How are you supposed to clean up fire retardant dropped from a plane over a large area?

With water? Like, hose it down? It's mostly ammonium phosphate anyway and afaik it's water soluble.

Edit: yes it moves it around, and just like the cleaning person at the office does you move it into the water table or drainage system. Or do you separate your dirt when you mop a floor or wash your clothes?

That isn't actually removing anything, it's just spreading it around.

Removing dirt from the carpet and washing it down the drain is fine because ordinary "dirt" (i.e. soil) is made of non-toxic or biodegradable stuff. By contrast, washing toxic materials or heavy metals into the water table is the place you don't want them. There's a reason it's illegal to pour used motor oil down the drain.

And there are plenty of things it's legal to pour down the drain, but illegal to put in rivers, because it (grey water) needs treatment before release into the environment.

Editing a comment is not the way to reply.

So the alternative is to let uninformed civilians clean it with their hose and bare hands?

Presumably some of the alternatives include informing them of what to do and devising less toxic means of fire suppression.