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Groundwater
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What is groundwater?
When rain falls to the ground, the water does not stop moving. Some of it flows along the land surface to streams or lakes, some is used by plants, some evaporates and returns to the atmosphere, and some seeps into the ground. Water seeps into the ground much like a glass of water poured onto a pile of sand.
As water seeps into the ground, some of it clings to particles of soil or to roots of plants just below the surface. This moisture provides plants with the water they need to live. Water not used by plants moves deeper into the ground. The water moves downward through empty spaces or cracks in the soil, sand, or rocks until it reaches a layer of rock through which water cannot easily move. This is called the saturation zone. The water then fills the empty spaces and cracks above that layer. The topmost layer of an aquifer is known as the water table. Water tables can be above ground, barely below the surface,or hundreds of feet underground. Other aquifers that are enclosed beneath this are called confined aquifers. Instead of flowing as an underground river, ground water is held in tiny crevices between soil or rock much like water is held in a sponge.
Figure: The Hydrologic Cycle
Water seeping down from the land surface adds to the ground water and is called recharge water. Ground water is recharged from rain water and snowmelt or from water that leaks through the bottom of some lakes and rivers. Ground water can also be recharged when water-supply systems (pipelines and canals) leak and when crops are irrigated with more water than the plants can use. Ground water moves through recharge areas to areas of low elevation, where the water is discharged into streams, agricultural ditches and ponds, bays, and the Atlantic Ocean. This is a very slow process. Ground water can remain in a water table aquifer for several decades and in confined aquifers for centuries or longer.
Aquifer is the name given to underground soil or rock through which ground water can easily move. The amount of ground water that can flow through soils or rock depends on the size of the spaces in the soil or rock and how well the spaces are connected. The amount of spaces is the porosity. Permeability is a measure of how well the spaces are connected.
Aquifers typically consist of gravel, sand, sandstone, or fractured rock such as limestone. These types of materials are permeable because they have large connected spaces that allow ground water to flow through. The spaces in the a gravel aquifer are called pores. The spaces in a fractured rock aquifer are called fractures. If a material contains pores that are not connected, ground water cannot move from one space to another. These materials are said to be impermeable. Materials such as clay or shale have many small pores, but the pores are not well connected. Therefore, clay or shale usually restrict the flow of groundwater. Understanding these properties enables land use planners to determine the best methods of extracting ground water for human use.
Ground water is an important natural resource. More than 50 percent of the people in the United States, including almost everyone who lives in rural areas, use ground water for drinking and other household uses. Ground water is also used in some way by about 75 percent of cities and by many factories. The largest use of ground water is to irrigate crops. We can run out of ground water if more water is discharged than recharged. For example, during periods of dry weather, recharge to the aquifers decreases. If too much ground water is pumped during these times, the water table may fall and wells may go dry.
Ground water can become unusable if it becomes polluted and unsafe to drink. It can become polluted by seepage through landfills, septic tanks and sewage systems, hazardous waste sites, landfills, underground storage tanks, road salt, waste oil disposal, and fertilizers or pesticides used on farms and homes. However, with careful use and by reducing sources of pollution, ground water can continue to be an important natural resource in the future.