To your surprise, there is an ocean of water beneath the ground on which you stand. This groundwater is actually the purest form of water. There are many sources through which the groundwater gets accumulated. While much of it gets collected below the ground from rains or melting snow, it also gets collected trapped in deep layers of porous rock during their formation.
The water percolates through pores and cracks in the soil and even solid rock. It seeps through these pores till they reach an “impermeable” layer. An impermeable layer is something that doesn’t allow liquids to pass through. This is a very slow process which may even take years to accumulate depending on the type of region. The contents of such a collection below the ground are the ground water. This water is tapped by wells, springs, hand pumps etc.
A permeable formation that holds water is called an aquifer. Aquifer in Latin means ‘water bearer’. Aquifers are normally composed of gravel, sand or rocks. Most rocks are capable of holding water but porous types such as limestone and sandstone have the greatest capacity.
Above the layer of aquifer, there is a much saturated zone called the water table. Above the water table, only a thin film of water clings to the soil particles and the pore spaces are filled with air. The depth of water table varies in different regions. How deep it lies below the ground depends on the amount of rainfall the region receives and how much gets tapped by humans. At some places, water may be close to the ground surface elsewhere it is at a much greater depth.