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Water first enters an aquifer through gravity-induced downward movement (percolation), but it also flows in a lateral direction within an aquifer. This flow can occur due to the dip (slope) of the aquifer or artificial conditions induced within the aquifer, such as pumping or recharging. Natural groundwater flow through an aquifer is relatively slow and is about a few centimeters a day.
The flow of water through a granular formation is governed by Darcy’s law which states that the flow through a porous media (Q) is directly proportional to the hydraulic gradient (L/H), where the permeability (K) of the media is the constant of proportionality. However, in hard rock areas, water flow through fracture zones does not follow Darcy’s law as the flow is similar to the flow through an open pipe.
When water from a surface water body like a reservoir or a river contains water at a higher level than the groundwater table, some water flows down as per the permeability of the intervening formation to join the aquifer. The process is known as influent seepage. Conversely, if any part of the water table is located at an elevation higher than a nearby surface water body or a stream, water flows from that part of the aquifer into the water body. The process is referred to as effluent seepage. Many seasonal streams and rivers are known to carry a base flow even long after the rainy season due to effluent seepage.
Hilly regions are known to have poor groundwater storage. This happens because water from rainfall and other surface water bodies that seep underground tend to flow out when exposed to steep mountain sides. It is pretty common in hilly regions to find that water is seeping or spurting out like a spring from along an exposed side of a hill.
Sometimes we find small dug wells at the top of a hill temple that contains water round the year, attributing the phenomenon as a miracle. This happens because the well at the hilltop eventually receives groundwater recharge through fractures or delayed seepage from a recharge area at an even higher elevation in the adjoining hills. Besides, such wells do not get dry in most cases as only a very small quantity of water is extracted for regular use. Nature indeed is a miracle by itself.
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