It is not always accessible, or fresh enough for use without treatment, and it's sometimes difficult to locate or to measure and describe. This water may occur close to the land surface, as in a marsh, or it may lie many hundreds of feet below the surface, as in some arid areas of the western United States.
Water at very shallow depths might be just a few hours old; at moderate depth, it may be years old; and at great depth or after having flowed long distances from places of entry, the water may have been in the ground for several thousand years. Groundwater is a part of the natural water cycle check out our interactive water cycle diagram. Some part of the precipitation that lands on the ground surface infiltrates into the subsurface.
The part that continues downward through the soil until it reaches rock material that is saturated is groundwater recharge. Water in the saturated groundwater system moves slowly and may eventually discharge into streams, lakes, and oceans.
Here is a simplified diagram showing how the ground is saturated below the water table the solid blue area at the bottom. The ground above the water table the pink area may be wet to a certain degree, but it does not stay saturated. The dirt and rock in this unsaturated zone contain air and some water and support the vegetation on the Earth. The saturated zone below the water table has water that fills the tiny spaces pores between rock particles and the cracks fractures of the rocks.
Nothing surprising here - gravity pulls water and everything else toward the center of the Earth. That means that water on the surface will try to seep into the ground below it.
The rock below the Earth's surface is the bedrock. If all bedrock consisted of a dense material like solid granite, then even gravity would have a hard time pulling water downward. But Earth's bedrock consists of many types of rock, such as sandstone, granite, and limestone. Bedrocks have varying amounts of void spaces in them where groundwater accumulates.
Bedrock can also become broken and fractured, creating spaces that can fill with water. And some bedrock, such as limestone, are dissolved by water -- which results in large cavities that fill with water. Here is groundwater seeping out from between rock layers and freezing in the winter temperatures.
In many places, if you looked at a vertical cross-section of the earth you would see that rock is laid down in layers, especially in areas of sedimentary rocks.
Some layers have rocks that are more porous than others, and here water moves more freely in a horizontal manner through the earth. Sometimes water can be seen seeping out through exposed rock layers as seen in the photo. Try as it might, gravity doesn't pull water all the way to the center of the Earth. Deep in the bedrock there are rock layers made of dense material, such as granite, or material that water has a hard time penetrating, such as clay.
These layers may be underneath the porous rock layers and, thus, act as a confining layer to retard the vertical movement of water. Since it is more difficult for the water to go any deeper, it tends to pool in the porous layers and flow in a more horizontal direction across the aquifer toward an exposed surface-water body, like a river.
Visualize it this way: get two sponges and lay one on top of the other. Pour water precipitation on top and it will seep through the top sponge downward into the bottom sponge.
If you stopped adding water, the top sponge would dry up and, as the water dripped out of the bottom sponge, it would dry up too. Now, put a piece of plastic wrap between the sponges, creating your "confining layer" making the bottom sponge an impermeable rock layer that is too dense to allow water to flow through it. Now when you pour water on the top sponge, the water will seep downward until it hits the plastic wrap.
The top sponge will become saturated, and when the water hits the plastic wrap it won't be able to seep into the second sponge. Instead, it will start flowing sideways and come out at the edges of the sponge horizontal flow of groundwater.
The top of the surface where groundwater occurs is called the water table. In the diagram, you can see how the ground below the water table is saturated with water the saturated zone.
Aquifers are replenished by the seepage of precipitation that falls on the land, but there are many geologic, meteorologic, topographic, and human factors that determine the extent and rate to which aquifers are refilled with water.
Rocks have different porosity and permeability characteristics, which means that water does not move around the same way in all rocks. Thus, the characteristics of groundwater recharge vary all over the world.
I hope you appreciate my spending an hour in the blazing sun to dig this hole at the beach. It is a great way to illustrate the concept of how at a certain depth the ground, if it is permeable enough to allow water to move through it, is saturated with water. The top of the pool of water in this hole is the water table. The breaking waves of the ocean are just to the right of this hole, and the water level in the hole is the same as the level of the ocean.
Of course, the water level here changes by the minute due to the movement of the tides, and as the tide goes up and down, the water level in the hole moves, too. Just as with this hole, the level of the water table is affected by other environmental conditions.
In a way, this hole is like a dug well used to access groundwater, probably saline in this case. But, if this was freshwater, people could grab a bucket an supply themselves with the water they need to live their daily lives.
You know that at the beach if you took a bucket and tried to empty this hole, it would refill immediately because the sand is so permeable that water flows easily through it, meaning our "well" is very "high-yielding" too bad the water is saline. To access freshwater, people have to drill wells deep enough to tap into an aquifer. The well might have to be dozens or thousands of feet deep. But the concept is the same as our well at the beach—access the water in the saturated zone where the voids in the rock are full of water.
In an aquifer, the soil and rock is saturated with water. 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 , such as roads and paved areas, 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.
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 the water level 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.
As these charts show, even though the amount of water locked up in groundwater is a small percentage of all of Earth's water, it represents a large percentage of total freshwater on Earth. The pie chart shows that about 1. As the bar chart shows, about 5,, cubic miles mi 3 , or 23,, cubic kilometers km 3 , of groundwater exist on Earth.
About 54 percent is saline, with the remaining 2,, mi 3 10,, km 3 , about 46 percent, being freshwater. How fast can glaciers travel? What is a slime mold? What are their different types? Username or Email Address. Remember Me. Aquifers contain pores—large, interconnected spaces—through which water can flow.
Without interconnected pores in the rock, groundwater cannot permeate an aquifer. For example, shale and clay contain pores, but groundwater cannot easily flow through these pores because they are not interconnected. Groundwater supplies are naturally replenished by rainfall, snowmelt, and water from the bottom of lakes and rivers.
The amount of rainfall or snowmelt that replenishes groundwater depends on local geology and climate, and water from rainfall or snowmelt is distributed in other ways. For example, some water from rainfall or snowmelt evaporates into air, which is taken in by plants through plant respiration. Rainfall or snowmelt can also be collected by streams. Aquifers containing groundwater may be also artificially replenished.
This process, known as artificial recharge, involves spreading treated surface water onto land so it seeps into aquifers or involves directly injecting treated water into aquifers through a well. Groundwater is brought to the surface naturally through springs or lakes and can also be extracted by digging or drilling wells into aquifers. A well, which is generally a single pipe, delivers groundwater to the surface through a pump.
Another type of well, known as an artesian well, is drilled into artesian aquifers, which are found within two impermeable rock layers.
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