Aquifers

• Aquifers are underground water reservoirs from which people pump out water, for agricultural, industrial, or municipal uses
• Most land areas on Earth have some form of aquifer underlying them. Sometimes they are close to the surface, and sometimes they are at significant depths. They either take the form permeable rocks where water is stored in the pores, or of unconsolidated materials like gravel, sand, silt, or clay. Underground rivers, where water flows freely are rare.
• Although water covers more than seventy percent of the earth’s surface, only about one percent is accessible fresh water, and this includes the water stored in aquifers. Water stored in aquifers is critical for a variety of reasons. Not only do they form a huge mass of the accessible fresh water, they are also free from impurities.
• Aquifers have always been critically important in sustaining human habitation, agriculture, and irrigation. Many civilizations and settlements have been established and sustained around aquifers. In many areas, where there are no rivers, lakes, or streams, aquifers are the only source of freshwater
• Even today, rivers do not provide adequate freshwater to many cities, which then depend on water from aquifers for their survival. In modern day cities with its piped water supply, many people still prefer water from aquifers, drawn up via wells. This is because water from aquifers is more reliable and free from chemicals and impurities.

The Various Types of Aquifer are-

1. Unconfined Aquifer:

• An aquifer which is not overlain by any confining layer but has a confining layer at its bottom is called unconfined aquifer. It is normally exposed to the atmosphere and its upper portion is partly saturated with water.
• The upper surface of saturation is called water table which is under atmospheric pressure therefore this aquifer is also called phreatic aquifer.

2. Perched Aquifer:

• It is a special case of an unconfined aquifer. This type of aquifer occurs when an impervious or relatively impervious layer of limited area in the form of a lens is located in the water bearing unconfined aquifer.
• As shown in Fig. 5.4 the water storage created above the lens is perched aquifer and its top layer is called perched water table.

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3. Confined Aquifer:

• It is also called artesian aquifer. It is a type of aquifer overlain as well as underlain by confining layers
• The water within the aquifer is therefore held under pressure. It is sometimes called pressure aquifer also.
• If the aquifer has high outcrop laterally than the ground surface there will be positive hydrostatic pressure to create conditions for a flowing well.
• Water from such well comes to the surface without pumping. The imaginary level upto which the water will rise is called piezometric surface.

4. Leaky Aquifer:

• In nature, truly confined aquifers are rare because the confining layers are not hundred per cent impervious.
• An aquifer which is overlain or underlain by a semi- pervious layer (aquitard) through which vertical leakage takes place due to head difference is called leaky aquifer or semi-confined aquifer.
• The permeability of the semi-confining layer is usually very small as compared to the permeability of the main aquifer
• Thus the water which seeps vertically through the semi-confining layer is diverted internally to proceed horizontally in the main aquifer.