(96).Groundwater

Vocabulary

aquifer

capillary action

impermeable

permeability

porosity

spring

subsidence

water table

well

Introduction

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

Groundwater

Groundwater is the largest reservoir of liquid fresh water on Earth and 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 groundwater 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 groundwater aquifer can be from minutes to thousands of years. Groundwater 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 groundwater reaches is the water table.

[Figure 1]

Groundwater is found beneath the solid surface. Notice that the water table roughly mirrors the slope of the land’s surface. A well penetrates the water table.

The Water Table

For a groundwater 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 groundwater; 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 groundwater 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 groundwater 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.

[Figure 2]

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).

[Figure 3]

In Florida, groundwater is sometimes not underground.

Groundwater Use

Groundwater is an extremely important water source for people. Groundwater 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 groundwater involves interaction between the Earth and the water, the study of groundwater 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.

[Figure 4]

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).

[Figure 5]

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).

[Figure 6]

The San Joaquin Valley of California is one of the world’s major agricultural areas. So much groundwater has been pumped that the land has subsided many tens of feet.

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

Groundwater 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.

[Figure 7]

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

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).

[Figure 8]

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 groundwater. 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 (Figurebelow).

[Figure 9]

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

Lesson Summary

Groundwater 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 groundwater.

An aquifer needs good porosity and permeability.

Where groundwater intersects the ground surface, a spring can form.

People dig or drill wells to access groundwater.

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