While chloride is having negative effects on some lakes, it’s not too late to act.

“We haven’t hit the salt tipping point yet, and we have a better understanding of which lakes are close to their tipping points,” Sauer said. “We still have time to do something.”

Take action to reduce chloride contamination:

• Smart Salting – Minnesota Pollution Control Agency
• Salt Sustainability – Minnesota Department of Transportation
• Salt Watch – Izaak Walton League of America

Funding for this research project and this article was provided by the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources (LCCMR). The Trust Fund is a permanent fund constitutionally established by the citizens of Minnesota to assist in the protection, conservation, preservation, and enhancement of the state’s air, water, land, fish, wildlife, and other natural resources. Currently 40% of net Minnesota State Lottery proceeds are dedicated to growing the Trust Fund and ensuring future benefits for Minnesota’s environment and natural resources.

Greg Seitz is the founder and editor of St. Croix 360, a news website focused on the St. Croix River and its watershed. His work has also been printed in the Minnesota Conservation Volunteer and many other publications. Greg manages Agate’s website and email newsletter, and writes for the publication occasionally. He lives in May Township, Minnesota with his wife and two children.