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 If water is withdrawn from the ground at a faster rate that it is replenished, either by infiltration from the surface or from streams , then the water table can become lower,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. Want to learn more about aquifers and groundwater? There is water somewhere beneath your feet no matter where on Earth you live. Groundwater starts as precipitation, just as surface water does, and once water penetrates the ground, it continues moving, sometimes quickly and sometimes very slowly.
Eventually groundwater emerges How much do you know about the water below your feet? The ground stores huge amounts of water and it exists to some degree no matter where on Earth you are. Lucky for people, in many places the water exists in quantities and at depths that wells can be drilled into the water-bearing aquifers and withdrawn to server the many needs people have.
As a non-existent proverb states: " Humans don't live by surface water alone. Groundwater is invaluable for many uses, from irrigation to drinking-water supply. But, you can't see groundwater, so how do water scientists know where it is in order to be able to drill wells and pump it out for use Millions of cubic miles of water exists in the ground.
You can't see it, but not only is it there, it is always moving around -- mostly downward, but also horizontally. Moving groundwater helps keep rivers full of water and allows for people to draw out water via wells.
Moving groundwater is an important part of the water cycle. Wells are extremely important to all societies. In many places wells provide a reliable and ample supply of water for home uses, irrigation, and industries.
Where surface water is scarce, such as in deserts, people couldn't survive and thrive without groundwater, and people use wells to get at underground water.
Groundwater is a valuable resource both in the United States and throughout the world. Groundwater depletion, a term often defined as long-term water-level declines caused by sustained groundwater pumping, is a key issue associated with groundwater use. Water flows over the particles into cracks but cannot get through the particles. As water seeps deeper into the ground, it will eventually reach an impermeable layer and either collect or flow sideways.
This creates underground layers of permeable soil that are saturated with water. Saturated ground has all its holes or pores filled with water. These layers are called aquifers, and they can be small or massive. Unlike surface water collected in rivers and lakes, groundwater is often clean and ready to drink.
This is because the soil actually filters the water. The soil can hold onto pollutants—such as living organisms, harmful chemicals and minerals—and only let the clean water through.
Observations and results Did you notice how the aquifer in the container with only corn kernels pebbles filled almost instantly whereas the one with a layer of cornmeal sand filled slower and the one with a layer of cornstarch clay took a long time? This is to be expected. The larger holes between the kernels or pebbles allow water to seep through quickly—the water drains fast. Cornmeal has smaller particles, just like sand. These particles pack close together and leave little holes in between.
The water can still seep through but takes a little longer. Cornstarch is similar to clay. It consists of very small particles packed closely together. The water has a very hard time getting through this material.
Did you also notice the kernels only filtered out black pepper whereas the cornmeal filtered out most of the cacao powder and a little bit of the food coloring, too? If you were patient, you could see that the cornstarch filtered out all the cacao power and more food coloring. This is similar to what happens when dirty rainwater seeps through the soil and gathers in an aquifer.
The soil filters the dirty water. Contaminants get stuck in the soil, and clean water reaches the aquifer. Although groundwater is usually clean, soils are not perfect filters. Some contaminants still make their way through the soil and contaminate the groundwater. This is a serious problem; once polluted, it is hard to clean an aquifer.
Cleanup The content of the soil containers can go into the composting bin. The plastic bottles can be recycled. This activity brought to you in partnership with Science Buddies. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital.
Key concepts Filtration Aquifers Permeable and impermeable soil Groundwater Introduction Have you ever noticed the claim on a bottle of water that it contains "spring water"?
Newly developed techniques uses bacteria to clean the groundwater in a process called bioremediation. Contamination is best prevented by managing land uses. Landfills now require lining the bottom of the landfill with impermeable clay and plastic liners. Underground storage tanks require double-lining to prevent leakage. Still the best practice is to require that contaminants not be allowed into the groundwater system.
Sinkholes - If the roof of a cave or cavern collapses, this results in a sinkhole. Sinkholes, likes caves, are common in areas underlain by limestones.
For example, in Florida, which is underlain by limestones, a new sinkhole forms about once each year, gobbling up cars and houses in process. Karst Landscapes - In an area where the main type of weathering is dissolution like in limestone terrains , the formation of caves and sinkholes, and their collapse and coalescence may result in a highly irregular topography called karst landscapes see figure Examples of questions on this material that could be asked on an exam.
Physical Geology. Stephen A. But, groundwater makes up about 35 times the amount of water in lakes and streams. Groundwater occurs everywhere beneath the Earth's surface, but is usually restricted to depths less that about meters.
The volume of groundwater is a equivalent to a 55 meter thick layer spread out over the entire surface of the Earth. It is an important resource for potable water, irrigation, and industry.
Because it is largely hidden from view, it is often forgotten and subject to contamination by careless humans. Groundwater is a primary agent of chemical weathering and is responsible for the formation of caves and sinkholes. The Groundwater System Groundwater resides in the void spaces of rock, sediment, or soil, completely filling the voids. Porosity In sediments or sedimentary rocks the porosity depends on grain size, the shapes of the grains, and the degree of sorting, and the degree of cementation.
Well-rounded coarse-grained sediments usually have higher porosity than fine-grained sediments, because the grains do not fit together well. Poorly sorted sediments usually have lower porosity because the fine-grained fragments tend to fill in the open space. Since cements tend to fill in the pore space, highly cemented sedimentary rocks have lower porosity. In igneous and metamorphic rocks porosity is usually low because the minerals tend to be intergrown, leaving little free space.
Highly fractured igneous and metamorphic rocks, however, could have high porosity. A thin layer of water will always be attracted to mineral grains due to the unsatisfied ionic charge on the surface. This is called the force of molecular attraction. If the size of interconnections is not as large as the zone of molecular attraction, the water can't move. Aquifers An aquifer is a large body of permeable material where groundwater is present and fills all pore space.
Rain that falls on the surface seeps down through the soil and into a zone called the zone of aeration or unsaturated zone also called the vadose zone , where most of the pore spaces are filled with air. In more humid regions it reaches the surface at streams and lakes, and generally tends to follow surface topography. The depth to the water table may change, however, as the amount of water flowing into and out of the saturated zone changes. During dry seasons, the depth to the water table increases.
During wet seasons, the depth to the water table decreases. Movement of Groundwater Groundwater is in constant motion, although the rate at which it moves is generally slower than it would move in a stream because it must pass through the intricate passageways between free space in the rock.
The same thing happens beneath the surface of the Earth, where pressure is higher beneath the hills and lower beneath the valleys. The Earth's surface can be divided into areas where some of the water falling on the surface seeps into the saturated zone and other areas where water flows out of the saturated zone onto the surface.
The rate at which groundwater moves through the saturated zone depends on the permeability of the rock and the hydraulic head. The hydraulic head is defined as the difference in elevation between two points on the water table.
Springs A spring is an area on the surface of the Earth where the water table intersects the surface and water flows out of the ground. Wells A well is human-made hole that is dug or drilled deep enough to intersect the water table.
Changes in the Groundwater System When discharge of groundwater exceeds recharge of the system, several adverse effects can occur. Water Quality and Groundwater Contamination Water quality refers to such things as the temperature of the water, the amount of dissolved solids, and lack of toxic and biological pollutants.
Remediation of Groundwater Contamination Problems In order to begin remediation , contaminant characterization is first done.
Prevention of Groundwater Contamination Contamination is best prevented by managing land uses. Geologic Activity of Groundwater Dissolution - Recall that water is the main agent of chemical weathering. Groundwater is an active weathering agent and can leach ions from rock, and, in the case of carbonate rocks like limestone, can completely dissolve the rock. Chemical Cementation and Replacement - Water is also the main agent acting during diagenesis.
It carries in dissolved ions which can precipitate to form chemical cements that hold sedimentary rocks together. Groundwater can also replace other molecules in matter on a molecule by molecule basis, often preserving the original structure such as in fossilization or petrified wood. Caves and Caverns - If large areas of limestone underground are dissolved by the action of groundwater these cavities can become caves or caverns caves with many interconnected chambers once the water table is lowered.
Once a cave forms, it is open to the atmosphere and water percolating in can precipitate new material such as the common cave decorations like stalactites hang from the ceiling , stalagmites grow from the floor upward , and dripstones, and flowstones. Define the following: a porosity, b permeability, c hydraulic head, d hydraulic gradient, d confined aquifer, f unconfined aquifer, g perched water body, h sinkhole, i stalactite, j stalagmite.
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