Artesian Aquifer Explained

An aquifer is an underground layer of permeable rock, sediment, or soil that holds and transmits groundwater. Aquifers are crucial sources of water.

The question describes a specific type of aquifer that has particular characteristics related to its structure and the pressure of the water it contains. Let's look at the options:

• Perched aquifer: This is a localized zone of saturation above the main water table, typically created by a layer of impermeable material (like clay or rock) within a larger permeable layer. The water in a perched aquifer is not under pressure greater than atmospheric pressure.
• Artesian aquifer: This type of aquifer is confined between two impermeable layers, often called aquicludes or aquitards. Water enters the aquifer in a recharge area, often at a higher elevation. Due to the confinement and the elevation difference, the water in the artesian aquifer is under significant pressure, known as hydrostatic pressure, which is greater than atmospheric pressure. When a well penetrates this aquifer, the pressure can cause water to rise above the top of the aquifer or even flow out onto the surface without pumping (if the pressure is high enough and the ground surface is low enough, creating a flowing artesian well).
• Phreatic aquifer: This is another term for an unconfined aquifer or water table aquifer. In a phreatic aquifer, the upper surface of the saturated zone is the water table, which is directly exposed to atmospheric pressure.
• Water table aquifer: Similar to a phreatic aquifer, this is an unconfined aquifer. The water table marks the level where the water pressure is equal to atmospheric pressure. The water in this aquifer is not confined between impermeable layers in the same way as an artesian aquifer.

Comparing the description in the question with the definitions above, the aquifer confined between two impervious beds with water held at a pressure greater than atmospheric pressure perfectly matches the description of an artesian aquifer.

Therefore, an aquifer which is confined between two impervious beds such as aquicludes, having water held at a pressure greater than atmospheric pressure is known as an artesian aquifer.