What is an aquifer?

An aquifer is a geologic formation, group of formations, or part of a formation that is capable of yielding usable amounts of water to a well, spring, or other point of discharge. There are two basic types of aquifers: unconfined and confined.  In unconfined aquifers, the groundwater only partially fills the aquifer and the upper surface of the groundwater (the water table) is free to rise and decline. A confined aquifer is sandwiched between two confining beds (layers of impermeable materials such as clay which impede the movement of water into and out of the aquifer). Because of the confining beds, groundwater in these aquifers is under high pressure. The high pressure causes the water level in a well to rise to a level higher than the top of the confined aquifer.