Tourist towns along the shores of Lake Michigan love to proclaim the giant body of water “unsalted and shark free.” The slogan is stuck on t-shirts, magnets and bumper stickers – but, according to a new study, just one of those claims holds up.
Together, the Great Lakes represent about 20 percent of all available surface freshwater in the world. While it’s true that sharks don’t swim in this vast reservoir, the lakes aren’t as cool as they used to be. Over the past 200 years, primarily through the use of road salt to keep roads and winter surfaces free of ice from the 1940s, freshwater in the Great Lakes has seen salinity levels rise. regularly.
While other studies have shown increased salinity levels in small bodies of water, “we know that large lakes are not immune to human pollution,” says Hilary Dugan, associate professor at the Center of Limnology at the University of Wisconsin-Madison and lead author of the study recently published in the journal Limnology and Oceanography Letters.
Dugan and his team used a combination of current and historical water quality data and computer models to analyze the amount of salt carried into Lake Michigan by 234 different tributaries, from large rivers to small streams.
Their research found that these tributaries bring just over one million metric tons of chloride – an indicator of the presence of salt – into Lake Michigan each year. Where the lake’s salinity level was around 1 to 2 milligrams of chloride per liter of water in the 1800s, current levels are now closer to 15 milligrams per liter. And almost half of that increase has occurred in the past 40 years alone. If business continues as usual, says Dugan, Lake Michigan can expect an increase of 1 milligram of chloride per liter every two to three years.
While these amounts do not approach the roughly 250 milligrams per liter that are known to harm freshwater plants and animals and impair drinking water supplies, any increase in salinity can be problematic.
“The living things in these lakes have evolved to thrive in freshwater conditions, and we have now pushed those conditions into less cool territory, which can cause stress,” said Dugan.
However, there is good news, she said. The Clean Water Act, which began regulating many types of pollution in the Great Lakes in the 1970s, has proven that with the right legislation, time, and effort, even water systems as large as the Great Lakes can be cleaned.
“If we pay attention to salt pollution, it’s a problem we can solve,” says Dugan.
To solve the problem, however, policymakers need to know not only how much salt is entering the Great Lakes, but also where all that salt is coming from. This is where the efforts of study co-author Rob Mooney come in.
For previous research, Mooney spent a few summers driving the entire length of the Lake Michigan shore and taking water samples from every river and stream he crossed. This work initially led to a study of how tributaries transport nutrients to the lake and trigger algal blooms. But when Mooney started working with Dugan’s lab, Dugan remembered that Mooney kept extra water samples and that they could test them for chloride concentrations as well.
When they analyzed all of this data and compared it to historical water quality readings, it revealed an interesting dilemma for any salt reduction management strategy.
On the one hand, urban areas are obvious hot spots of salt concentration. According to the study, the main predictor of high chloride levels in a tributary is the amount of impermeable (paved) surface in its watershed. This means that large metropolitan areas like Milwaukee send a lot of salt to Lake Michigan. In fact, the highest salt concentrations detected by the study were astronomical readings of a storm-carrying flow and snowmelt from General Mitchell International Airport in Milwaukee in Lake Michigan just a few miles away.
But the high salt concentrations in a tributary are only part of the problem, Mooney says. It is also important to consider the amount of water that a tributary delivers to the lake.
Five of Lake Michigan’s 300 tributaries are responsible for more than 70 percent of the salt that drains into the lake. It’s largely the result of the flow, Mooney says. These are five major rivers that do not necessarily have high concentrations of chloride, but carry a lot of water through the system. Even though their chloride levels were only a few milligrams of chloride per liter, the millions of liters they bring to the lake can raise salinity levels.
This raises a difficult question for resource managers, according to Mooney.
“The larger tributaries are the biggest contributors to the lake in terms of chloride load, but their concentrations are well below any sort of threat (toxic) concentration, so they are not necessarily a priority for chloride management.” , he said. “While these little tributaries don’t add much, but their chloride concentrations are so high that they get a lot of attention. There is therefore this mismatch between the watersheds to be favored for the management of chloride: are you thinking of the watercourse or are you thinking of the lake? “
Whatever decision resource managers make, says Dugan, the only way to fix the problem is to use less salt in the first place. And that’s something she thinks she can do, pointing out that municipalities and states in the Great Lakes region are already updating winter road maintenance plans and turning to materials like sand or technologies like brine to reduce the amount of salt they use.
Since lakes currently receive enough salt each year to barely push them to higher salinity levels, even modest reductions in use have the potential to move lakes in a “cooler” direction – which , from Dugan’s perspective, is a big deal.
“The Great Lakes are one of the world’s most precious freshwater resources,” she says. “Would they still be excellent if they weren’t fresh?” “