4 Different Types of Geological Formations of Groundwater

Groundwater occurs in many types of geological formations under the ground, some of which can be accessed from the surface, while some cannot.

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The major four types of water-bearing formations under the ground are:

1. Aquifer
2. Aquiclude
3. Aquitard
4. Aquifuge

1. Aquifer

An aquifer is a geological formation with groundwater within a permeable membrane that stores and allows transmission of water through it under ordinary field conditions. Unconsolidated sands and gravels are typical examples of aquifer formation. An aquifer is also called a water-bearing formation or groundwater reservoir.

Fig.2. Aquifer- Types

An aquifer contains saturated material that yields significant quantities of water to wells and springs. Generally, an aquifer formation extends within a larger area and may overlie or be underlain by a confining bed or a relatively impervious material.

An aquifer can be an unconfined or confined aquifer. As aquifer is the major source of water compared; to aquiclude, aquitard and aquifuge; aquifer forms a major study of groundwater hydrology. When we dig a well penetrating a confined aquifer, we call it an artesian well, and a well penetrating an unconfined aquifer is called a water-table well. Read more on Groundwater Hydrology and Aquifers.

2. Aquitard

An aquitard is a geological formation with a poorly permeable membrane (semi-pervious) that permits the storage of water but is not capable of transmitting water in sufficient quantity. Sandy clay, shale, and silty clays (stratigraphic units) are examples of such formations.

Fig.3. Aquifer and Aquitards

An aquitard does not yield appreciable quantities of water to wells. As it is a semi-pervious nature, it transmits water at a slower rate compared to aquifers. 

Figure 3 is a representation of how aquifers and aquitards spread within a ground. An aquitard transmits appreciate quantity of water to or from adjacent aquifers. 

3. Aquiclude

Aquiclude is a geological formation of relatively impermeable material that permits the storage of water, but it is not capable of transmitting water in sufficient quantity. It does not yield appreciable quantities of water to wells. Clay is an example of such a formation.

Fig.4. Aquitard and Aquiclude

An aquiclude is formed when an aquifer is confined by an impervious material like clay.

4. Aquifuge

An aquifuge is a geological formation of relatively impermeable material that neither contains water nor transmits water. Solid granite belongs to this category of formation. 

Also Read: Water Supply Engineering