A groundwater aquifer is an underground layer that holds water within soil, porous rock, or similar material. Groundwater is the term used to refer to water that exists in the ground, as opposed to surface water, which occurs in ponds, streams and other bodies of water. Groundwater can be found in nearly all areas of the Earth’s surface, but the term groundwater aquifer usually refers to layers that are saturated with water; that is to say that all of the available space, such as between soil particles or in pores in the rock, are filled with water.
A groundwater well may be used to extract water from a groundwater aquifer and may make up a very important part of a region’s water resources, particularly in arid areas or deserts where surface water may be scarce. As a source of sustainable drinking water, a groundwater aquifer normally requires a means of recharging. Most groundwater aquifers are replenished via rivers or rain. This water may seep into the groundwater aquifer as groundwater flow through the soil. Sometimes an aquifer may fill up during a rainy season, and then gradually dry out during the summer.
The depth at which a groundwater aquifer is found in a particular geographic region usually depends on the level of the water table in the area. The water table refers to the depth under the ground at which the pressure of groundwater is equal to the pressure of the atmosphere, and usually is regarded as the top level of accessible groundwater in a given area. It is important to realize that the water table depth changes with time, and there may be a significant difference in the water table depth from season to season. This may affect the depth at which local groundwater aquifers are accessible.
Given the important human reliance on groundwater aquifers in some areas, the management and maintenance of these water sources may be a vital consideration. Some of the issues facing groundwater aquifers may include surface subsidence; water depletion, if water is constantly being removed from the aquifer too quickly for it to naturally replenish; and groundwater contamination. Contamination may occur in freshwater aquifers, especially near the sea, if too much water is removed and saltwater is drawn in. This problem is also sometimes called saltwater intrusion.