Typically when we talk about groundwater sustainability, we talk about it either being sustainable or being mined. Technically that means that if you're withdrawing at a rate where if you stop withdrawing water, the levels recover in time—typically taking about the same length of time you were pumping—it's considered sustainable. If you pump at a rate that exceeds that, it's mined. In other words, it's not being replenished.
On May 13th, 2009. See this statement in context.