The Great Lakes hold about one-fifth of the Earth's fresh water, but a new report indicates they're getting saltier - and says that's reason for concern.
Lake Michigan used to sit at a salinity level of one to two milligrams of chloride per liter of lake water. Now, that concentration has risen to nearly 15 milligrams per liter.
Hilary Dugan, an assistant professor in the Center for Limnology at University of Wisconsin-Madison, said it's still a relatively low level for a water body as large as Lake Michigan. But rising chloride levels could pose problems for plants and animals in the smaller lakes and tributaries that feed into it.
"If anything, we're just hurting those native freshwater species that are adapted to, you know, Wisconsin waters that basically have no salt in them," said Dugan. "And now, you know, we've increased the salt ten-, hundredfold, depending on what body of water we're looking at."
She said salt levels have to hit roughly 250 milligrams per liter to pose a serious risk to plants, animals and humans, and notes virtually all the chloride pollution is from road salt.
Dugan added that people can reduce pollution in lakes and streams this winter by limiting their use of salt on roads, driveways and sidewalks.
Unlike other chemical pollutants, Dugan said salt pollution isn't irreversible. She explained that over the course of their life cycle, lakes and streams will gradually flush it out of their system.
"Salt stays dissolved in water, and so lakes and rivers will naturally flush themselves out," said Dugan. "So what it takes to reduce the salinity is just to stop the amount of salt that's going into these bodies of water."
The report found about 70% of the salt flowing into Lake Michigan comes from just five of the body's 300 tributaries.
Dugan said if nothing's done to address the issue, salt concentration in the lake will rise by about one milligram per liter, every two to three years.
