Aquifers correspond to geological formations (rocks and geological materials) that are characterized by being permeable, that is, they have the capacity to store and transmit water easily.
However, it should be noted that there is a variety of geological formations capable of storing water and that its transmission capacity is variable.
From the point of view of hydrogeology, these geological formations can be divided into four fundamental groups.
- Aquifers
- Aquitards
- Aquicluded
- Aquifugos
Aquifers
The aquifers They are characterized by being able to store and transmit water easily, that is, they have a high drainage capacity.
Therefore, they are of great importance to satisfy the needs of water supply in agriculture, industry and for human consumption.
The geological materials that are apt to be considered as a aquifer, generally correspond to sands, gravels and sandstones due to their characteristic primary porosity.
Instead, aquifers that are formed in calcareous materials, volcanic or plutonic rocks are usually due to their secondary porosity due to fractures.
Aquitards
Aquitards have the capacity to store a large amount of water, but their drainage capacity is medium to low and their water transmission capacity is usually low.
Therefore, they are not usually the focus of the study to find water sources that supply the needs of the human being.
However, aquitards play a very important role in the transmission of water in vertical recharges of aquifers in great extensions.
Common examples of geological formations that serve as aquitards are: silts (siltstones, mudstones), silty sands, clayey sands, and any material that is contaminated with clays.
Aquicluded
Aquacludes are geological materials that can contain large amounts of water in their pores.
However, they are characterized by the fact that their drainage capacity is very low and their capacity to transmit water is null.
Technically, aquicludes are known as impermeable materials, but in geological engineering they are studied to avoid problems in civil works.
Some examples are clays (shales) and silty clays.
Aquifugos
The aquifugos correspond to the geological materials that are incapable of containing and transmitting water, therefore, they are considered impermeable.
They are represented by very compact rocks that are not fractured or karstified, such as: granites and basalts (igneous rocks), limestones (calcareous rocks).
Types of aquifers
To understand the different types of aquifers It is correct to understand the concept of the water table, for this we recommend you read the article on the water table.
From the point of view of hydrogeology and hydraulic-structural characteristics, we have the following types of aquifers :
- Free
- Confined
- semi-confined
- perched
Free aquifer
The aquifer Free is one that occurs very close to the surface, that is, it is clearly affected by atmospheric pressure.
Its base is usually mainly an impermeable rock (aquifugos), while its roof is the same water table.
The water table will fall if water is extracted from the aquifer just like removing water from a swimming pool.
Confined aquifers
The aquifer Confined is one that is located between two impermeable geological formations and its water level is equal to the piezometric level.
That is to say that at its base and roof there will be impermeable rocks that subject the water to pressure greater than atmospheric.
The water level in this case is usually above the aquifer so when a well is made, the water comes out under pressure.
If water is pumped (withdrawn) from the aquifer, the pores are not emptied until the piezometric level drops to such a point that it begins to be less than the limit of the upper layer.
That is to say that when the aquifer confined, subsidence phenomena or ground settlement could occur.
Semi-confined aquifer
The aquifer semi-confined is more common than confined in nature.
It is surrounded by rock formations or impermeable materials at its base and semi-permeable at the top (aquitards).
The recharge of this aquifer It occurs by vertical descent of water from an aquitard.
Therefore, this aquifer is also subjected to a pressure greater than atmospheric and the water level corresponds to a piezometric level.
Perched aquifers
Perched aquifers are those that have been isolated in a rock formation.
They can be generated by lateral and vertical changes in the rock composition and sedimentary composition.