A trial set to begin next week in federal court in South Carolina was seen as a test case for those lawsuits. In that case, the City of Stuart, Fla., sued 3M and several other companies, claiming that firefighting foam containing PFAS — used for decades in training exercises by the city’s fire department — had contaminated the local water supply.

The announced settlement is “an incredibly important next step in what has been decades of work to try to make sure that the costs of this massive PFAS ‘forever chemical’ contamination are not borne by the victims but are borne by the companies who caused the problem,” said Rob Bilott, an environmental lawyer advising plaintiffs in the cases.

Environmental groups were cautious, however. Erik D. Olson, a lawyer with the Natural Resources Defense Council, said the settlement, combined with money recently appropriated by Congress to help with contamination, would “take a bite out of the problem.” But, he added, “it’s not going to fully solve it.”

The preliminary settlement with Chemours, DuPont and Corteva, all of which declined to comment beyond the announcement, may not be the end of the costs for those companies, either. The deal, which requires approval by a judge, would resolve lawsuits involving water systems that already had detectable levels of PFAS contamination, as well as those required to monitor for contamination by the Environmental Protection Agency.

But it excludes some other water systems, and it would not resolve lawsuits resulting from claims of environmental damage or personal injury from individuals already sickened by the chemicals. And state attorneys general have filed new suits, some as recently as this week, over the matter.