Groundwater System: Definition & Geological Role

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  • 0:07 Groundwater
  • 1:35 The Role of Groundwater
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Surprisingly, most of Earth's liquid freshwater is not where you might think! In this video lesson, you will learn about groundwater, as well as the important roles it plays in sustaining life and shaping Earth.


Believe it or not, groundwater, or water below the surface, makes up most of Earth's liquid freshwater. All of the streams, lakes, rivers, and ponds (so all of the Great Lakes, the Mississippi, the Amazon, the Nile, and all of the other fresh surface water) only make up 1.5% of Earth's liquid freshwater. The other 98.5% is underground as groundwater!

Water gets underground because when it precipitates, like with rain or snow, the ground soaks up the falling water like a sponge. Some soils are better at absorbing groundwater than others. Sand, for example, is really good at soaking up water, and you've seen this if you've ever been to the beach. Clay, however, is not very good at taking in water, and rocks are even worse.

Where groundwater completely fills any open spaces underground is called the saturated zone; it's literally saturated with water. There is space above this in the soil, where moisture exists, but it doesn't completely fill all of the open spaces. The water here is called soil moisture, and this region where the soil is not saturated is called the unsaturated zone. You can tell where these two meet if you dig a hole. Once the hole hits the saturated zone you will see the bottom fill with water, and this boundary where the saturated and unsaturated zones meet is called the water table.

The Role of Groundwater

Groundwater is an important component of the water cycle, which is the natural cycling of water through phases and locations on Earth. The water that soaks into the ground sometimes comes back out above ground in other locations, feeding the world's rivers, lakes, streams, and oceans.

Much of the world's groundwater is stored in aquifers, which are underground water reservoirs, usually made of rock like limestone. The limestone acts like a giant rock sponge, soaking up the water and holding it for thousands of years. One of the world's largest aquifers is the Ogallala Aquifer in the Great Plains of the U.S. This aquifer is one large connected system, stretching from South Dakota to Texas and from Colorado to Arkansas! Because aquifers hold so much water, they take a very long time to fill, sometimes thousands or even millions of years.

Groundwater also acts like a cementing agent, helping sedimentary rocks form. As it moves, it carries sediments along with it, and then over millions of years, the water glues those sediments together into rocks.

Geysers are the result of groundwater; if the water underground is above a near-surface magma source, this magma underneath heats the water. As the water heats, the pressure builds up, sometimes escaping at the surface. When this happens, the result is the same as heating a kettle of water on the stove. Steam escapes through a small opening in the surface and we get a geyser, like Old Faithful.

Natural ecosystems depend on groundwater because, as mentioned before, it's a source of freshwater for surface water systems, like wetlands and rivers. Both plants and animals depend on groundwater because plants take it up through their roots in the soil, and animals (like us!) use it as a source of drinking water.

We pump groundwater not only for drinking, but also for things like agricultural irrigation. The problem is that, as mentioned before, it takes a long time to fill or refill an aquifer. So, if we pump too much groundwater out, the land can be affected in various ways.

Sinkholes are funnel-shaped cavities in the ground. They are literally holes that occur from the ground sinking. Groundwater holds the land underground in place, and when it's removed, that ground just sinks downward to fill the empty space. Sinkholes can be small, or they can be very large like Lake Eola in downtown Orlando. Once a sinkhole, Lake Eola has since been turned into a park in the middle of the city, and this open-air ground hole is an incredible 23 acres in size!

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