Groundwater is water beneath the Earth's surface. We don't often think about groundwater, as we usually can't see it. However, groundwater makes up 94 percent of all the liquid freshwater on Earth, and is the largest source of available freshwater for living things. (Most of the Earth's freshwater is glacial ice.) This means that it's very important to have sources of unpolluted groundwater.

Where does groundwater come from?

When rain falls, some of the water evaporates and some water becomes runoff, eventually forming rivers and streams. The rest of the water soaks into the ground.

Some of this water forms a film over the surface of the soil. Plants use this water for nourishment. They then return some of it, through their leaves, stems and flowers, back into the air. The process by which plants return water to the atmosphere is known as transpiration. Some of the water on top of the soil evaporates directly into the air.

The rest of the water percolates downward into the Earth, where it collects in spaces between particles of sediment and in fractures in bedrock. It continues to move downward until there is an area where every open space is filled with water. Groundwater is the water in this area, which is known as the zone of saturation.

The water table is the upper boundary of the zone of saturation. The water table isn't level. It usually follows the contours of the surface and is higher where there are hills and lower where there are valleys or plains.

A wetland is a place where the water table is on the surface.

Groundwater movement

Groundwater moves because of the force of gravity. It moves from places where the water table is higher to places where the water table is lower, often towards a spring, stream or lake. Groundwater moves slowly. The rate depends on the material it's traveling through. It's difficult for groundwater to move through non-porous material, such as clay. This type of material is known as an aquitard. Groundwater moves more quickly through an aquifer, a layer of a material with large pores, such as sand or gravel.

As groundwater moves through an aquifer, many of the impurities in the water are removed. Because of this, groundwater tends to be cleaner than water flowing on the surface. However, if an aquifer is near the surface, the groundwater flowing through it is vulnerable to pollution.

Springs

When the water table meets the ground, a spring forms.

A hot spring forms when groundwater from deep below ground rises to the surface. Water usually becomes hotter the deeper it percolates into the Earth.

Hot springs often have large amounts of minerals dissolved in them. Many people think that bathing in water with such a high mineral content is good for your health. Consequently, spas are often built around hot springs.

Mineral water comes from springs with large amounts of minerals.

Geysers

A geyser is a hot spring in which water spurts up through the surface, with great force, at regular intervals. A column of steam erupts after each jet of water.

Geysers form within hot igneous rock deep underground. Water percolates below the ground and enters a chamber in the rock. The rock heats the water; however, the immense pressure on the water at the bottom of the chamber increases its boiling point. This prevents the water at the bottom from boiling.

As the heat causes the water to expand, some of it is rises through the surface. This decreases the pressure on the rest of the water in the chamber. The decrease in pressure lowers the boiling point, so some of the water boils and turns to steam. This steam then erupts from the ground.

How Groundwater Shapes the Earth

Although groundwater moves very slowly, it is an important agent of erosion.

Carbon dioxide that dissolves in water forms carbonic acid, which dissolves limestone.

As the limestone is dissolved, caverns, or caves, form.

Sinkholes are depressions in the Earth's surface that form when groundwater dissolves limestone.

A Karst landscape is a large area underlain by limestone. The action of groundwater on the limestone creates a distinctive landscape, full of sinkholes.

Karst landscapes have very few above ground streams. When it rains, runoff runs into the ground then flows through caverns until it reaches the water table.

Wells

A well is a hole dug in the ground to reach the zone of saturation. People have been digging wells to obtain freshwater for at least 7,000 years.

An artesian well is a well where water naturally rises toward the surface. An artesian well forms when an aquifer is slanted so that one end opens at the surface and there are aquitards above and below the aquifer. Pressure on the water in the aquifer causes it to move up to the surface.