Does Lake Michigan have a Mercury pollution problem?

Last week I went to the Chicago Gold Coast beach. I was at the beach across from the pedestrian over pass near Fullerton Ave. It is early Spring, and the beach remains rarely used through the winter(compared to Summer). I noticed little scattered black rocks. I picked one up and examined it.

It was extremely light in weight and coal black. The pieces were about the size of a half dollar coin. The old story is that mercury contaminates water from coal burning exhaust emissions. So obviously mercury is naturally contained within coal. I looked very closely at these pieces and noticed tiny scattered specs of a mercury color on many of these little black rocks.

I collected over ten of these rocks. Today I noticed they City of Chicago had done there beach combing.

How could pieces of coal landed on the Gold Coast beach?

My first answer to the problem was that there is probably ships in some part of Lake Michigan that are transporting coal across Lake Michigan to coal burning power plants probably located in Indiana, or the Chicago vicinity. If they transport coal the same way that its transported on freight trains, then the top is uncovered, and coal pieces could be falling into the water of Lake Michigan.

That or the rate of coal burning from power plants is so powerful that small pieces of coal are being flung into mid air and are landing in Lake Michigan and washing ashore.

Since I am not a geologist or a chemical scientist these little rocks could be something else. They could have eroded from rocks elsewhere in the Great Lakes, and naturally washed ashore. Then again, There have been reports in the news of Lake Michigan having high mercury contamination.