Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of geologic formations. A formation of rock/soil is called an aquifer when it can yield a useable quantity of water. The depth at which soil pore spaces become saturated with water is called the water table. Groundwater is recharged from, and eventually flows to, the surface naturally; natural discharge often occurs at springs and seeps and can form oases or wetlands. Groundwater is also often withdrawn for agricultural, municipal and industrial use by constructing and operating extraction wells.   Groundwater can be a long-term 'reservoir' of the natural water cycle (with residence times from days to millennia), as opposed to short-term water reservoirs like the atmosphere and  fresh surface water (which have residence times from minutes to years). Groundwater is naturally replenished by surface water from precipitation, streams, and rivers when this recharge reaches the water table. It is estimated that the volume of groundwater is fifty times that of surface freshwater. Groundwater makes up about twenty percent of the world's fresh water supply, which is about .61 percent of the entire world's water supply. The temperature of groundwater averages out climactic fluctuations to maintain a relatively steady temperature.   Groundwater is a highly useful and abundant resource, but in arid or semi-arid regions it is in a pre- development state. 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 some places in India and Rajasthan the water table has dropped hundreds of feet due to well pumping. A lowered water table may, in turn, cause other problems such as subsidence.   Sometimes the water movement from the recharge zone to the place where it is withdrawn may take centuries (see figure above). When the usage of water is greater than the recharge, it is referred to as mining water (the water is often called fossil water, due to its geologic age). Under those circumstances it is not a renewable resource.

Groundwater pollution   Iron oxide staining caused by reticulation from an unconfined aquifer in karst topography. Pollutants released to the ground can work their way down into groundwater. Movement of water within the aquifer is then likely to spread the pollutant over a wide area, making the groundwater unsafe to use. One example of widespread groundwater pollution is in the Ganges Plain of northern India and West Bengal  where severe natural pollution by arsenic affects 25% of water wells in the shallower of two regional aquifers. The pollution occurs because aquifer sediments contain organic matter that generates anaerobic and release of the arsenic, normally strongly bound to iron oxides, into the water. In some parts of Rajasthan and northern India fluoride contents are found that is not safe for drinking use.

The recharging process of ground water

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