From the USGS:
The areal and vertical location of the major aquifers is fundamental to the determination of groundwater availability for the Nation. An aquifer is a geologic formation, a group of formations, or a part of a formation that contains sufficient saturated permeable material to yield significant quantities of water to wells and springs.
A two-dimensional map representation of the principal aquifers of the Nation is shown below. The map, which is derived from the Ground Water Atlas of the United States, indicates the areal extent of the uppermost principal aquifers on a national scale. In this map, a principal aquifer is defined as a regionally extensive aquifer or aquifer system that has the potential to be used as a source of potable water. (For study or mapping purposes, aquifers are often combined into aquifer systems.)