I'm currently building a fort on a map with a major + minor salt river meeting in a waterfall (major river drops 5 levels). In that drop span is a salty aquifer.
In 31.08 I built a wooden cistern beside the Major river before it went over the falls, and was able to get clean water, without a well. (so Z+1 from the "Salt Plane"). Had to abandon that fort, but save-scummed the embark save as I really liked the location.
In 31.12 I tried to build a stone Cistern out of Rock Salt, on top of the Major River, below the falls (the cistern itself ended up more or less in the middle of the aquifer layers, height-wise). Got Salt water. Put a well in, still not drinkable. Figured, well, you know, I did built it out of large pieces of salt... and rebuilt it out of Sylvite Blocks. Still not drinkable. I've been planning on rebuilding it out of wood, as that had worked up above, but I've been unable to try yet (my DF time is pretty much limitted to weekends these days).
This could support:
-the "salt plane" affects cisterns/wells built within it.
-certain materials do/don't work at purification (in wells and/or cisterns)
I'm going to experiment with building higher up and with different materials, and see if I get any different results.