A recent report from the Alliance for the Great Lakes details the importance of remaining vigilant against contaminating the Great Lakes waters for a number of obvious reasons and a few that might be less obvious. 
From their news room
“An Alliance report released today notes that since the production of synthetic chemicals took off after World War II, the waters of Lake Michigan – which take a century to refresh — have yet to see a complete turnover.  
Halfway through this cycle, scientists are beginning to see alarming trends of an increasing multitude of chemicals found in the water. In southern Lake Michigan, one of the most urbanized and industrialized areas in the Great Lakes Basin and home to approximately a third of the Great Lakes population, these contaminants are a steady source of chemical exposure for aquatic species, and affect the quality of the waters we rely upon for drinking and look to for recreation.”  
Read more at the link above, which includes information on some of the contaminants that pose risks to Lake Michigan. Among them are common landscaping chemicals, pharmaceuticals, and other common substances.