An aquifer, essentially, is an underground water reservoir. They are particularly important geological phenomena because large aquifers can be rich sources of very clean water for human habitation above. One of the world’s largest aquifers, the massive Ogallala Aquifer, lies beneath the Great Plains and supplies water to people and farms in eight American states.
In geological terms, an aquifer is not necessarily an area in which water simply exists as a large subsurface lake or sea. Instead, it may be found within an area in which rock is unusually porous and becomes saturated with water. Sand, gravel, and fissures in the bedrock, for instance, can all become saturated with water. In contrast, around the aquifer may be found areas of clay or solid rock which cannot conduct or hold water, known as “aquitards.” Most surface areas on the Earth are within reach of an aquifer, although the size and depth of that aquifer can vary tremendously.
From a human perspective, aquifers are essential because they represent a source of clean freshwater in areas which cannot be easily supplied by moving surface water (i.e. from rivers and lakes). This makes them highly useful both for drinking water as well as irrigation water. Wells can be dug to access water in aquifers, which is then pumped up for distribution on the surface. Traditionally, more because of technological limitations and cost-effectiveness than any other reason, cities and farms were most commonly supplied by reaching aquifers which were relatively close to the surface. Massive aquifers supply water to large populations in many countries. For instance, the Great Artesian Basin lies beneath almost one-quarter of Australia, while the Guarani Aquifer provides water to Argentina, Brazil, Paraguay, and Uruguay. Underneath the Great Plains, the massive Ogallala Aquifer provides drinking water and irrigation water to Colorado, Kansas, Nebraska, New Mexico, Oklahoma, Wyoming, South Dakota, and Texas.
In relatively recent history, aquifers have thus been a great boon to human development. However, at the same time it is important to recognize the limitations of this resource. Aquifers can replenish, but typically only very slowly. There are exceptions: for instance, the Edwards Aquifer in Texas provides a relatively sustainable water supply for several million people because it is continually being recharged by river flow. However, other places are not so fortunate. The Ogallala Aquifer last filled up during the most recent Ice Age, and currently probably only replenishes a small fraction of what is withdrawn from it each year. The aquifer being used by the Great Manmade River Project in Libya is not being replenished at all, although at current rates of use it can last for centuries.
