Lesson Objectives

  • Define ground water.
  • Explain the location, use, and importance of aquifers.
  • Define springs and geysers.
  • Describe how wells work and why they are important.

Introduction


Although this may seem surprising, water beneath the ground is commonplace. Usually ground water travels slowly and silently beneath the surface, but in some locations it bubbles to the surface at springs and geysers. The products of erosion and deposition by ground water were described in the Erosion and Deposition chapter.

Ground water


Ground water is the largest reservoir of liquid fresh water on Earth.

Ground water is found in aquifers, porous rock and sediment with water in between. Water is attracted to the soil particles and capillary action, which describes how water moves through a porous media, moves water from wet soil to dry areas.

Aquifers are found at different depths. Some are just below the surface and some are found much deeper below the land surface. A region may have more than one aquifer beneath it and even most deserts are above aquifers. The source region for an aquifer beneath a desert is likely to be far from where the aquifer is located; for example, it may be in a mountain area.

The amount of water that is available to enter ground water in a region is influenced by the local climate, the slope of the land, the type of rock found at the surface, the vegetation cover, land use in the area, and water retention, which is the amount of water that remains in the ground. More water goes into the ground where there is a lot of rain, flat land, porous rock, exposed soil, and where water is not already filling the soil and rock.
The residence time of water in a ground water aquifer can be from minutes to thousands of years. Ground water is often called “fossil water” because it has remained in the ground for so long, often since the end of the ice ages.

Aquifers

Features of an Aquifer


To be a good aquifer, the rock in the aquifer must have good:
  • porosity: small spaces between grains
  • permeability: connections between pores

This animation shows porosity and permeability. The water droplets are found in the pores between the sediment grains, which is porosity. When the water can travel between ores, that’s permeability. http://www.nature.nps.gov/GEOLOGY/usgsnps/animate/POROS_3.MPG

To reach an aquifer, surface water infiltrates downward into the ground through tiny spaces or pores in the rock. The water travels down through the permeable rock until it reaches a layer that does not have pores; this rock is impermeable (Figure below). This impermeable rock layer forms the base of the aquifer. The upper surface where the ground water reaches is the water table.
external image images?id=309487
Ground water is found beneath the solid surface. Notice that the water table roughly mirrors the slope of the land

The Water Table


For a ground water aquifer to contain the same amount of water, the amount of recharge must equal the amount of discharge. What are the likely sources of recharge? What are the likely sources of discharge?

In wet regions, streams are fed by ground water; the surface of the stream is the top of the water table (Figure below). In dry regions, water seeps down from the stream into the aquifer. These streams are often dry much of the year. Water leaves a ground water reservoir in streams or springs. People take water from aquifers, too.

What happens to the water table when there is a lot of rainfall? What happens when there is a drought? Although ground water levels do not rise and fall as rapidly as at the surface, over time the water table will rise during wet periods and fall during droughts.
external image images?id=325778

The top of the stream is the top of the water table. The stream feeds the aquifer.

One of the most interesting, but extremely atypical types of aquifers is found in Florida. Although aquifers are very rarely underground rivers, in Florida water has dissolved the limestone so that streams travel underground and above ground (Figure below).

external image images?id=325776

In Florida, ground water is sometimes not underground.

Ground Water Use


Ground water is an extremely important water source for people. Ground water is a renewable resource and its use is sustainable when the water pumped from the aquifer is replenished. It is important for anyone who intends to dig a well to know how deep beneath the surface the water table is. Because ground water involves interaction between the Earth and the water, the study of ground water is called hydrogeology.

Some aquifers are overused; people pump out more water than is replaced. As the water is pumped out, the water table slowly falls, requiring wells to be dug deeper, which takes more money and energy. Wells may go completely dry if they are not deep enough to reach into the lowered water table.

The Ogallala Aquifer supplies about one-third of the irrigation water in the United States (Figure below). The aquifer is found from 30 to 100 meters deep over about 440,000 square kilometers! The water in the aquifer is mostly from the last ice age.

The Ogallala Aquifer is widely used by people for municipal and agricultural needs.
external image images?id=315518

The Ogallala Aquifer is found beneath eight states and is heavily used.

About eight times more water is taken from the Ogallala Aquifer each year than is replenished. Much of the water is used for irrigation (Figure below).

external image images?id=317808

Farms in Kansas use central pivot irrigation, which is more efficient since water falls directly on the crops instead of being shot in the air. These fields are between 800 and 1600 meters (0.5 and 1 mile) in diameter.

Lowering the water table may cause the ground surface to sink. Subsidence may occur beneath houses and other structures (Figure below).

external image images?id=315514

The San Joaquin Valley of California is one of the world

When coastal aquifers are overused, salt water from the ocean may enter the aquifer, contaminating the aquifer and making it less useful for drinking and irrigation. Salt water incursion is a problem in developed coastal regions, such as on Hawaii.

Springs and Geysers

Ground water meets the surface in a stream, as shown above, or a spring (Figure below). A spring may be constant, or may only flow at certain times of year.

external image images?id=309473

Big Spring in Missouri lets out 12,000 liters of water per second (Left). Other springs are just tiny outlets like this one (Right).

Towns in many locations depend on water from springs. Springs can be an extremely important source of water in locations where surface water is scarce (Figure below).

external image images?id=315517

In the dry Arizona desert, Oak Creek and many other streams are spring fed.

Wells

A well is created by digging or drilling to reach ground water. When the water table is close to the surface, wells are a convenient method for extracting water. When the water table is far below the surface, specialized equipment must be used to dig a well. Most wells use motorized pumps to bring water to the surface, but some still require people to use a bucket to draw water up (Figure below).

external image images?id=309492

An old-fashioned well that uses a bucket drawn up by hand.


Lesson Summary

  • Ground water is the largest reservoir of fresh water.
  • The water table is the top of an aquifer below which is water and above is rock or soil mixed with air.
  • Aquifers are underground areas of sediment or rock that hold ground water.
  • An aquifer needs good porosity and permeability.
  • Where ground water intersects the ground surface, a spring can form.
  • People dig or drill wells to access ground water.

Review Questions

1. What is ground water?
2. What is the water table?
3. What are aquifers and why are they so important?
4. Replenishing an aquifer is important because it makes the aquifer a resource that can last a long time. What do you think are ways to keep the amount of water used and the amount of water replenished the same?
5. How does a well work?
6. Since ground water is largely unseen from the surface, how might you monitor how humans are affecting the amount of ground water in an aquifer?

Further Reading / Supplemental Links


Inside Yellowstone http://www.nps.gov/archive/yell/insideyellowstone/0017oldfaithful3.htm
Earth's water distribution video, University of Waikato, New Zealand http://www.sciencelearn.org.nz/contexts/h2o_on_the_go/sci_media/video/earth_s_water_distribution

Vocabulary


water table The upper surface of ground water.
subsidence Sinking of the land surface because of the extraction of ground water.
spring A point on the Earth’s surface where ground water bubbles up.
porosity The small holes that exist between grains in a rock or sediment.
permeability The interconnectedness of the pores within a rock or sediment.
impermeable Something that water cannot penetrate.
capillary action Water moves from wet to dry regions in soil.
aquifer A layer of rock, sand, or gravel that holds large amounts of ground water.

Points to Consider

  • Is water from a river or from a well more likely to be clean to drink?
  • Why is overuse of ground water a big concern?
  • What policies might people put in place to conserve water levels in lakes and aquifers?

Maine Connections


Maine Geological Survey: Hydrogeology

Maine Geological Survey: Ground Water & Wells

Virtual Tour

Field Localities.

Frequently Asked Questions

Fact Sheets

Geologic Maps and Digital Data

Related Reading
Ground Water Regulatory Issues

Maine Bureau of Land and Water Quality



Maine Public Health: Protect Your Groundwater

Lesson Plans


Groundwater Contamination
Is Your Water Clean?

Webquests


Water Webquest
Water Systems: Ground Water

More Resources


EPA: Drinking Water and Ground Water
A Layman's Guide to Clean Water: Locating Groundwater
Groundwater Resources at UTexas
Groundwater Model Handbook - Understanding Groundwater and Using the Groundwater Model

Videos


Groundwater: The Hidden Source of Life
Groundwater Animation

Games

Water Island Game
Water Conservation Games
Groundwater Games at K-12 Resources, UTexas


some great information Coolest Marketer Traffic And Conversion