From Wikipedia, the free encyclopedia.
Groundwater is any water found below the land surface, including water found in aquifers, in the pore spaces of rocks, unconsolodated sediments, permafrost, and soil moisture. Groundwater flows to the surface naturally at springs, seeps and often forms oases or swamps. It may also be tapped artificially by the digging of wells. The top of an unconfined aquifer is the water table, where water pressure is equal to atmospheric pressure.
Groundwater is a long residence time 'reservoir' in the natural water cycle (as compared to the atmosphere or surface water), which is naturally replenished from above, as surface water from rain, rivers or lakes sinks into the ground. The region between the land surface and the water table is the vadose zone (latin for shallow); in this region, water is under pressure less than atmospheric pressure (suction).
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.
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 seawater 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Sometimes the water movement from the recharge zone to the place where it is withdrawn may take centuries.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, 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.
Water table conditions are frequently of importance to agricultural irrigation, waste disposal (including nuclear waste), and other ecological issues.
