Not sure if you're unaware or don't care, but sea salt contains microplastics. You can get untreated alpine salt (Alpensalz) pretty cheap (presuming you live in or around Germany)
On the surface. Not necessarily in the caverns, where it is 'mined'.
OTOH thinking about the large trucks, tractors, and other machinery operating down there, there is probably contamination too, just by abrasion from the rubber tires, or beltways.