Aquifer Definition

An aquifer is a geological formation that can store and provide water. It consists of one or more layers of permeable rocks where groundwater accumulates. The water contained in aquifers is of good quality.

How do aquifers form?

Aquifers form when rainwater seeps through various layers of permeable rocks until it reaches an impermeable layer. The impermeable layer can be made of clay, granite, quartzite, etc., and the aquifer’s water accumulates above it.

An aquifer’s water can remain underground or come up to the surface through a spring or well, or supply other bodies of water or watercourses (lakes, rivers, etc.).

What are the parts of an aquifer?

An aquifer essentially consists of two zones:

1. Zone of saturation: the area immediately above the impermeable layer, where the water completely inundates the rock. This is where the water reserve is located.
2. Zone of aeration: the pores, cracks, or spaces in the rock or ground that are not saturated with water. 

This is known as the water table level, the boundary between the saturated and unsaturated zones.

What types of aquifers are there?

There are many types of aquifers, and they can be classified in different ways. Here are a few types.

According to their structure:

• Free: also called unconfined or phreatic. These have a water surface that’s stored in contact with air and with the same atmospheric pressure conditions. 
• Confined: also called captive. The water is completely contained in the pores of the rock and is under pressure greater than atmospheric pressure. If water is extracted from this type of aquifer, the pores are not emptied, but the water pressure inside them decreases, which can cause settling and sinking of the land (subsidence).
• Semi-confined: these are more common than confined aquifers; some of their confining layers are semi-permeable. These layers are known as aquitards; they contain water but transmit it very slowly.
• Multilayer: these are common and are composed of layers with varying levels of permeability.

According to their lithological configuration or characteristics:

• Porous: these are permeable due to the intergranular porosity of the sediments or rocks of which they are composed. They can consist of sand, arcose, sandstone, gravel, conglomerates, and more.
• Fissure or karst: their permeability is due to the cracks and fissures (both of mechanical origin as well as by dissolution) of the rocks that make them up. They can be composed of limestone, dolomite, igneous rocks, metamorphic rocks, gypsum, granite, basalt, etc.
• Mixed: their permeability is a combination of the previous ones. They can be made up of limestone or calcarenite.

What are aquifers used for?

Aquifers contain 96% of the planet’s fresh water. They are an indispensable water reserve to meet the global demand for water. The process of forming an aquifer involves the slow, repeated filtering of water through various types of materials. This process purifies the water.

What are the biggest threats to aquifers?

The two main threats to aquifers are underground water pollution from industrial and agricultural processes, as well as the excessive extraction of water.

The process of creating and recharging an aquifer is slow, and the global demand for water is leading to extraction rates greater than the refill capacity. In order to minimize the effects of this problem (desertification, marine intrusion, etc.), aquifers can be recharged artificially. Some artificial recharge techniques date back centuries in semi-arid areas. Today, these techniques are grouped under the heading of Managed Aquifer Recharge (MAR).

What are the largest aquifers in the world?

There are 273 localized underground aquifers around the world. The three largest ones are:

1. Nubian Sandstone Aquifer System: it is located in Africa and is made of sandstone. This is a non-renewable aquifer because it contains fossil water reserves, which accumulated when rain in the region was more abundant. It spreads across Libya, Egypt, Chad, and Sudan in the eastern part of the Sahara Desert.
2. Great Artesian Basin in Australia: as the name suggests, it is an artesian aquifer; it is a confined aquifer where the water is under high pressure. It is the only large aquifer found in just one country, and it occupies 23% of Australia.
3. The Guarani Aquifer: strictly speaking, this is not a single aquifer but an aquifer system. It extends through Brazil, Argentina, Paraguay, and Uruguay, and its name refers to the Guarani, an indigenous ethnic group from the area.