Hey, Curt
I think that's about the only possibility you can rule out. Even if the tree had some property that would say precipitate salt or sodium or another chemical out of the water, remember that there were 1-2 million people there plus their animals, so no physical means other than a giant nuclear powered desalination plant could have turned that much bad water potable.
edit: The largest plant in the US can do 25 million gallons per day so it wouldn't have to be nuclear powered. But even assuming a minimum need of 2,000,000 gallons per day, no tree or chemical could have made a whit of difference, so it had to have been miraculous.