My assumption is that because salt is mined and stored in open air, plastic particles from the ocean's non-trivial cache of microplastics being brought up into

My assumption is that because salt is mined and stored in open air, plastic particles from the ocean's non-trivial cache of microplastics being brought up into the atmosphere by evaporation and weather, and from ashes/plastics from trash incineration are potentially finding their way onto the piles. If water is used during the processing of the salts it could be that as well.