An aquifer is a geologic unit that can store and transmit water at rates fast enough to supply reasonable amounts to wells. Unconsolidated sands and gravels, sandstones, limestones and dolostones, basalt flows, and fractured plutonic igneous and metamorphic rocks are examples of rock units known to be aquifers.* Aquifers can be unconfined, as depicted to the right, where a continous layer of highly permeable material extends from the land surface to the bottom of the aquifer, or they can be confined, where a layer of highly permeable material is overlain by a relatively impermeable layer. There is no water table in a confined aquifer as there is in an unconfined aquifer. The water table represents groundwater experiencing the same pressure underground that is felt on the earth's surface. Therefore, a pump must be used to retrieve groundwater from an unconfined aquifer. Since groundwater in a confined aquifer is under pressure, artesian wells can exploit the pressure (head) difference to draw the water to the surface.


Recharge refers to the process by which an aquifer is replenished with fresh water. As seen in the diagram to the right, rainwater falls of the surface and percolates down through an unsaturated zone ("vadose zone") of soil and rock. It continues through the ground until it reaches the water table, the boundary between the unsaturated zone and zone of saturation. Here, it ceases to percolate vertically downward, and it is incorporated into local groundwater flow. The direction of groundwater flow is determined by local topography and local or regional pressure gradients.

*Information courtsey of C.W. Fetter.Applied Hydrogeology, Prentice Hall, New Jersey. 2001

Return to Previous Page