Noted: A December 2021 study from the University of Wisconsin-Madison found that the lake’s chloride levels have risen from about 9 milligrams per liter in 1980 to about 15 milligrams per liter today, primarily due to the use of road salt. Chloride levels in Lake Michigan have been rising steadily since the 19th century, when the lake’s chloride levels reached only 2 milligrams per deciliter.
Rob Mooney, a postdoctoral researcher at UW-Madison who worked on the chloride study, said that although researchers don’t have a definitive answer as to why, it could be because Lake Michigan has a much longer water replacement time — the time it takes for the water in each lake to be completely replaced — than Erie and Ontario.