ARTESIAN WELLS

ARTESIAN WELLS

(b) Perched

Figure 15–7 shows a tilted sandstone aquifer sand-

water table

wiched between two layers of shale. An inclined aquifer bounded top and bottom by impermeable rock is an arte-

Layer of

sian aquifer . Water in the lower part of the aquifer is un-

impermeable shale

der pressure from the weight of water above. Therefore,

Spring

if a well is drilled through the shale and into the sand- stone, water rises in the well without being pumped. A well of this kind is called an artesian well. If pressure is sufficient, the water spurts out onto the land surface.

Main water table

䊳 15.3 USE OF GROUND WATER

(c)

Ground water is a particularly valuable resource because

Land surface

1. It is abundant. Sixty times more fresh water exists underground than in surface reservoirs.

Spring

2. It is stored below the Earth’s surface and remains available for use during dry periods.

3. In some regions, ground water flows from wet envi- ronments to arid ones, making water available in dry areas.

GROUND-WATER DEPLETION If ground water is pumped from a well faster than it can

flow through the aquifer, a cone of depression forms around the well (Fig. 15–8). If the aquifer has good per- meability, water flows back toward the well in a few days or weeks after the pump is turned off, and the cone

(d)

Spring

of depression disappears. Near the desert town of Cave Creek, Arizona, ground water is pumped onto a golf

Figure 15–6 Springs form where the water table intersects the land surface. This situation can occur where (a) the land surface intersects a contact between permeable and imperme- able rock layers; (b) a layer of impermeable rock or clay lies

Cavern “perched” above the main water table; (c) water flows from fractures in otherwise impermeable bedrock; and (d) water flows from caverns onto the surface.

264 CHAPTER 15 G RO U N D WAT E R

Water table

Non-artesian well

Artesian well

Impermeable rock

Sandstone aquifer

Figure 15–7 An artesian aquifer forms where a tilted layer of permeable rock, such as sandstone, lies sandwiched between layers of impermeable rock, such as shale. Water rises in an artesian well without being pumped. A hose with a hole shows why an artesian well flows spontaneously.

course so fast that the cone of depression sometimes the development of advanced drilling and pumping tech- leaves the wells of nearby homeowners dry.

nologies, human impact on ground water was minimal. If water is continuously pumped more rapidly than

Today, however, deep wells and high-speed pumps can it can flow through the aquifer, or if many wells extract

extract ground water more rapidly than the hydrologic water from the same aquifer, the water table drops. Before

cycle recharges it. In that case, an aquifer becomes de-

Original Water table

water table

Cone of depression

Lowered water table

Figure 15–8 (a) A well is drilled into an aquifer. (b) A cone of depression forms because a pump draws water faster than the aquifer can recharge the well. (c) If the pump continues to extract water at the same rate, the water table falls.

Use of Ground Water 265

R NH 276.2

___ 0.6 ___ 1.6 RI 78.4 NV

WY

74.3 CT IA NE PA NJ ___ CA 1.8 ___ 4.1 UT

DE 25.5 ___ CO 13.9 WV VA MD

___ 0.4 OK NC 10.8 ___

33.1 Consumption as a percentage of renewable supply

___ 5.6 Consumption (billion gal/day) 233.5 Renewable water supply (billion gal/day) 60%

Figure 15–9 Ground-water consumption compared to recharge rates. Arizona consumes ground water faster than the aquifers are recharged. In Georgia, recharge is much faster than consumption.

pleted and is no longer able to supply enough water to the arrival of irrigation technology. Electric power lines support the farms or cities that have overexploited it.

were built to service rural regions, and affordable pumps This situation is common in the arid and semiarid west-

and irrigation systems were developed. With the specter ern United States (Fig. 15–9).

of drought fresh in people’s memories and the tools to avert future calamities available, the age of modern irri-

gation began.

The High Plains and the

Figure 15–10 shows a map and cross section of the

Ogallala Aquifer

STUDY

Ogallala aquifer beneath the central high plains. The aquifer extends almost 900 kilometers from the Rocky

Most of the high plains in western and midwestern North Mountains eastward across the prairie and from Texas America receive scant rainfall, yet the soil is fertile.

into South Dakota. It consists of a layer of permeable Early farmers prospered in rainy years and suffered dur-

sand and gravel 50 to 100 meters thick. The top of the ing drought. In the 1930s, two events combined to change

aquifer lies 15 to 100 meters below the surface. Before agriculture in this region. One was a widespread drought

intensive pumping began, the aquifer contained more that destroyed crops and exposed the soil to erosion. Dry

than 3 billion acre-feet of water (an acre-foot is an acre winds blew across the land, eroding the parched soil and

of water 1 foot deep—325,851 gallons). carrying it for hundreds and even thousands of kilome-

The Ogallala aquifer filled when the last Pleistocene ters. Thousands of families lost their farms, and the re-

glaciers melted in the Rocky Mountains hundreds of gion was dubbed the Dust Bowl. The second event was

kilometers to the west. Water now flows southeastward

266 CHAPTER 15 G RO U N D WAT E R

Unsaturated

2.5 centimeters per year. In many places, however, the

1500 zone

aquifer is being drawn down by tens of centimeters each year.

High Plains aquifer

Currently, more than 3 million hectares of land are

1000 Base

irrigated from the Ogallala aquifer—an area about the

Water table

Altitude (m) of

size of Massachusetts, Vermont, and Connecticut com-

500 aquifer Bedrock

bined. Approximately 40 percent of the cattle raised in the United States are fed with corn and sorghum grown

Distance (km)

East

in this region, and large quantities of grain and cotton are grown there as well. About 150,000 pumps extract irri-

(a)

gation water from the aquifer 24 hours a day during the growing season.

Hydrologists estimate that half of the water has

SOUTH DAKOTA

already been removed from parts of the aquifer and that it would take 1000 years to recharge the aquifer if

WYOMING

pumping were to cease today. But pumping rates are increasing. Under these conditions, deep ground water

is, for all practical purposes, a nonrenewable resource. Some hydrologists predict that, as the aquifer’s water is used up during the next two decades, the amount of irri- gated land in the central high plains will decline by 80

Excessive removal of ground water can cause subsi-

MEXICO

dence , the sinking or settling of the Earth’s surface. When water is withdrawn from an aquifer, rock or soil particles may shift to fill space left by the lost water. As

a result, the volume of the aquifer decreases and the overlying ground subsides (Fig. 15–11). Removal of oil

TEXAS

from petroleum reservoirs has the same effect. Subsidence rates can reach 5 to 10 centimeters per year, depending on the rate of ground-water removal and the nature of the aquifer. Some areas in the San Joaquin Valley of California, one of the most productive agricul- tural regions in the world, have sunk nearly 10 meters. Subsidence creates particularly severe problems when

it affects a city. For example, Mexico City is built on an old marsh. Over the years, as the weight of buildings

(b)

Figure 15–10 The Ogallala aquifer supplies water to much and roadways has increased and ground water has been of the High Plains. (a) A cross-sectional view of the aquifer

extracted, parts of the city have settled as much as 8.5 shows that much of its water originates in the Rocky

meters. Many millions of dollars have been spent to Mountains and flows slowly as ground water beneath the High

maintain this complex city on its unstable base. Simi- Plains. (b) A map showing the extent of the aquifer.

lar problems are found in Phoenix, Arizona, in the Houston–Galveston area of Texas, and in other U.S. cities.

Unfortunately, subsidence is not a reversible process. through the aquifer at a rate of about a meter per day, and

When rock and soil contract, their porosity is perma- the recharge rate (the rate at which natural processes

nently reduced so that ground-water reserves cannot be would raise the water level in the aquifer if no water

completely recharged even if water becomes abundant were being extracted) varies from 0.4 centimeter to about

again.

Ground-Water Pollution 267

Well Water table

Fresh water

(a)

Salt water

A dropping water table caused subsidence New water table and structural damage in Jacksonville, Florida. (Wendell Metzen/Bruce Coleman, Inc.)

Fresh water

(b)

Salt water

Ocean

SALT-WATER INTRUSION Two types of ground water are found in coastal areas:

fresh water and salty water that seeps in from the sea. Fresh water floats on top of salty water because it is less

Figure 15–12 Salt-water intrusion can pollute coastal dense. If too much fresh water is pumped to the surface,

aquifers. (a) Fresh water lies above salt water, and the water in the salty water rises into the aquifer and contaminates the well is fit to drink. (b) If too much fresh water is removed, the water table falls. The level of salt water rises and contami-

wells (Fig. 15–12). Salt-water intrusion has affected

nates the well.

many of south Florida’s coastal ground-water reservoirs.