Question

Which type of aquifer has an impervious layer above and below it?

• A. Confined aquifer
• B. Unconfined aquifer
• C. Perched aquifer
• D. Alluvial Aquifer

Hint:

• Confined Aquifer: Bounded above and below by impervious layers (aquitards or aquicludes). Water is under pressure.
• Unconfined Aquifer: Bounded below by an impervious layer; its upper surface is the water table (open to atmospheric pressure).
• Perched Aquifer: A local saturated zone above the main water table, underlain by a lens of impervious material.
• Alluvial aquifer refers to the geological material (river deposits).

Answer 1

The Correct Option is A

Aquifers are underground layers of water-bearing permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). They are classified based on their hydrogeological characteristics:

• Confined Aquifer (Artesian Aquifer) (option a): A confined aquifer is an aquifer that is sandwiched between two impervious layers (also called aquicludes or confining layers), one above and one below it. The water in a confined aquifer is typically under pressure greater than atmospheric pressure. When a well penetrates a confined aquifer, the water level in the well will rise above the top of the aquifer. If the pressure is high enough, water may flow freely from the well (an artesian well).
• Unconfined Aquifer (Water Table Aquifer) (option b): An unconfined aquifer has an impervious layer at its base, but its upper surface is the water table, which is free to rise and fall and is at atmospheric pressure. It is directly recharged by precipitation infiltrating from the ground surface.
• Perched Aquifer (option c): A perched aquifer is a localized zone of saturation that occurs above the main regional water table. It forms when an impermeable or slowly permeable layer (e.g., a clay lens) within the unsaturated zone intercepts percolating water, causing it to "perch" or accumulate. It has an impervious layer below it (the perching layer) but is typically unconfined above.
• Alluvial Aquifer (option d): This term refers to an aquifer formed in alluvial deposits (sediments like sand and gravel deposited by rivers). Alluvial aquifers can be either unconfined or confined, depending on the presence and nature of overlying and underlying layers. It describes the material, not necessarily the confinement status.

The description "an impervious layer above and below it" specifically defines a confined aquifer. \[ \boxed{\text{Confined aquifer}} \]