A confined aquifer happens when water in porous layers is trapped by layers that are relatively impermeable, like granite or dense clay. Confined layers tend to be under pressure and typically are saturated through out.
Unconfined layers are where ground water is located. This is water right under the soil, the top of this is considered the water table. The amount of water stored in an aquifer can vary from season to season. Depending on its permeability, aquifers can gain water at a rate of 50 feet per year to 50 inches per century.
They have both recharge and discharge zones. A recharge zone usually occurs at a high elevation where rain, snowmelt, lake or river water seeps into the ground to replenish the aquifer.
A discharge zone can happen anywhere. Natural springs, wetlands and streams can all be areas where the aquifer is losing water.
Many people with wells are accessing an unconfined aquifer layer for drinking water. This can be an issue when there is a drought. If little or no water is entering the ground, the water table can drop and pumps will then go dry. Problems can also occur when industrial, agricultural and other chemicals leach down through the soil and enter the water.
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. Schematic showing a cone of depression around the well, usually the result of overpumping. Water movement in aquifers is highly dependent of the permeability of the aquifer material.
Permeable material contains interconnected cracks or spaces that are both numerous enough and large enough to allow water to move freely. In some permeable materials groundwater may move several meters in a day; in other places, it moves only a few centimeters in a century. Groundwater moves very slowly through relatively impermeable materials such as clay and shale.
Source: Environment Canada. After entering an aquifer, water moves slowly toward lower lying places and eventually is discharged from the aquifer from springs, seeps into streams, or is withdrawn from the ground by wells. Groundwater in aquifers between layers of poorly permeable rock, such as clay or shale, may be confined under pressure.
If such a confined aquifer is tapped by a well, water will rise above the top of the aquifer and may even flow from the well onto the land surface.
Water confined in this way is said to be under artesian pressure, and the aquifer is called an artesian aquifer. Here's a little experiment to show you how artesian pressure works. Fill a plastic sandwich baggie with water, put a straw in through the opening, tape the opening around the straw closed, point the straw upward but don't point the straw towards your teacher or parents!
Artesian water is pushed out through the straw. Do you think you know about groundwater? Quiz icon made by mynamepong from www. At the same time, along the coast, particularly in the highly agricultural Pajaro and Salinas valleys of Central California, seawater can also intrude into depleted aquifers.
Similarly, in some cases, salty groundwater can come from ancient seawater that was isolated in the subsurface sediments until pumping began. The northern San Joaquin Valley near Stockton has problems with pockets of seawater that are drawn via pumps into the freshwater aquifers.
In the northern Sacramento Valley, some pumpers are at risk of tapping into ancient deposits of sea water, which can degrade the quality of the groundwater. Man-made pollutants, once thought to be filtered out by the soil, are now known to harm water quality as well. These contaminants include industrial solvents and pesticides.
In some cases, the challenges to aquifer sustainability are being addressed by recharging aquifers with surface water rather than waiting for them to refill naturally through a process called conjunctive use. In other situations, proper management of aquifers is still in development, particularly as courts have established that groundwater may be pumped and used on land far from the original source. The Water Education Foundation is a nonprofit, tax-exempt, c 3 organization, federal tax ID Header link June 22, Read more.
Header link September 15, Header link November 3, Header link May 15, Water Education Foundation. Aquapedia background January 30, Aquifers Overview Aquifers play an important role as a source of freshwater for urban areas and agricultural irrigation.
0コメント