#06 | Typologies and variety of aquifers

Author: Eva Mena (EMUASA)

LIFE Nirvana is a European project co-funded by the LIFE Programme that aims to address the environmental problem of groundwater pollution by developing innovative, effective and sustainable technology for the in situ remediation of nitrate-contaminated aquifers.

The experimental phase of the LIFE Nirvana project will be developed in Murcia, specifically in an area that is included in the groundwater body 070.036 – Vega Media and Baja del Segura.

Aquifers are geological formations that contain groundwater. From a practical point of view, an aquifer must be able to store and transmit water in the quantity that can be exploited economically. Clay formations, for example, are capable of storing large amounts of water, but they do not transmit it easily, so they cannot be considered aquifers.

An aquifer behaves as if it were a reservoir, where it is necessary to consider; an inlet flow, an output flow and a storage and regulation capacity.

The flow of water inflow or recharge is generally constituted by infiltrated water from rainfall, surface water, irrigation, wastewater, etc.

In the general regime of operation of the aquifer, the outflow or discharge of water is produced by the surface upwelling in sources and springs or by underground discharge into the channels of rivers, other neighbouring aquifers or the sea. In the case of aquifers exploited by humans, the outflow of water occurs, also by pumping.

The storage capacity of an aquifer is determined by its volume (defined by its extent and thickness) and by its porosity and cracking.

The reserve is the amount of water stored in the aquifer.  It is necessary to differentiate between variable reserve, which can vary according to the inputs and outputs of water, and the invariable reserve, which is independent of the amount of the inputs and outputs, the variable reserve can be exploited indefinitely, while the invariable reserve can be exploited only once unless a recharge is made with surface waters.

There are numerous classifications of aquifers, the most common are presented below that are based on their hydrodynamic, hydraulic and compositional behaviour.

Types of aquifers according to their hydrodynamic behaviour:

From the hydrodynamic point of view we can distinguish the following:

• Aquifers: Geological formations with good characteristics for storing and transmitting water. For example, sand and gravel formations or karstified limestones.
• Aquitaids: These are geological formations with good capacities to store water, but water is transmitted slowly. For example, silts.
• Aquacludes: These are geological formations that can contain water, but their transmission power is very low or none due to their low permeability. For example, clay formations.
• Aquafuges: Geological formations that cannot store water and therefore not transmit it. For example, igneous rocks not fissured or fractured.

 

Types of aquifers according to hydraulic behaviour:

Depending on the characteristics, different types of aquifers can be considered:

• Porous aquifers: Made up of unconsolidated loose materials: water circulates easily through the pores or spaces left by solid particles
• Fissured aquifers: They are made up of consolidated rocks, where water circulates through fissures and cracks.
• Free, unconfident or phreatic aquifers: The surface of the water is at atmospheric pressure because they do not have any land on top that acts as a boundary.
• Captive, confined or pressurized aquifers: The roof of the aquifer is limited by a layer of impermeable soil, so the water is subjected to greater pressure than the atmosphere.
• Semi-captive or semi-confined aquifers: The roof of the aquifer is limited by a semi-permeable terrain, which gives intermediate characteristics between the two types mentioned above.

 

Types of aquifers according to their lithology:

Traditionally, depending on the characteristics of the formation that forms the aquifer, we can differentiate two types or classes of aquifers:

• Carbonate aquifers: These are aquifers linked to carbonate rocks, which are mainly made up of minerals from the carbonate group.
• Detrital aquifers: Detrital aquifers are formed by granular materials or clasts of various nature and size such as sands, gravels, conglomerates, clays, etc.

For the LIFE Nirvana Project, the typology of the aquifer influenced the choice of the pilot, it is important to be able to carry out the project so that it can be carried out to a successful conclusion.  This  chosen aquifer is composed,

• Aquifer in the Vega Media corresponds to a detrital set that reaches 250 m of minimum thickness
• Aquifer of the Vega Baja, the stratigraphic series begins with a metapelitic substrate on which permotriasic materials of carbonate character are arranged mostly that seems to lose permeability on the surface

Two permeable geological formations have been defined in the 070.036_Vega Middle and Lower Segura groundwater body:

• Gravels and sands Plio-Quaternario “Vega del Segura”.
• Limestones and Triassic dolomites “Vega del Segura”.