Instead, groundwater drips slowly and gently through the small spaces within rocks, between rocks, and between loose materials such as sand and gravel.
In fact, water in aquifers can take years to centuries to flow back to the surface, as shown in the figure. A typical flow rate for water in aquifers is ten feet per year. For this reason, if a region experiences no rain for a few weeks, the wells will not immediately run dry.
New water, such as from rain or melting snow, drips down into the ground through the pores and cracks in the rocks and soil. Some of the water sticks to the dirt and rocks close to the surface and some of it continues to drip downward. The layer of ground just below the surface is a mixture of rock, soil, water, and air bubbles.
When gravity pulls the water in the ground deep enough, it fills all of the possible pores and cracks, forcing the air bubbles up. At this depth, the ground becomes saturated with water. The boundary between the unsaturated ground and the saturated ground is called the water table. The exact location of the water table depends on how much new water there is, how quickly the water is flowing away, and how permeable the ground is.
For example, the Ogallala Aquifer — a vast, , square-mile , square kilometers groundwater reservoir — supplies almost one-third of America's agricultural groundwater, and more than 1. Similarly, Texas gets almost 60 percent of its water from groundwater; in Florida, groundwater supplies more than 90 percent of the state's freshwater. But these important sources of freshwater are increasingly endangered.
By , about 30 percent of the Ogallala Aquifer's groundwater had been tapped, according to a study from Kansas State University. Some parts of the Ogallala Aquifer are now dry, and the water table has declined more than feet in other areas. More than two-thirds of this Ogalalla aquifer groundwater could be drained in the next several decades, the study found.
We're taking out old water that isn't being replenished. The same problem is increasingly found throughout the world, especially in areas where a rapidly growing population is placing greater demand on limited aquifer resources — pumping can, in these places, exceed the aquifer's ability to recharge its groundwater supplies. When pumping of groundwater results in a lowering of the water table, then the water table can drop so low that it's below the depth of a well.
In those cases, the well "runs dry" and no water can be removed until the groundwater is recharged — which, in some cases, can take hundreds or thousands of years. When the ground sinks because of groundwater pumping, it is called subsidence. In California's southern San Joaquin Valley, where farmers rely on wells for irrigation, the land surface settled 28 feet 8.
In addition to groundwater levels, the quality of water in an aquifer can be threatened by saltwater intrusion a particular problem in coastal areas , biological contaminants such as manure or septic tank discharge, and industrial chemicals such as pesticides or petroleum products. After the water requirements for plant and soil are satisfied, any excess water will infiltrate to the water table --the top of the zone below which the openings in rocks are saturated.
Below the water table, all the openings in the rocks are full of water that moves through the aquifer to streams, springs, or wells from which water is being withdrawn.
Natural refilling of aquifers at depth is a slow process because ground water moves slowly through the unsaturated zone and the aquifer. The rate of recharge is also an important consideration. It has been estimated, for example, that if the aquifer that underlies the High Plains of Texas and New Mexico--an area of slight precipitation--was emptied, it would take centuries to refill the aquifer at the present small rate of replenishment.
In contrast, a shallow aquifer in an area of substantial precipitation may be replenished almost immediately. Aquifers can be replenished artificially.
For example, large volumes of ground water used for air conditioning are returned to aquifers through recharge wells on Long Island, New York.
Aquifers may be artificially recharged in two main ways: One way is to spread water over the land in pits, furrows, or ditches, or to erect small dams in stream channels to detain and deflect surface runoff, thereby allowing it to infiltrate to the aquifer; the other way is to construct recharge wells and inject water directly into an aquifer as shown on page The latter is a more expensive method but may be justified where the spreading method is not feasible.
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