Geological formations and their classification

Aquifer

An aquifer is an underground geological formation which contains water and sufficient amount of water can be extracted economically using water wells. Aquifers comprise generally layers of sand and gravel and fracture bedrock.

When water table serves as the upper boundary of the aquifer, the aquifer is known as unconfined aquifer (Fig. 1.5 (a)). As discussed in the earlier section, there exists a capillary zone above the water table. However, in most of the analysis, the capillary zone is neglected and water table is considered as the upper boundary of the aquifer. The unconfined aquifer is also known as water table aquifer and phreatic aquifer. An impervious layer is generally served as the bottom boundary of an unconfined aquifer. Sometime, the bottom of an unconfined aquifer may be semipervious and water may gain and lose through the semipervious bottom layer. The aquifer is then known as leaky unconfined aquifer (Fig. 1.5 (b)).

Fig. 1.5 (a) Unconfined aquifer, (b) Leaky unconfined aquifer

An aquifer which is bounded by two impervious layers at top and bottom of the aquifer is called confined aquifer (Fig. 1.6 (a)). In case of confined aquifer, if we insert a piezometer into the aquifer, the water level will rise above the top impervious layer as the pressure in the aquifer is more than the atmospheric pressure. As such, the confined aquifer is also known as pressure aquifer. Top and bottom layer of a confined aquifer is generally impervious. However, sometimes these layers may be semipervious in nature. In such a situation, the water may gain or lose through these semipervious layers. The aquifer is then called leaky confined aquifer (Fig. 1.6 (b)).

Fig. 1.6 (a) Confined aquifer (b) Leaky confined aquifer

When piezometric surface of a confined aquifer is above the ground level, the confined aquifer is then called an artesian aquifer. For artesian aquifer, if you put a well, the water will come out of the well automatically.