“It keeps you up at night,” Ms. Schweinzger said. “You don’t sleep, because you’re wondering, ‘What don’t we know yet?’”

The scope of the problem remains an open question. Wary locals have been reporting potential dump sites — 76 so far, state officials say — in backyards and hillsides in this area just north of Grand Rapids, where the suburbs fade into the countryside. Wolverine officials said they were helping investigate those sites, but said many were not theirs.

Kathleen Shirey, who is helping lead the Michigan Department of Environmental Quality’s response, said her agency had not been fully aware of Wolverine’s past dumping and was continuing to investigate whether other reported dumpsites were related to Wolverine. She blamed shoddy record-keeping and loose dumping regulations in past generations.

“We’ve asked the company — they apparently don’t have too much, either, in way of historical files,” Ms. Shirey said. “So what we’re primarily relying on is citizen memory and reports.”

Wolverine officials acknowledged dumping waste, but said they had complied with applicable laws at the time, and were committed to working with regulators and cleaning up the mess. The officials emphasized that the health risks of PFAS were not known at the time of the dumping, and that the extent of those hazards remains unknown.

Several nervous residents pointed out flaws in Michigan’s response in Flint, about 120 miles east, where the state’s environmental quality agency gave assurances about a 2014 change in the city’s water source that ultimately poisoned people with lead and put them at risk for Legionnaires’ disease. That debacle led to involuntary manslaughter charges against the state’s former top drinking water official, as well as the state’s chief medical executive, the director of the Michigan Department of Health and Human Services, and other government workers.