Types of Aquifers

 Depending on the presence or absence of water table, an aquifer can either be classified as confined or unconfined. A special kind of aquifer known as leaky aquifer represents a combination of the two. A perched aquifer occurs where a groundwater body is separated from the main groundwater body by an impermeable layer of a limited extent.

  • Unconfined aquifers

These are aquifers whose upper surface of saturation zone is under the influence of the atmospheric pressure. The lower boundary is usually defined by a less permeable or impermeable layer. They are also known as water table aquifers, free aquifers or phreatic aquifers. 

  • Confined aquifers

They are characterized by complete saturation, with the upper and lower layers being impermeable. They are not under direct influence of the atmospheric pressure since they are separated from the atmosphere by an impermeable stratum. Water in this aquifer imparts an upward hydrostatic pressure on the upper confining layer. Borewells tapping this aquifers are known as artesian wells. In such wells water flows and rise way above the confining stratum. Unlike in unconfined aquifers, recharge does occur in a specific area.


  • Leaky Aquifers

They occur in situations where a permeable medium is overlain or underlain by a stratum that is semi-permeable. During pumping, water flows by horizontal flow and by vertical flow. 

  • Idealized Aquifers

These are aquifers that are assumed to be homogeneous and isotropic  for purposes of mathematical calculations of the storage and groundwater flow. In real life however, idealized aquifers do not exist.

 Geological framework
To a hydro-geologist, the following key questions should be answered:

  • Why is it important to understand the geologic setting?
  • Is the area structurally complex?
  • Are the rocks metamorphic, igneous, or sedimentary?
  • What geologic mapping has been published or conducted in the area?
  • Is there faulting or structural lineaments in the area?
  • Are the formations flat lying or tilted?

Extrusive Igneous rocks have a greater water holding capacity because of their higher porosity (80%) as compared to intrusive igneous rocks (<1%). Intrusive igneous rocks can produce significant amounts of water only if their secondary porosity (resulting from fracturing and faulting) is substantial.

Metamorphic rocks are those that have undergone a change of form through temperature and pressure changes. Foliation is important in identifying the direction of groundwater flow and potential groundwater zones. Metamorphic rock's ability to transmit water will be determined by the degree of metamorphism. Migmatite, which is a type of metamorphic rock is one type of rock that has poor water holding and transmission capacities. Structurally, migmatites are complex and can be termed as a mixed rock.


Sedimentary rocks are the most common types of water bearing materials that produce potable water. They have high primary porosity and have high hydraulic conductivities (depending on the depositional environment).

The strains caused by different tectonic movements affect the physical properties of formations which can change the direction of flow, porosity, and fluid conductivity. This in turn will affect the well yield.



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