Earth & Space Science
Students learn how groundwater collects. Students explore the relationship between porosity, permeability, groundwater, and the water table.
After completing this tutorial, you will be able to complete the following:
Water below the Earth's surface is called groundwater. After a rainfall or snow melt, some of the water soaks into the soil and becomes groundwater. Groundwater occupies open spaces, called pores, between grains of soil.
The ratio of open pore space to solid material in the soil (or another material) is called the porosity. A layer of gravel or sand can actually be as much as 40 percent open pore space. Solid igneous rock, on the other hand, can have essentially no open pore space.
In order for water to get into the pores, though, there needs to be connections between the pores. In other words, there needs to be an unbroken pathway for the water to follow. This leads to the idea of permeability.
Permeability is defined as the rate at which water flows through a material. Permeability depends on porosity, since a rock with no open space will not allow water to flow through, but it also depends on the connections between those pores. The permeability of different materials is generally measured experimentally.
Groundwater collects in permeable layers underground, called aquifers. Aquifers represent an important source of water around the world. The Ogallala Aquifer, for example, underlies parts of eight states in the western United States, with a total ground area of approximately 450,000 square kilometers. Water from the Ogallala Aquifer has been estimated to support close to 20 percent of the wheat, corn, cotton, and cattle produced in the United States.
Occasionally, especially in areas where layers within the Earth have been tilted, permeable layers may be exposed at the Earth's surface in one location, but deep underground somewhere else. If covered by an impermeable layer, water that soaks into the permeable layer where it reaches the Earth's surface may flow down the tilted layer, becoming pressurized by the weight of the water above it. A well drilled into the pressurized layer is called an artesian well. Because of the pressure, groundwater rises in an artesian well without pumping. In some cases, the water may even shoot out of the ground in a fountain.
|Approximate Time||20 Minutes|
|Pre-requisite Concepts||Students should be familiar with the properties of materials and with soil types.|
|Course||Earth & Space Science|
|Type of Tutorial||Concept Development|
|Key Vocabulary||clay, drill, drought|