Aquifers are often formed by water seeping through pores in soil and cracks in rocks, and is held as groundwater within the underground soil or rock. If the ground below consists of permeable material that allows water to penetrate easily, at a certain depth the ground will become saturated with water. The top of this water saturated layer is referred to as the water table, while the saturated layer below is referred to as an aquifer.
Aquifers are permeable geological formations that trap water above or between impermeable layers of underground substrate, including loose unconsolidated material such as sand and gravel, and permeable solid bedrock, such as limestone, chalk, and sandstone. Because the substrate of the aquifer is permeable, water moves freely through it, and consequently it can be extracted for human use with the aid of water wells, providing a valuable source of water for drinking and irrigation, especially in areas where surface water is scarce. While water held in an aquifer does not flow at the same rate as a river, stream, or lake, it is not static, but rather moves through the underground spaces from a high point to a low point due to the forces of gravity; or it will move as a result of pressure gradients, from a high pressure to a low pressure until evenly dispersed. One of the greatest aquifers in the U.S is the Ogallala aquifer located under the central states.
Types of Aquifers
Aquifers can be classified into two types: confined aquifers, and unconfined aquifers. Unconfined aquifers consist of a saturated layer of permeable substrate situated below an unsaturated layer of permeable substrate, which allows water to seep down to the aquifer directly from the surface above. Confined aquifers, on the other hand, consist of a saturated layer of permeable substrate situated between two impermeable layers of substrate; the upper impermeable layer effectively prevents water from seeping down into the aquifer directly from the ground above. Instead, the water enters the aquifer further afield where the upper substrate consists of permeable material, and moves laterally within the permeable layer, filling up the spaces until saturation is achieved.
Consequently, unconfined aquifers are generally found in shallow surface layers, where they are more vulnerable to contamination from accidental spills, runoff, and leaching of pollutants. They also do not have the protection of an upper impermeable layer to protect them from pollutants that may readily leach through the permeable substrate above to contaminate the water.
Aquifers situated near the coast can also become contaminated with saltwater due to saltwater intrusion. This can occur as a result of water being pumped from the aquifer, reducing the water pressure, which allows seawater to be drawn into the aquifer, moving from a high pressure to a low pressure, contaminating the freshwater held in underground reservoirs as it does so.
Accessing Groundwater Stored in Aquifers
Groundwater stored in an underground aquifer is exploited by constructing a well. An artesian aquifer is where groundwater is held in a confined aquifer under pressure, which allows it to move readily up a well shaft – sometimes even flowing freely to the surface of the well without being pumped out, if the force of the pressure is high enough. More typically water is brought to the well surface with the assistance of pumps, but more primitive methods include the use of a pulley and bucket system to bring groundwater to the surface.