An aquifer is a geologic formation containing water. This body of water occurs within permeable saturated strata of gravel, rock, or sand, and contains water which may supply streams, wells, etc. Aquifers are commonly referred to using the general term groundwater.
Two main types of aquifers exist: unconfined and confined. An unconfined aquifer is not covered by impermeable material which allows recharge. Recharge occurs when precipitation (i.e. rain, snow, ice) can saturate to the water table. This replaces, or recharges, some of the water which has been removed by evaporation and/or consumption (i.e. drinking, irrigation, etc.). Examples of unconfined aquifers include rivers, streams, lakes, ponds.
A confined aquifer, also known as an artesian aquifer, is covered on the top and bottom by aquatards, relatively impermeable materials. A confined aquifer may have a limited area where the geologic formation forms an outcrop which allows recharge. Other confined aquifers have no area allowing recharge.
An aquifer may span a small area such as an above ground lake within a park, or a large area, such as the Ogallala aquifer in the Midwest United States, which underlies the states of Nebraska, Oklahoma, Texas, New Mexico, Kansas, Colorado, South Dakota, and Wyoming.
Aquifers are measured in acre-feet. An acre-foot equals 325,851 gallons of water, or the amount of water needed to cover one acre of land one foot deep.
Although it may seem to fit the basic definition, the oceans aren't normally classified as aquifers. A sea though, may be considered an above ground aquifer.
Aquifers may consist of freshwater, saltwater, or brackish water. Brackish water occurs when saltwater and freshwater mix together, such as occurs when a river meets the ocean.
About four percent of the water contained in the hydrologic cycle, the movement of water between the atmosphere, oceans and other surface water bodies, and land, is groundwater, a sobering thought considering between 40 and 50 percent of the population depends on groundwater as their primary drinking water source. Because some aquifers have no area allowing recharge, they can go dry. If an area containing an aquifer experiences a drought, an extended period of below normal precipitation, the water table of an aquifer can drop or, in some cases, become depleted. The effect of a drought on an aquifer depends on the severity of the drought and the level and type of water use taking place.