Minnesota Valley National Wildlife Refuge found cause of death for numerous waterfowl (swans, mallards & Canada geese) was lead poisoning. Lead shot fragments have been at the bottom of Blue Lake for decades, as lead shot was banned in the 1980's.
https://bringmethenews.com/minnesota-news/lead-bullets-shot-decades-ago-likely-culprit-in-case-of-poisoned-blue-lake-waterfowl1) The Minnesota Pollution Control Agency said it doesn't track the number of lakes with incidences of lead-poisoned waterfowl. However, the issue is common enough for the agency to launch a "Get the Lead Out" campaign, which encourages fishers to switch to non-lead tackle. According to the MPCA, national research indicates lead poisoning accounts for about 14 percent of loon deaths in Minnesota.
2) In January, Minnesota state Sen. Charles Wiger and Rep. Peter Fischer, both Maplewood Democrats, introduced a bill to ban the sale and use of lead jigs and sinkers.