This presentation by ACWADAM on aquifers states that a systematic approach to groundwater studies is required to understand how water exists in each type of rock or sediment for which hydrogeological mapping of aquifers becomes significant in every watershed programme. The necessity of studying the scientific aspects of groundwater such as its properties and distribution below the earth’s surface is highlighted.
Aquifers are defined as the saturated material (sediment or rock) that is capable of yielding water in quantities sufficient enough to be economically useful. In simple words, aquifers are groundwater bearing formations or groundwater reservoirs. The zone of saturation is described as the zone or layer in the subsurface in which all pores are completely filled with water. An aquifer must possess enough porosity to get saturated and must also be able to release this stored water to wells and springs which can then serve as good sources of water supply.
The two different physical conditions under which water can exist in aquifers i.e., confined and unconfined aquifers is explained. The most common condition is when the water table is exposed to the atmosphere through openings in the overlying regolith and is known as unconfined aquifers. On the other hand confined groundwater has no connection to the atmosphere because the aquifer is bounded by impermeable geological formations at its point of discharge i.e., where wells are located.
This rise and fall of the water table in an unconfined aquifer is related to the changes in the storage of the aquifer. The rise in water table (recharge) indicates that water is added to the aquifer while the fall in water table (discharge) indicates that water is removed from the aquifer. The relationship between an unconfined aquifer and the ground surface is described.
The contribution of groundwater to the ground surface through springs and seepages, which then flows as surface flow is called as baseflow. Baseflow contributes significantly to the surface runoff in a stream or river. Streams, in many parts of India, continue to flow during the dry season due to baseflow contributions. Surface runoff, however, may also include other components such as pure runoff generated by rain, snow melt, releases from reservoirs upstream etc.
Fracture zones and dykes, which act as zones of recharge to confined aquifers is explained. So is the point that the same layer of rock can act as unconfined and confined aquifers for wells in different locations. Perched aquifers that hold a limited amount of groundwater and possess all the qualities of a conventional unconfined aquifer are explained.
This presentation is part of the training modules on planning, development and management of groundwater with special reference to watershed management programmes by ACWADAM. Please write to ACWADAM at email@example.com for sourcing these presentations.