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There are approximately 1.385 billion cubic kilometers of water on, above, and in the Earth. However, the vast majority of the Earth's water (around 97 percent) is found in its oceans. Groundwater, though constituting less than 2 percent of the world's water, accounts for more than 98 percent of its available freshwater resources. In the United States, more than half of the population depends on groundwater, while in semiarid to arid regions the percentage of people using groundwater is almost 100 percent.

Groundwater is water found in pore spaces (pores) in soil and rocks. It differs from soil water in that groundwater is found below a water table--the demarcation separating saturated pores from unsaturated pores. Soil or rock that is saturated with water and is capable of supporting a well is called an aquifer. An aquifer can comprise one or multiple rock or sediment layers, ranging in size from a few meters to several hundred kilometers long, several meters thick, and meters to kilometers in width. An aquifer could be unconsolidated material, such as sand, or consolidated material, such as sandstone and other rocks. Limestone also makes good aquifers.

Aquifers may be divided into two main types. Open aquifers enjoy direct access to surface water. Closed aquifers sit underneath confining layers of semi- or nonpermeable material, which prevent such access. Water recharges aquifers by infiltration or seepage through permeable materials, and the recharge rate depends on the type of surface material, land cover, slope, and amount and intensity of precipitation melt.

The effect of global warming on groundwater varies from place to place, with differing effects on water quality and quantity. In semiarid to arid regions, the higher rate of evaporation increases the concentration of dissolved salts in the groundwater, as is the case in Lake Chad, in Africa, where the surface water in some closed areas is becoming more saline. This saline water may recharge the African groundwater system, making the water saline.

With global warming, snow and ice caps will melt, resulting in rapid sea-level rise. This rise may force saline waters into aquifers, reducing the water's suitability for human and ecological use. Already, such seawater intrusion has occurred along some coastal aquifers, including one in El Paso, Texas.

Global temperature rise will lead to an increase in precipitation in the Northern Hemisphere. As snow and glaciers melt, the groundwater table will rise in most places. Sinkhole numbers may increase in limestone terrains with higher rates of solution weathering resulting from acid rain. These sinkholes could become conduits through which contaminants from the land surface could reach the groundwater.

Groundwater resident time (the length of time water stays in an aquifer) varies from a few days to several thousand years. Generally, the shorter the resident time, the greater the infiltration rate or the shallower the aquifer. In areas such as Libya, where resident time can stretch to hundreds or thousands of years, drought-induced demands for groundwater can cause faster depletion of that water. In areas where the land surface is highly permeable and where rainfall or snowmelt is heavy, the resident time may be low; such areas may experience a dramatic rise in groundwater level. Assuming the anthropogenic effect upon groundwater is minimized or maintained at its current rate, a reduction in rainfall would lead to the lowering of water tables. Such lower water tables could cause ground compaction, loss of permeability of the land surface, and an increased runoff that could lead to flooding. Groundwater withdrawal could also lead to ground subsidence, as has occurred in Texas and California. Heavy cracks, rills, or gullies may develop on the surface, as in Lake Chad, where the soil is clayey. Global warming is likely to increase flood and drought conditions in different parts of the world. This in turn would affect the rate of evaporation, lowering water levels in rivers and lakes during the summer months. Some of these water bodies could become groundwater recharging zones. Nearsurface water tables will lose increasing amounts of water to evaporation as surface temperatures increase. In addition, surface water infiltration will lead to a reduction of groundwater flows to lakes.

Bibliography:

1) Anderson, M. P. "Introducing Groundwater Physics." Physics Today 60, no. 5 (May, 2007): 42.

2) Fetter, C. W. Applied Hydrogeology. 4th ed. Upper Saddle River, N.J.: Prentice Hall, 2001.

3) Hudak, Paul F. Principles of Hydrogeology. 3d ed. Boca Raton, Fla.: CRC Press, 2005.

4) Todds, David Keith, and Larry W. Mays. Groundwater Hydrology. Hoboken, N.J.: Wiley, 2005.

5) Younger, P. L. Groundwater in the Environment: An Introduction. Malden, Mass.: Blackwell, 2007.

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