Polluted sediments sit at the bottom of rivers and lakes across the Great Lakes region. They can affect water quality, wildlife and human health. More than 40 highly contaminated areas in the region have been identified by the EPA’s Great Lakes Office. But so far, only about half of those sites have been cleaned up. This summer, dredging is taking place in at least three of those hot spots, all on rivers. The Great Lakes Radio Consortium’s Karen Kelly reports on the challenges of cleaning up a river bottom: