Formation of underground water is mainly due to two processes:
1. Seeping of river and stream water
2. Direct seeping of rain water
The number of intermolecular spaces between the soil particles is quite high and water percolates down through a region called unsaturated zone (the region is not fully saturated with water but contains lot of air with water).
Water in the unsaturated zone is called Vadose water. Due to instauration this water cannot be pumped out, although plants utilize it fully.
Below the unsaturated zone, there is a saturated zone where all the spaces between the soil particles are occupied solely by water molecules. This water in the saturated zone is underground water. The boundary above the saturated zone is known as Water Table.
The transition region between the unsaturated zone (Vadose water) and saturated zone (groundwater) is called the Capillary Frinze.
In this region, water rises into the unsaturated zone in small cracks, due to intermolecular attraction between water and soil-rock particles.
An aquifer can be defined as the saturated geologic layer, which is permeable enough to allow water to flow with ease through it.
An aquifer sits on the top of less permeable layer which greatly restricts the movement of ground water. This less permeable layer is called confining bed or aquitard or aquiclude.
When ground water is sandwiched between two confining layer it is known as confined aquifers.
In a well drilled into unconfined aquifer, the water pressure is same as atmospheric pressure. On the other hand, in a confined aquifer the water pressure is greater than atmospheric pressure and a well drilled into this confined aquifer water will rise above the upper surface of the confined aquifer, such well is called an artesian well.
The surface up to which water would rise in an artesian well is known as piezometric surface or potentiometric surface.
In certain cases, excessive pressure might occur in a confined aquifer causing water to flow automatically, i.e., pumping is not required. Such well is called flowing artesian well.