With respect to water retention and discharge capacity, geological formations are classified as follows:
1) AQUIFIER
2) AQUICLUDE
3) AQUITARD
4) AQUIFUGE
1) AQUIFIER:-
Aquifer is a type of geological formation which have sufficient porosity and sufficient permeability.
i.e., Aquifer can retain the water and we can discharge water from these aquifer.
Example: Formations with fine sand and coarse silt.
2) AQUICLUDE:-
Aquiclude is a type of geological formation which are highly porous but are practically impermeable.
Due to impermeability of these formations we cannot discharge the water through them.
Example: Formation with clay.
3) AQUITARD:-
The geological formation which are having sufficient porosity but are less permeable, then these formations are called as "Aquitard".
Due to sufficient porousness the water seeps through them and we cannot take any discharge from them.
Example: Formations with sandy clay and silty clay.
4) AQUIFUGE:-
These are the geological formations which are neither porous nor permeable. The water does not seeps through it as well as the discharge from these is not possible.
Example: Formations with granite, sand stone, marble.