I’ve been getting this wrong.
The Bellagio fountains are mostly using well water that exists beneath the Bellagio landscape. That well water was used previously to maintain a golf course that previously existed there. The beauty of that is the current water use, because of the lake, represents only two-thirds of the water that was used before when the golf course existed, so in reality, the Bellagio uses less water than the golf course that use to be there before.
(I got it wrong in a talk I gave this morning, and a gentleman who knows Vegas water kindly corrected me in the hall after.)