Groundwater is any water found below the land surface. It is found in aquifers, in the pore spaces of rocks, in unconsolidated sediments, as permafrost, and as soil moisture. Groundwater flows to the surface naturally at springs and seeps and can form oases or swamps. It may also be tapped artificially by the digging of wells.
Groundwater is a long-term 'reservoir' of the natural water cycle, as opposed to short-term water reservoirs like the atmosphere and fresh surface water. It is naturally replenished from above, as surface water from precipitation sinks into the ground.
Groundwater is often contained in aquifers, which are subterranean areas (or layers) of permeable material that channel the groundwater's flow. Aquifers can be confined or unconfined. If a confined aquifer follows a downward grade from a recharge zone, groundwater can become pressurized as it flows. This can create artesian wells that flow freely without the need of a pump. The top of an unconfined aquifer is called the water table or phreatic groundwater, where water pressure is equal to atmospheric pressure. The region between the land surface and the water table is called the vadose zone (Latin for shallow). Within the vadose zone water is held to soil particles by adhesion and in pore spaces by capillary action.
Groundwater is a highly useful and abundant resource, but it does not renew itself rapidly. If groundwater is extracted intensively, as for irrigation in arid regions, it may become depleted. The most evident problem that may result from this is a lowering of the water table beyond the reach of existing wells. Wells must consequently be deepened to reach the groundwater; in places like India, the water table has dropped hundreds of feet due to over-extraction. A lowered water table may, in turn, cause other problems such as subsidence.
The film of ground water around particles of an aquifer of unconsolidated sediment actually holds the particles apart, and the removal of this water will compact the sediment. Thus the aquifer is permanently reduced in capacity, and the surface of the ground may also subside.