Many people draw their water from an underground region of soil or porous rock saturated with water, called an aquifer. These stores receive water from rainfall and snowmelt that soaks into the ground through infiltration.
As water trickles down, filling every available space in the aquifer, it becomes naturally filtered by rocks and soil. The top of this water-saturated region, referred to as the water table, rises and falls due to seasonal weather changes.
Aquifers remain dependable sources of water only as long as removal rates don't exceed replenishment rates. Wells can "run dry" if too much water is removed; this often happens if a deeper well produces a cone of depression, an area where the water table is much lower than surrounding areas.
Overpumping wells in coastal aquifers can result in contamination of the aquifer. Normally, "full" freshwater aquifers will not take in seawater, but if the aquifer becomes depleted by overwithdrawing water, salt water can push in and contaminate the aquifer.