What is an aquifer: definition and how it is formed

According to its etymological origin, aquifer comes from the Latin “aqua” which means “water” and “fero” which literally means “I carry”, thus in Latin it literally means “I carry water”. In terms of geology, an aquifer is an underground geological formation that houses water . Aquifers allow water to pass through fissures and crevices in rocks to such an extent that water can sometimes reach the earth’s surface and other surface water bodies.

In aquifers, several key zones are differentiated: the water table, the saturation zone and the impermeable layer . The water table is the level where the water is and corresponds to the upper sector. The saturation zone is the space where the pores of the rocks are found that can be flooded when the water table rises. Finally, there is the impermeable layer, which is the layer of rocks that allows water to accumulate and move horizontally and vertically. Between the water table and the surface there may be another zone known as the aeration zone.

Groundwater is one more part of the water cycle . Part of the precipitation infiltrates the subsoil and descends until it reaches the rocky material below. Rocky material can be more or less permeable; if it is permeable, it will let water through, but if it is impermeable, the passage of water will be interrupted and it will accumulate.

Two important factors are involved in the formation of groundwater and aquifers :

• Gravity: gravity pulls water towards the center of the Earth, so the water that falls on the surface will try to seep into the ground.
• Lithology: the rock below the Earth’s surface influences the formation of aquifers. If all the bedrock consists of dense material even gravity cannot cause the water to leak out. Bedrock also contains varying amounts of void spaces where groundwater collects and can also break apart creating more spaces. On the other hand, when the bedrock is limestone, the water dissolves the rock, resulting in large cavities that are filled with water.

No matter how intense the force of gravity is, it cannot drag all the water down. In the depth of the earth’s crust there are materials such as granite or clays that prevent water from passing through them. In many cases, these layers are located below others that do allow water to pass through and, therefore, act as a confining layer for groundwater. Since it is more difficult for water to go deep, it tends to accumulate in the porous layers and to flow slowly in a more horizontal direction (parallel to the rock layers) towards the surface.

Precipitation eventually recharges the aquifer. However, the recharge rate is not the same for all aquifers, and that must be considered when drawing water from a well. Pumping too much water in a short space of time can cause its depletion if, in addition, the precipitation does not recharge the aquifer enough.

Aquifers can be classified in different ways according to their characteristics.

Structure

According to their structure, three types of aquifers can be distinguished : free, confined and semi-confined . Free aquifers are those in which there is a water table, that is, a level at which the water is found in the subsoil. This level is determined by the place where the impermeable layer that is located in the subsoil is located and where the earth’s surface is. In some cases the water table can reach the surface and then create springs, wetlands, streams or other bodies of water.

On the other hand, confined aquifers are those in which groundwater is stored under pressure due to the existence of several layers of impermeable rocks. Sometimes the rock layers bend or tilt inside the earth’s crust and this can cause a less porous rock layer to lie both above and below the porous layer, confining the water inside. In this case, the rocks surrounding the aquifer limit the pressure on the porous rock and its water. In these aquifers, the pressure, called artesian pressure, is greater than atmospheric. Consequently, if a well is drilled in this “pressurized” aquifer, the internal pressure could be sufficient (depending on the ability of the rock to carry water) to push the water into the well and to the surface.

Finally, in semi-confined aquifers, the permeability is intermediate but the pressure is equal to atmospheric.

Porosity

According to their porosity, they can be classified into porous aquifers and fissure aquifers . Porous aquifers have numerous interconnected pores where water accumulates. They are not very permeable and in this category you can find alluvial clay or gravel formations. Fissural aquifers are characterized by the existence of cracks in the rock, which are also interconnected but allow a better passage of water. These are the best known aquifers and for example are those that occur in karst areas.

Lithology

They can also be classified based on their lithology or type of rock on which it was formed, but in this case there may be hundreds of types of aquifers. They can settle, for example, on limestone rocks ( karst landscape ), sandstones or volcanic stones.

Water behavior

According to the behavior of the water, they can be differentiated into four types: aquifers, aquitards, aquícludes and aquifuges . Aquifers can be considered as stores and transmitters of groundwater. The aquitards are good stores, but bad transmitters, the acuícludos are only transmitters and the aquifuges do not allow neither the storage nor the transmission of water.

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