My feewings is that Bonneville is important enough that off-season forest fire tankers should airlift saltwater brine from the Coast.
I believe the answer to your question is that The Flats floods in Winter so everything gets averaged out annually. The video suggests there are salt tailings in big piles that could be redistributed, if the potash compan(y/ies) was/were held to account they could remediate the situation, but $$$$$.