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. The study of the distribution and behavior of groundwater is hydrogeology.

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, streams, and rivers infiltrates into the ground. It is estimated that groundwater is fifty times that of surface freshwater[1].

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 the upper 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 unsaturated or vadose zone ("vadose" is Latin for "shallow"). Within the vadose zone water is held to soil particles by adhesion (funiculary groundwater) and in pore spaces by capillary action (capillary groundwater).

Problems with groundwater

Groundwater is a highly useful and abundant resource, but it does not renew itself rapidly. If groundwater is extracted intensively from water wells, 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 some places in 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. The city of New Orleans, Louisiana is actually below sea level today, and its subsidence is partly caused by removal of ground water under it.

Generally (but not always) ground water flows in the same direction as the slope of the surface. The recharge zone of an aquifer near the seacoast is likely to be inland, often at considerable distance. In these coastal areas, a lowered water table may induce sea water to reverse the flow toward the sea. Sea water moving inland is called a saltwater intrusion. Alternatively, salt from mineral beds may leach into the groundwater of its own accord.

Sometimes the water movement from the recharge zone to the place where it is withdrawn may take centuries. When the usage of water is greater than the recharge, it is referred to as mining water. Under those circumstances it is not a renewable resource.

In India and Bangladesh, 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 or in landfills 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. Areas of karst topography on limestone bedrock are especially vulnerable to surface pollution. Sinkholes and underground caverns allow direct groundwater flow without the filtering effect of a permeable aquifer. See environmental engineering and remediation.

Water table conditions are of great importance to agricultural irrigation, waste disposal (including nuclear waste), and other ecological issues.