Aquifers are geologic formations capable of yielding significant amounts of groundwater. The groundwater in most aquifers is stored in the open spaces, known as pores, between solid particles comprising the skeletal framework of the sediment or rock of the aquifer. In some cases, groundwater accumulates in open cracks within rock or, more rarely, in cave-like voids in limestone or dolostone. The definition of an aquifer in terms of its ability to yield significant amounts of water makes the definition relative. Whereas a formation may yield enough water to supply a few households in a rural setting and thereby be classified as an aquifer, the same formation may be incapable of supplying the amount of water needed to support a large city or industrial complex. In the second case, the formation would not be considered an aquifer.

The ability of an aquifer to transmit groundwater is known as its hydraulic conductivity or transmissivity (transmissivity is the hydraulic conductivity of an aquifer multiplied by its thickness). Hydrogeologists also use several different measures of the capacity of an aquifer to store and yield water. The simplest of these measures is specific yield, which is the ratio of the volume of water that will flow freely from an aquifer to the total volume of the aquifer. Typical values of specific yield range from 5 to 30 percent.

Aquifers occurring in layered sediments or sedimentary rocks are in many cases separated by less conductive layers known as aquitards. Aquitards can store large amounts of water but, because of their low hydraulic conductivity, yield water very slowly when pumped. Taken together, a series of aquifers and aquitards comprise an aquifer system. Some older books and reports use the term aquiclude to denote a geologic formation that is impermeable, but that word has fallen out of use.

Aquifers can be classified according to their connection with the atmosphere. Water within an unconfined aquifer (sometimes referred to as a phreatic or water-table aquifer) is in direct contact with the atmosphere through open pore spaces of the soil or rock above the water. Water at the top of the saturated portion of an unconfined aquifer, commonly referred to as the water table, is at atmospheric pressure and is free to move vertically in response to water level changes within the aquifer. When an aquitard separates the water within a water-bearing formation from the atmosphere, the aquifer is said to be confined. The aquitard restricts the upward movement of the water within the aquifer and causes the pressure at the top of the aquifer to be at levels greater than atmospheric pressure, giving rise to artesian conditions. Perched aquifers are unconfined aquifers that occur above aquitards that are underlain by unsaturated soil or rock.

Aquifers contain underground layers of water-bearing permeable rock or materials for storing water. (stihii/Shutterstock) (stihii/Shutterstock)

If water is pumped from a confined aquifer system at a rate greater than it can be replenished by infiltration of rain or melted snow, water will be removed from storage in the aquitards. If the amount withdrawn is small, the aquifer system will respond elastically and return to its initial condition when the rate of withdrawal decreases. If the amount withdrawn is large (a condition often referred to as groundwater overdraft), the pore space within the aquifer can permanently collapse and reduce the ability of the aquifer system to store or transmit water in the future even if pumping is curtailed. Permanent collapse of deformation of an aquifer system often gives rise to land subsidence and the development of large cracks known as earth fissures. The term safe yield is often used to refer to the amount of water that may be pumped without significant consequences but, like the definition of an aquifer, the inclusion of significance makes safe yield a relative term.

Aquifers can be susceptible to contamination that may render the water unusable. The susceptibility of an aquifer to contamination depends on whether it is confined or unconfined, its depth, the kind of soil or rock above the aquifer, the kind of sediments or rock comprising the aquifer, the amount of infiltration or recharge to the aquifer, and the topography above the aquifer. Common sources of groundwater contamination include septic systems, leaking gasoline storage tanks, poorly designed landfills or dumps, and illegal disposal of hazardous waste. Aquifers in coastal areas are also susceptible to seawater intrusion if pumping allows buoyant fresh water to be replaced by dense saltwater.

Groundwater is also facing decline and depletion. Aquifer depletion occurs when the rate of water loss/usage outstrips the recharge. As a valuable resource where surface water is scarce, it is important to take into consideration the effects of excessive pumping. For example, the depth of the water table in the basalt well in Umatilla County, Oregon, has gone from 160–260 ft (49–79 m) in the span of less than 50 years. Taking out too much water from underground can lead to the drying up of wells, the reduction of water in streams and lakes, and the deterioration of water quality.

One of the country's most important and one of the world's largest aquifers, the Ogallala Aquifer, which is located beneath the Great Plains, is experiencing depletion. The aquifer spans eight states from South Dakota to Texas, a primary agricultural region, and is a victim of overpumping.