This module explains about the water-bearing geological formations called as aquifers and their types.

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AQUIFER TYPES
by
Prof. A. Balasubramanian
Centre for Advanced Studies in Earth
Science
University of Mysore,India

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Introduction:
Any geological formation that is water-bearing
is called as an aquifer. Such rocks may readily
transmit water to wells and springs.
Wells can be drilled into the aquifers and water
can be pumped out.
An Aquifer is a permeable geologic formation
that stores and transmits water.

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An aquifuge is a rock formation containing no
interconnected openings and therefore neither
transmits nor stores water (Ex. Massive
granite).
An aquiclude can only store water and can not
transmit water (Ex. Clay, and shale).
Aquitard is an impervious and semiconfined
geological formation that transmits water very
slowly (Ex. Shale or clay lenses interbedded
with sand).

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Based on the nature and distribution of water
bearing zones, aquifers could be classified into
four types.
They are
1. Unconfined
2.confined
3.semi-confined and
4.semi- unconfined.
Artesian aquifers are special category of
aquifers.

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Groundwater occurs in the saturated soil and
rock below the water table.
If the aquifer is shallow enough and permeable
enough to allow water to move through it at a
rapid- rate, then people can drill wells into it
and withdraw water.
The level of the water table can naturally
change over time due to changes in weather
cycles and precipitation patterns, streamflow
and geologic changes.

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It may also be due to human activities such as
the increase in impervious surfaces on the
landscape.
The pumping of wells can have a great deal of
influence on water levels below ground.

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Unconfined aquifer (Water table aquifer):
It is a permeable bed saturated with water table
serves as the upper surface of the zone of
saturation, it is other wise be called as water
table aquifers.
Water which is present in an unconfined aquifer
is called as unconfined or phreatic water. Water
table undulates in form of depending upon the
recharge and discharge, pumpage of wells and
permeability.

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Rises and falls correspond to the changes in the
volume of water in storage. This aquifer is
directly accessible to the atmosphere.

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Confined aquifer (Artesian):
It is a completely saturated aquifer which is
underlain and overlain by impervious layers
.The pressure of water is higher than that of the
atmosphere. Water in wells stand above the top
of the aquifer rather than storage changes.
Confined aquifers exhibit only minor changes in
storage and act as conduits from zones of
recharge to those of discharge.

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The imaginary surface to which water rises in
wells tapping an artesian aquifer is known as ‘
Piezomertric surface ‘.

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A well will be “ free flowing” when this
piezometric level rises above the ground
surfaces. Such wells are called ‘artesian wells’
Semi confined ( Leaky ) aquifer:
This type is completely saturated aquifer
bounded above by a semi pervious and below
by an impervious layer.
A semi pervious layer is defined as a layer
which has a low permeability.

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Lowering of piezometric level in leaky aquifers
for example by pumping will generate a vertical
flow of water from semipervious layer into the
pumped aquifer.
Horizontal flow component in the semipervious
layer is negligible since its permeability of is
very low.

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Semi unconfined aquifer:
If the hydraulic conductivity of the
semipervious layer of the above case is very
high an aquifer intermediate between
semiconfined and unconfined aquifers may exit,
and is called as semi unconfined.

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Semi-confined Aquifer
An aquifer that is partly confined by layers of
lower permeability material through
which recharge and discharge may occur.

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Perched Aquifers:
Perched aquifers, are special kinds of phreatic
aquifers occurring whenever an impervious (or
semi-pervious) layer of limited extent is
located between the water table of a phreatic
aquifer and the ground surface, thereby making
a groundwater body, separated from the main
groundwater body, to be formed.

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Sometimes, these aquifers exist only during a
relatively short part of each year as they drain to
the underlying phreatic aquifer. Therefore wells
taping such aquifers yield only temporary or
small quantities of water.

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Artesian aquifer
An artesian aquifer is a confined aquifer
containing groundwater under positive pressure.
This causes the water level in a well to rise to a
point where hydrostatic equilibrium has been
reached.
An artesian aquifer is confined between
impermeable rocks or clay which causes this
positive pressure.

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An artesian aquifer is confined
between impermeable rocks or clay which
causes this positive pressure.
Not all the aquifers are artesian, because
the water table must reach the surface .

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A flowing artesian well is one that has been
drilled into an aquifer where the pressure within
the aquifer forces the groundwater to rise above
the land surface naturally without using a pump.
Water movement in aquifers
Water movement in aquifers is highly
dependent of the permeability of the aquifer
material.

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Permeable material contains interconnected
cracks or spaces that are both numerous enough
and large enough to allow water to move freely.
In some permeable materials groundwater may
move several metres in a day; in other places, it
moves only a few centimeters in a century.
Groundwater moves very slowly through
relatively impermeable materials such as clay
and shale.

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After entering an aquifer, water moves slowly
toward lower lying places and eventually is
discharged from the aquifer from springs, seeps
into streams, or is withdrawn from the ground
by wells.
An aquifer is a valuable geologic layer of
porous and permeable material such as sand and
gravel, limestone, or sandstone, through which
water flows and is stored.