Some aquifers are overused; people pump out more water than is replaced. As the water is pumped out, the water table slowly falls, requiring wells to be dug deeper, which takes more money and energy. Wells may go completely dry if they are not deep enough to reach into the lowered water table.Other problems may stem from groundwater overuse. Subsidence and saltwater intrusion are two of them.
When water is pumped from an aquifer, the water table declines and wells must be drilled deeper.
The Ogallala Aquifer was filled in the ice age but is being used to irrigate the farms of the Midwestern U.S. at a rate far greater than it is being replenished.