Groundwater
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Groundwater is water flowing within aquifers below the water table. Within aquifers, the water flows through the pore spaces in unconsolidated sediments and the fractures of rocks. Groundwater is recharged from, and eventually flows to, the surface naturally; natural discharge often occurs at springs and seeps and can form oases or swamps. Groundwater is also often withdrawn for agricultural, municipal and industrial use through man-made wells. The study of the distribution and movement of groundwater is hydrogeology.
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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). The figure shows how deep groundwater (which is quite distant from the surface recharge) can take a very long time to complete its natural cycle. 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; the icecaps and glaciers are the only larger reservoir of fresh water on earth.
Usable groundwater is contained in aquifers, which are subterranean areas (or layers) of permeable material (like sand and gravel) 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 surface, where water pressure is equal to atmospheric pressure.
Typically groundwater is thought of as liquid water flowing through shallow aquifers, but technically it can also include soil moisture, permafrost (frozen soil), immoble water in very low permeability bedrock, and deep geothermal or oil formation water. Groundwater is believed to provide lubrication and buoyancy which allow thrust faults to move. Nearly any point in the Earth's subsurface has water in it, to some degree (it may be very dry or mixed with other fluids). Groundwater is not confined only to the Earth, either; subsurface water on Mars is believed to have given rise to some of the landforms observed there.
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Problems with groundwater
Groundwater overdraft
Groundwater is a highly useful and abundant resource, but in arid or semi-arid regions it cannot renew itself as rapidly as it is being withdrawn by humans. If groundwater is extracted intensively from water wells, as for irrigation or municipal use, it may not recover to its 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 (e.g., California, Texas and India) 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.
Subsidence
In its natural equlibrium state, the groundwater in the pore spaces of the aquifer supports some of the weight of the overlying sediments. When groundwater is depressurized or even removed from aquitards, where the materials are very compressible and pore pressures can be high, compaction may occur. This compaction, typically called subsidence, may be partially recoverable if pressures rebound, but much of it is not. 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.
Seawater intrusion
Generally, in very humid or undeveloped regions, the shape of the water table mimics 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.
Mining groundwater
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
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.
See also
- Aquifer — what groundwater is typically found in
- Hydrogeology — descriptions of groundwater and aquifer properties
- Vadose zone — the region between the water table and the land surface
- Soil moisture — the water in the vadose zone (above the water table)de:Grundwasser
es:Agua subterránea he:מי תהום ja:地下水 nl:Grondwater pl:Woda gruntowa ru:Подземные воды fi:Pohjavesi