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Arthur T Knackerbracket has found the following story:

Plastic debris makes up about 80% of the litter on Great Lakes shorelines. Nearly 22 million pounds enter the Great Lakes each year—more than half of which pours into Lake Michigan, according to estimates calculated by the Rochester Institute of Technology. Regardless of size, as plastics linger in the water, they continue to break down from exposure to sunlight and abrasive waves.

Microplastics have been observed in the guts of many Lake Michigan fish, in drinking water and even in beer. Perhaps the most worrisome aspect is that the impact of microplastics on human health remains unclear. Plastics are known to attract industrial contaminants already in the water, like PCBs, while expelling their own chemical additives intended to make them durable, including flame retardants.

[...] Matthew Hoffman, the lead author of the Rochester Institute of Technology estimate, said population centers like Chicago and Milwaukee are large contributors to plastics pollution in Lake Michigan. In addition to trash that can drift into the water from beaches, wastewater treatment facilities are significant sources of microplastics.

Before a federal ban in 2017, some soaps and facial scrubs contained microbeads that were rinsed down the drain into waterways. The majority of microplastics are tiny fibers that break off from synthetic fabrics when people do laundry.

Once plastics enter the lake, they follow lake currents, potentially migrating to other states but largely remaining trapped at the southern end.

"Things from Chicago might end up on the shores in the state of Michigan," Hoffman said. "In the Great Lakes, plastic could move to different states, different lakes, different countries. So that can be an interesting challenge if you want to clean up. Now you have to look at interstate regulations."

What goes into Lake Michigan typically stays there. While water from the other Great Lakes moves downstream, Lake Michigan's only major outflow is the Chicago River (and the water it intermittently exchanges with Lake Huron at the Straits of Mackinac). As a result, a drop of water that enters Lake Michigan stays for about 62 years, according to the National Oceanic and Atmospheric Administration.

Because some municipal sewage sludge is applied to farm fields, agricultural runoff can also be a significant contributor. Farmers who may use plastic materials to cover their seed beds (to regulate the soil temperature and moisture) may also be partly responsible for microplastics.

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