What is an aquifer?

Aquifers have an essential role in the water cycle.

1 . Definition of an aquifer

An aquifer is defined as a geological formation made up of one or more layers of rock; water can be stored and flow out of aquifers. They are located in the "saturated zone" of the Earth's crust, where all available spaces are filled with water.

Aquifers are characterised by certain properties such as permeability and porosity, which define the hydraulic characteristics of the aquifer, that is, the movement of water depends on the type of rocks below the soil surface.

2 . How are aquifers formed?

Aquifers fill when stormwater that originates from rain and snow melt infiltrates into the ground until it reaches impermeable rock layers that do not let the water flow through, so it is stored underground. Stormwater that infiltrates into the soil recharges the porous rocks of aquifers.

3 . Types of aquifers

Aquifers may be classified in several ways, depending on the criteria used:

• The characteristics of the rock: either carbonate rock or detrital rock (made o particles derived from preexisting rocks by weathering).
• Type of cracks or spaces: porous, karst and/or fractured.
• Hydrostatic pressure: unconfined (in contact with the atmosphere and separated by the unsaturated zone), confined or artesian (groundwater is subject to pressure greater than atmospheric pressure) and semiconfined (partially overlain or underlain by a semi-pervious layer).
• Extension: Local (small area) and regional (regionally extensive) aquifer or aquifer system.

Water flow through an aquifer. Source: Hans Hillewaert (Wikipedia).

4 . How are aquifers used

When water flows through aquifers, they serve to convey groundwater to recharge areas such as lakes, swamps, springs, wells or other water abstraction structures. However, when water meets impermeable rock, it leads to water stores, which can be used for water withdrawal as long as it does not exceed recharge, because if that happens they would end up drying up.