1. Aquifer

An aquifer is a saturated formation of the earth. It not only stores the water but also yields it in adequate quantity. Aquifers are highly permeable formations and hence they are considered as the main source of groundwater applications. Unconsolidated deposits of sand, silt, and gravel are examples of an aquifer.

Aquifers are classified into two types based on their occurrence:

a.Unconfined aquifer

b.Confined Aquifer

2. Aquiclude

An aquiclude is a geological formation that is impermeable to the flow of water. It contains a large amount of water in it but it does not permit water through it and also does not yield water. It is because of its high porosity. It can also be defined as - A body of relatively impermeable rock that is capable of absorbing water slowly but does not transmit it rapidly enough to supply a well or spring. Clay is an example of an aquiclude.

3. Aquifuge

An aquifuge is an impermeable geological formation that is neither porous nor permeable - which means it cannot store water in it and at the same time it cannot permit water through it. An aquiclude is a formation that has very low hydraulic conductivity and hardly transmits water. Compact rock(or bedrock) is an example of an aquifuge.