Groundwater: Definition & Conservation

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  • 0:00 What Is Groudwater?
  • 0:44 Groundwater Basics
  • 2:28 Movement of…
  • 3:18 The Dangers to Groundwater
  • 4:24 Lesson Summary
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Lesson Transcript
Instructor: Ryan Hultzman
In this lesson, we learn how groundwater is formed and moves beneath the earth. We also discuss why groundwater is such an important resource and the current dangers facing our groundwater supplies.

What Is Groundwater?

Fresh water is one of the most important necessities for human life. Currently, approximately 30% of the freshwater on Earth is groundwater, or water that is contained beneath the earth's surface within pore spaces of underground bedrock and soils. Because we cannot readily see groundwater, it is often not fully appreciated or preserved.

Groundwater is a dynamic and important source of freshwater around the world; in fact, 20% of all water used in the United States is groundwater. Groundwater is also highly susceptible to pollution and overuse, which we will discuss further in this lesson.

Groundwater Basics

Groundwater forms when water from the surface seeps into the ground. This process is called recharge. The water is able to move underground through the rock and soil due to connected pore spaces. These pore spaces can be tiny spaces within particles in the rock or soil, fractures in the bedrock, or dissolved cavities in limestone, like caves.

A common misperception of groundwater is that it exists as a large reservoir or lake underground. Instead, groundwater fills the small spaces left within the rock and soils that make up the subsurface, much like water fills the spaces in a sponge.

During recharge, water is pulled downward into the earth by gravity through two zones. The upper zone, called the zone of aeration, is where a mixture of water and air fills the pore spaces. Below the zone of aeration is the zone of saturation, where the pore spaces are completely filled by water. The upper boundary of the zone of saturation is known as the water table.

The area that is saturated with water is called an aquifer. An unconfined aquifer is where the aquifer is connected to the surface through pore spaces. A confined aquifer is an aquifer that is restricted between two impermeable rock units. Confined aquifers are only accessed using a well or where the aquifer meets the surface.

Groundwater moves extremely slowly through the earth. While rivers can flow at the rate of several kilometers per hour, groundwater can move as slowly as a meter per year. This means it can take several thousands of years for underground aquifers to become replenished.

Movement of Groundwater and Discharge

Whereas water on the earth's surface moves under the force of gravity, groundwater moves from areas of high pressure, where the water table is high, to areas of low pressure, where the water table is low. The flow often follows the slope of the water table, which normally mimics the overlying land topography. Water is naturally discharged from the groundwater system at locations where the water table intersects the earth's surface at lakes, rivers, and swamps.

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