"A water elemental can’t venture more than 180 feet from the body of water from which it was conjured."
"Dust of Dryness
This special dust has many uses. If it is thrown into water, a volume of as much as 100 gallons is instantly transformed to nothingness, and the dust becomes a marble-sized pellet, floating or resting where it was thrown. If this pellet is hurled down, it breaks and releases the same volume of water. The dust affects only water (fresh, salt, alkaline), not other liquids.
If the dust is employed against an elemental with the water subtype, the creature must make a DC 18 Fortitude save or be destroyed. The dust deals 5d6 points of damage to the creature even if its saving throw succeeds.
Moderate transmutation; CL 11th; Craft Wondrous Item, control water; Price 850 gp."
Now, one might argue that without the body of water, the Elemental is free to travel anywhere, but common sense dictates "what are you smoking and can I have some?"
[-0800 07/02/2010 Sun 03:40:16] When I posted the description of the "Dust of Dryness", I didn't see the part about what it did to Water Elementals. Chuck enough of them and they're utterly destroyed (on that 5% chance of a nat.-1), MWAHAHAHAHAHAAAAAAAAAAAAAAAAAAAAA!
[spoiler]Aside, prepared dust of dryness pellets (marble-sized) is now fresh in my memory again alongside immovable rods and Quall's feather tokens in terms of usefulness.
[-0800 07/02/2010 Sun 03:45:57] I've just wowed myself on my
remembrance anamnesis of "Quall".[/spoiler]
Reference(s):
Water Elemental -
http://www.d20srd.org/srd/monsters/elemental.htmDust of Dryness wondrous item -
http://www.d20srd.org/srd/magicItems/wondrousItems.htm#dustofDryness