What is Groundwater? - Definition & Explanation

Instructor: Jeff Fennell

Jeff has a master's in engineering and has taught Earth science both domestically and internationally.

Groundwater accounts for up to 20% of all the fresh water on Earth. This lesson will explain what ground water is and its important aspects. Once you've completed the lesson, test your understanding with a quiz.


The simplest definition of groundwater is that it is water that is underground. Water takes up the available spaces between particles underground and moves in the direction of least pressure. Of all the fresh water on Earth, about 20% is groundwater.

As water seeps into the ground, it will continue downward due to gravity until the surrounding ground is saturated with water. The line where saturated and unsaturated layer meet is called the water table. In order for a well to be able to pump water, it must be as deep as the water table.

Water can be confined underground for thousands of years

Porosity and Permeability

The ability for the ground to hold water depends on two main factors, porosity and permeability.

Porosity is the amount of free space within a material (the ground in this example). Imagine if you pour water onto sand at the beach, it will sink right in; sand has high porosity. On the other hand, if you pour water onto a sidewalk, it will stay there until it evaporates away; cement, and cities in general, have very low porosity.

Permeability is how connected the free spaces in a material are, which is important for the ability for movement or transfer of water through the ground.

For example, a cloth is permeable because water is able to pass through the cloth. On the other hand, a plastic bag is not permeable because water cannot pass through it.


Groundwater doesn't stay in the ground forever, at some point it comes back to the surface or flows into the ocean. An aquifer is the location where groundwater comes back to the surface from underground. An aquifer can be man-made or natural. Many towns around the country use aquifers for water supply. There are many naturally occurring aquifers as well, in some instances, a lake can be fed solely by an aquifer (as shown below).

Hanging Lake in Colorado is fed by an aquifer
hanging lake

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