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Text on this page is printable and can be used according to our Terms of Service. Any interactives on this page can only be played while you are visiting our website. You cannot download interactives. An aquifer is an underground layer of rock that holds groundwater. Groundwater is water that has infiltrated the ground to fill the spaces between sediments and cracks in rock.
Groundwater is fed by precipitation and can resurface to replenish streams, rivers, and lakes. This is a great way to illustrate the concept of how the ground, if it is permeable enough, can hold water but still stay solid. The upper surface of this water-filled area, or "zone of saturation", is called the water table.
The saturated area beneath the water table is called an aquifer, and aquifers are huge storehouses of water. In our sand hole example, you have essentially dug a "well" that exposes the water table, with an aquifer beneath it. At the beach, the level of the water table is always at the same level as the ocean , which is just below the surface of the beach.
As you may have read, most of the void spaces in the rocks below the water table are filled with water. These rocks have different porosity and permeability characteristics, which means that water does not move around the same way in all rocks below ground. When a water-bearing rock readily transmits water to wells and springs , it is called an aquifer. Wells can be drilled into the aquifers and water can be pumped out.
Precipitation eventually adds water recharge into the porous rock of the aquifer. The rate of recharge is not the same for all aquifers, though, and that must be considered when pumping water from a well. Pumping too much water too fast draws down the water in the aquifer and eventually causes a well to yield less and less water and even run dry. In fact, pumping your well too much can even cause your neighbor's well to run dry if you both are pumping from the same aquifer. In the diagram below, you can see how the ground below the water table the blue area is saturated with water.
The "unsaturated zone" above the water table the gray area still contains water after all, plants' roots live in this area , but it is not totally saturated with water.
You can see this in the two drawings at the bottom of the diagram, which show a close-up of how water is stored in between underground rock particles. Sometimes the porous rock layers become tilted in the earth. There might be a confining layer of less porous rock both above and below the porous layer. This is an example of a confined aquifer. In this case, the rocks surrounding the aquifer confines the pressure in the porous rock and its water. If a well is drilled into this "pressurized" aquifer, the internal pressure might depending on the ability of the rock to transport water be enough to push the water up the well and up to the surface without the aid of a pump, sometimes completely out of the well.
This type of well is called artesian. The pressure of water from an artesian well can be quite dramatic. A relationship does not necessarily exist between the water-bearing capacity of rocks and the depth at which they are found. A very dense granite that will yield little or no water to a well may be exposed at the land surface.
Conversely, a porous sandstone may lie hundreds or thousands of feet below the land surface and may yield hundreds of gallons per minute of water. Rocks that yield freshwater have been found at depths of more than 6, feet, and salty water has come from oil wells at depths of more than 30, feet. On the average, however, the porosity and permeability of rocks decrease as their depth below land surface increases; the pores and cracks in rocks at great depths are closed or greatly reduced in size because of the weight of overlying rocks.
The illustration shows an artesian well and a flowing artesian well, which are drilled into a confined aquifer, and a water table well, which is drilled into an unconfined aquifer. Also shown are the Piezometric surface in the confined aquifer and the impermeable, confining layer between the confined and unconfined aquifer. Groundwater occurs in the saturated soil and rock below the water table.
If the aquifer is shallow enough and permeable enough to allow water to move through it at a rapid-enough rate, then people can drill wells into it and withdraw water. The level of the water table can naturally change over time due to changes in weather cycles and precipitation patterns, streamflow and geologic changes, and even human-induced changes, such as the increase in impervious surfaces on the landscape. The pumping of wells can have a great deal of influence on water levels below ground , especially in the vicinity of the well, as this diagram shows.
If water is withdrawn from the ground at a faster rate that it is replenished, either by infiltration from the surface or from streams , then the water table can become lower, resulting in a "cone of depression" around the well.
Depending on geologic and hydrologic conditions of the aquifer, the impact on the level of the water table can be short-lived or last for decades, and it can fall a small amount or many hundreds of feet. Excessive pumping can lower the water table so much that the wells no longer supply water—they can "go dry.
A perched aquifer's water table is usually highly sensitive to the amount of seasonal recharge so a perched aquifer typically can go dry in summers or during drought years.
Why is Groundwater So Clean? Aquifers are natural filters that trap sediment and other particles like bacteria and provide natural purification of the ground water flowing through them. Like a coffee filter, the pore spaces in an aquifer's rock or sediment purify ground water of particulate matter the 'coffee grounds' but not of dissolved substances the 'coffee'.
Also, like any filter, if the pore sizes are too large, particles like bacteria can get through. This can be a problem in aquifers in fractured rock like the Snake River Plain, or areas outside the sediment-filled valleys of southeast Idaho. Clay particles and other mineral surfaces in an aquifer also can trap dissolved substances or at least slow them down so they don't move as fast as water percolating through the aquifer.
Natural filtration in soils is very important in recharge areas and in irrigated areas above unconfined aquifers, where water applied at the surface can percolate through the soil to the water table. For example, in the lower Portneuf River valley Figure 1 , a protective layer of silt in the southern valley provides natural protection to the aquifer from septic systems, pesticide application, and accidental chemical spills.
Despite natural purification, concentrations of some elements in ground water can be high in instances where the rocks and minerals of an aquifer contribute high concentrations of certain elements. In some cases, such as iron staining, health impacts due to high concentrations of dissolved iron are not a problem as much as the aesthetic quality of the drinking water supply.
In other cases, where elements such as fluoride, uranium, or arsenic occur naturally in high concentrations, human health may be affected. How is an Aquifer Contaminated? In other areas, where the rock and soil are looser and more permeable, groundwater can move several feet in a day.
The water in an aquifer can be held beneath the Earth's surface for many centuries: Hydrologists estimate that the water in some aquifers is more than 10, years old meaning that it fell to the Earth's surface as rain or snow roughly 6, years before Egypt's Great Pyramid of Giza was built.
The oldest groundwater ever found was discovered 2 miles 2. But the deeper one digs for water, the saltier the liquid becomes, Phillips said. Much of the drinking water on which society depends is contained in shallow aquifers. 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.
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