Aquifers bear the most important natural resource known to man - groundwater. They are the source of fresh water for drinking and agricultural irrigation; and in desert and urban areas, the only source. Aquifers are underground reservoirs formed when permeable rock transmits water to a well. The wells are bound by impermeable and permeable layers. The water that falls on the earth through precipitation is gathered and stored in surface water, such as lakes and rivers, as well as these underground reservoirs. In aquifers the water is much cleaner because most contaminates are filtered out through the sediment walls. And because aquifers are areas of pressure and water, nothing can live in them, including bacteria.
There are two main types of aquifers - unconfined and confined. Unconfined aquifers are also called water table aquifers because they are bordered directly by the water table. The water table is the saturated surface sediment where the pressure of the groundwater matches the atmospheric pressure. Because these aquifers are located just below the surface they are more susceptible to being over-exploited, especially in highly populated areas. This happens when more water is taken out then can be replenished by precipitation or nearby bodies of surface water. When this occurs the water table lowers. This leaves the fresh groundwater open to contamination. Due to overpopulation in Israel and Libya, many aquifers have been diluted with salt water, known as saline intrusion.
Confined aquifers are generally found below unconfined aquifers. They are bordered by a layer of impermeable rock, which is called an aquitard. Confined aquifers tend to have purer water because the water content is further protected by more layers of sediment. Artesian wells are found in confined aquifers. When the pressure is so great within the water compartment, that when a hole is drilled, the water automatically rushes to the surface, you have pure artesian water.
The rate of water recharge is different for all aquifers. It depends on the permeability of the sediment. Where gravel is highly porous, allowing the water to move quickly, a clay aquifer would slow down the flow of water. The ability of water to move through pores is called hydraulic conductivity. Some groundwater travels at the rate of many meters per day, while some, only inches every hundred years. This of course needs to be considered to preserve aquifers. As long as the water that is taken out is equalled by the water recharge, an aquifer will continue to be a source of fresh water, sustaining a community, or entire region indefinitely. On the contrary, once an aquifer dries up due to over-exploitation, it is gone forever. The science of water flow in aquifers is called hydrogeology.
Aquifers are found in every region of the world, some as deep as 6,000 feet beneath the surface. The largest aquifer, the Guarani Aquifer covers 1.2 million kilometers and supplies fresh water to Brazil, Argentina, Paraguay, and Uruguay. They are an essential component of human civilization, providing our most simple and greatest need - water.