After the snow from last week’s bomb cyclone melts, cities and towns across the Northeast will find their roads and sidewalks still a ghostly white.

That fine white powder is salt, America’s favorite de-icing chemical. More than 15 million tons of the stuff is applied annually, only to wash away with snowmelt or spring rains. But just because it’s gone from your block doesn’t mean it’s gone for good. In fact, it may end up in your faucet.

Road salt is an influential part of a decades-long chemistry experiment local governments have unwittingly conducted on soil, waterways and infrastructure. Studying...