Connie Fortin’s job is to get the truck drivers who spread salt after Minnesota’s brutal winter storms to spread less salt. It’s not always that easy.

As an environmental consultant who trains the truck drivers, she aims to minimize the amount of road salt that ends up in the state’s waterways, where it can harm fish and contaminate drinking water supplies.

“When I first started teaching these classes—and nobody ever heard of this—people would say to me...