The Oglalla's been overdrawn for many years now, and to quote wikipedia: "Once depleted, the aquifer will take over 6,000 years to replenish naturally through rainfall".

Then there's paleowater (water laid down in the Paleolithic, not being naturally recharged) being used in parts of California. They're doing their best to artificially recharge, but still - and there are signs that rising sea levels can eventually 'salt' the aquifers beneath the Central Valley.

So very much not just a migration issue - at least for those of us who sometimes need to eat. :-)