The lake which the Bellagio fountains are part of is filled from a private well on the property. It does not pull from the mountain aquifers or from Lake Mead,

The lake which the Bellagio fountains are part of is filled from a private well on the property. It does not pull from the mountain aquifers or from Lake Mead, but from a shallow and rather polluted aquifer underneath the Strip. That aquifer is primarily replenished from ground infiltration; Vegas does get rain, just not a lot of it, and there is still some irrigation which filters into the shallow aquifer. Because it's irrigation and turf runoff, and so shallow that there hasn't been sufficient filtering by soil layers, this aquifer is "salty" (various salts, not just NaCl) and not potable water. It is trapped by clay layers in the soil and does not refill the deeper aquifers or Lake Mead. So they are using that lower-quality groundwater for nonpotable use in the Bellagio lake and fountains. The shallow aquifer is replenished at a sustainable rate, it's just not the high-quality mountain water in the deep aquifer. In fact, replenishment of the shallow aquifer has INCREASED with development - ironically for a desert, the water table would be high enough in parts of Las Vegas to require sump pumps if it wasn't for the shallow-aquifer wells that feed the fountains and golf courses. There is a very limited number, accounted for in existing water rights, of these intensive nonpotable water uses that the Las Vegas Valley can support, but those few play an important role in the overall water cycle.