Groundwater is any water found underground, including aquifers, subterranean rivers and streams, permafrost, and soil moisture. Groundwater flows to the surface naturally at springs and oases. It may also be tapped artificially by the digging of wells. The upper limit of abundant groundwater is called the water table.
Groundwater is naturally replenished from above, as surface water from rain, rivers or lakes sinks into the ground. Some groundwater also comes from below, as water from the mantle enters the lithosphere.
Problems with groundwater
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.
In coastal areas, a lowered water table may induce seawater to flow into the ground and mix with the groundwater. This is called a saltwater intrusion. Alternatively, salt from minerals may leach into the groundwater of its own accord
In India, a drop in the water table has been associated with arsenic contamination. It is thought that irrigation for rice production since late 1970s resulted in the withdrawal of large quantities of groundwater, which caused the local water table to drop, allowing oxygen to enter the ground and touching off a reaction that leaches out arsenic from pyrite in the soil. The actual mechanism, however, is yet to be identified with certainty.
Not all groundwater problems are caused by over-extraction. Pollutants dumped on the ground may leach into the soil, and work their way down into aquifers. Movement of water within the aquifer is then likely to spread the pollutant over a wide area, making the groundwater unusable.