Understanding Aquifer Types: Confined Aquifers Explained

The question asks to identify the type of aquifer that is situated between two impervious layers of rock. Let's explore the different types of aquifers to understand which one fits this description. An aquifer is a geologic formation that contains sufficient groundwater to yield significant quantities to wells and springs. The nature of the confining layers significantly influences how an aquifer behaves.

Defining Different Aquifer Classifications

Here's a breakdown of the aquifer types mentioned in the options:

• Unconfined Aquifer: This type of aquifer is directly connected to the surface and its upper level is the water table. It is not bounded by an impervious layer above. Water enters this aquifer through recharge areas on the surface.
• Non-artesian Aquifer: This term is often used synonymously with an unconfined aquifer, emphasizing that the water within it is not under pressure sufficient to cause it to rise above the top of the aquifer in a well.
• Confined Aquifer: This aquifer is found between two layers of impermeable material, often referred to as confining beds or aquicludes (like impervious rock formations). The water within a confined aquifer is under pressure (hydrostatic pressure) greater than atmospheric pressure. Wells drilled into confined aquifers may exhibit artesian pressure, where water rises above the aquifer's top.
• Perched Aquifer: This is a special case where a small, localized zone of saturation exists above the main regional water table, separated from it by an unsaturated layer. It typically sits on top of a lens-shaped impervious layer.

Identifying the Correct Aquifer Type

The question specifically states that the aquifer is confined "on its upper and undersurface, by impervious rock formation". This description perfectly matches the definition of a confined aquifer. The impervious rock formations act as confining layers, preventing the free recharge from the surface and trapping the water under pressure.

Therefore, when an aquifer is bounded above and below by impervious layers, it is known as a confined aquifer.