EARTH SCIENCE geology, the environment, and the universe 2008 (11)

10.1 Movement and Storage of Groundwater MAIN Idea Groundwater reservoirs provide water to streams and wetlands wherever the water table intersects the surface of the ground.. Loca

BIG Idea Precipitation

and infiltration contribute to

groundwater, which is stored

in underground reservoirs

until it surfaces as a spring

or is drawn from a well.

10.1 Movement and

Storage of Groundwater

MAIN Idea Groundwater

reservoirs provide water to

streams and wetlands wherever

the water table intersects the

surface of the ground.

10.2 Groundwater

Weathering and Deposition

MAIN Idea Chemical

weather-ing of limestone by water causes

the characteristic topography of

karst areas.

10.3 Groundwater Supply

MAIN Idea Water is not

always available in the

quanti-ties and in the locations where it

is needed and might be

compro-mised by pollution.

GeoFacts

• The Strokkur geyser in Iceland

erupts every 5 to 10 minutes.

• The eruptions of Strokkur

geyser reach heights of more

Beneath your feet, there are vast amounts of water

This water fills in the pore spaces and fractures in

rock and unconsolidated sediment In this activity,

you will model groundwater storage.

Procedure

1 Read and complete the lab safety form.

2 Fill a 250-mL graduated cylinder with

fine, dry sand.

3 Fill another 250-mL graduated cylinder

with water.

4 Pour water from the second cylinder into the

sand-filled cylinder until the water level is flush with the surface of the sand Measure and record the volume of saturated sand in the cylinder.

5 Measure and record how much water is left

in the second cylinder.

6 Repeat the experiment twice using coarse

sand and clay.

Analysis

1 Describe how much water is present in the

saturated fine sand, coarse sand, and clay.

2 Calculate the ratio of water volume to the

volume of fine sand, coarse sand, and clay, and express the value as a percentage.

3 Infer how many liters of water could be

stored in a cubic meter of each sediment.

Threats to the Water Supply

Make this Foldable to summarize the major problems that threaten groundwater supplies.

STEP 1 Fold a sheet of paper in half lengthwise.

STEP 2 Fold the sheet

in half and then into thirds.

STEP 3 Unfold and cut along the fold lines of the top flap to make six tabs.

STEP 4 Label the tabs

as you read.

F OLDABLES Use this Foldable with Section 10.3

As you read this section, summarize the lems that can threaten groundwater.

prob-Overuse

Chapter 10 • Groundwater 251

Visit glencoe.com to study entire chapters online;

• Interactive Time Lines

• Interactive Figures

• Interactive Tables access Web Links for more information, projects, and activities;

review content with the Interactive Tutor and take Self-Check Quizzes.

Doug Martin

Section 10 10.1 1

Objectives

◗ Describe how groundwater storage

and underground movement relate

to the water cycle.

◗ Illustrate an aquifer and an

aquiclude.

◗ Relate the components of aquifers

with the presence of springs.

Review Vocabulary

hydrologic cycle: a never-ending

natural circulation of water through

when it has not rained in a long time? Rainfall contributes to the flow in a stream, but much of the water comes from beneath the ground.

The Hydrosphere

The water on and in Earth’s crust makes up the hydrosphere, named

after hydros, the Greek word for water You learned about the

hydro-sphere in Chapter 1 in the context of Earth’s systems, including the geosphere, hydrosphere, atmosphere, and biosphere About 97 per-cent of the hydrosphere is contained in the oceans The water con-tained by landmasses — nearly all of it freshwater — makes up only about 3 percent of the hydrosphere

Freshwater is one of Earth’s most abundant and important renewable resources However, of all the freshwater, between 70 and

80 percent is held in polar ice caps and glaciers All the rivers, streams, and lakes on Earth represent only a small fraction of Earth’s liquid freshwater, as shown in Table 10.1 Recall from Chapter 9 that water in the hydrosphere moves through the water cycle

Location Percentage of

Total Water

Water Volume (km 3 )

Estimated Average Residence Time of Water Oceans 97.2 1,230,000,000 thousands of years

Ice caps and

tens of thousands of years and longer

Groundwater 0.31 4,000,000 hundreds to many

thousands of years

Rivers and

Interactive Table To explore more about Earth’s water supply, visit glencoe.com.

Well-sorted, large sand grains Unsorted sand grains Well-sorted, small sand grains

■ Figure 10.1 Porosity depends on the size and variety of particles in a material.

Compare the porosities shown in each sample.

Section 1 • Movement and Storage of Groundwater 253

Groundwater and Precipitation

The ultimate source of all water on land is the oceans Evaporation

of seawater cycles water into the atmosphere in the form of

invisi-ble water vapor and visiinvisi-ble clouds Winds and weather systems

move this atmospheric moisture all over Earth, with much of it

concentrated over the continents Precipitation brings atmospheric

moisture back to Earth’s surface Some of this precipitation falls

directly into the oceans and some falls on land

on land trickles into the ground and becomes groundwater Only a

small portion of precipitation becomes runoff and is returned

directly to the oceans through streams and rivers Groundwater

slowly moves through the ground, eventually returns to the surface

through springs and seepage into wetlands and streams, and then

flows back to the oceans

Groundwater Storage

Puddles of water that are left after it rains quickly disappear, partly by

infiltrating the ground On sandy soils, rain soaks into the ground

almost immediately Where does that water go? The water seeps into

small openings within the ground Although Earth’s crust appears

solid, it is composed of soil, sediment, and rock that contain countless

small openings, called pores spaces.

Pore spaces make up large portions of some of these materials The

amount of pore space in a material is its porosity The greater the

porosity, the easier water can flow through the material Subsurface

materials have porosities ranging from 2 percent to more than 50

percent For example, the porosity of well-sorted sand is 30 percent;

however, in poorly sorted sediment, smaller particles occupy some of

the pore spaces and reduce the overall porosity of the sediment, as

shown in Figure 10.1 Similarly, the cement that binds the grains of

sedimentary rocks together reduces the rocks’ porosity Because of

the enormous volume of sediment and rock beneath Earth’s surface,

enormous quantities of groundwater are stored in the pore spaces

■ Figure 10.2 The zone of saturation

is where groundwater completely fills all the

pores of a material below Earth’s surface.

Describe what is above the zone

of saturation.

254 Chapter 10 • Groundwater

The Zone of Saturation

The region below Earth’s surface in which groundwater completely

fills all the pores of a material is called the zone of saturation The upper boundary of the zone of saturation is the water table, shown in Figure 10.2. Strictly speaking, only the water in the zone of satura-

tion is called groundwater In the zone of aeration, which is above the

water table, materials are moist, but because they are not saturated with water, air occupies much of the pores

Water movement Water in the zone of saturation and zone

of aeration can be classified as either gravitational water or capillary water Gravitational water is water that trickles downward as a result

of gravity Capillary water is water that is drawn upward through capillary action above the water table and is held in the pore spaces

of rocks and sediment because of surface tension Capillary action can be seen when the tip of a paper towel is dipped into water and the water seems to climb up through the fibers of the paper towel

The water table The depth of the water table often varies depending on local conditions For example, in stream valleys, groundwater is relatively close to Earth’s surface, and thus the water table can be only a few meters deep In swampy areas, the water table is at Earth’s surface, whereas on hilltops or in arid regions, the water table can be tens to hundreds of meters or more beneath the surface As shown in Figure 10.2, the topography of the water table generally follows the topography of the land above

it For example, the slope of the water table corresponds to the shape of valleys and hills on the surface above

Because of its dependence on precipitation, the water table tuates with seasonal and other weather conditions It rises during wet seasons, usually in spring, and drops during dry seasons, often

fluc-in late summer

Aquifer

Precipitation and infiltration

Section 1 • Movement and Storage of Groundwater 255

■ Figure 10.3 An aquifer is a layer of meable subsurface material that is saturated with water This aquifer is located between two impermeable layers called aquicludes.

per-Groundwater Movement

Groundwater flows downhill in the direction of the slope of the water

table Usually, this downhill movement is slow because the water has to

flow through numerous tiny pores in the subsurface material The

ten-dency of a material to let water pass through it is its permeability.

Materials with large, connected pores, such as sand and gravel, have

high permeability and permit relatively high flow velocities up to

hundreds of meters per hour Other permeable subsurface materials

include highly fractured bedrock, sandstone, and limestone

Permeability Groundwater flows through permeable sediment

and rock, called aquifers, such as the one shown in Figure 10.3

In aquifers, the pore spaces are large and connected Fine-grained

materials have low permeabilities because their pores are small

These materials are said to be impermeable Groundwater flows so

slowly through impermeable materials that the flow is often

mea-sured in millimeters per day Some examples of impermeable

mate-rials include silt, clay, and shale Clay is so impermeable that a

clay-lined depression will hold water For this reason, clay is often

used to line artificial ponds and landfills Impermeable layers,

called aquicludes, are barriers to groundwater flow.

Flow velocity The flow velocity of groundwater depends on

the slope of the water table and the permeability of the material

through which the groundwater is moving The force of gravity

pulling the water downward is greater when the slope of the water

table surface is steeper Water also flows faster through a large

opening than through a small opening The flow velocity of

groundwater is proportional to both the slope of the water table

and the permeability of the material through which the water

flows

256 Chapter 10 • Groundwater

■ Figure 10.4 Springs occur when the

groundwater emerges at points where the

water table intersects Earth’s surface.

Springs

Groundwater moves slowly but continuously through aquifers and eventually returns to Earth’s surface In most cases, groundwater emerges wherever the water table intersects Earth’s surface Such inter-sections commonly occur in areas that have sloping surface topogra-phy The exact places where groundwater emerges depend on the arrangement of aquifers and aquicludes in an area

groundwater emerges.

As you learned on the previous page, aquifers are permeable ground layers through which groundwater flows easily, and aquicludes are impermeable layers Aquifers are commonly composed of layers of sand and gravel, sandstone, and limestone In contrast, aquicludes, such as layers of clay or shale, block ground water movement As a result, groundwater tends to discharge at Earth’s surface where an aquifer and an aquiclude are in contact, as shown in Figure 10.4.

under-These natural discharges of groundwater are called springs.

Emergence of springs The volume of water that is discharged

by a spring might be a mere trickle or it might form a stream In some regions called karst regions, an entire river might emerge from the ground Such a superspring is called a karst spring Karst springs occur

in limestone regions where springs discharge water from underground pathways In regions of nearly horizontal sedimentary rocks, springs often emerge on the sides of valleys at about the same elevation, at the bases of aquifers, as shown in Figure 10.5 Springs might also emerge at the edges of perched water tables In a perched water table, a zone of saturation that overlies an aquiclude separates it from the main water table below Other areas where springs tend to emerge are along faults, which are huge fractures along which large masses of rock have moved, and sometimes block aquifers In limestone regions, springs discharge water from underground pathways as karst springs

Visualizing Springs

Springs can be caused by a variety of situations.

Section 1 • Movement and Storage of Groundwater 257

To explore more about springs, visit

glencoe.com.

Cavern Spring

Water table Sandstone

Spring Water table

Shale

Fault

A spring forms where a permeable layer and impermeable layer

come together

A layer of impermeable rock or clay can create

a perched water table Springs can result where groundwater emerges from a perched water table

Karst springs form where groundwater weathers through stone bedrock, and water in the underground caverns emerges

lime-at Earth’s surface.

Some springs form where a fault has brought together two

different types of bedrock, such as a porous rock and a

non-porous rock.

Perched water table

Spring

Layer of impermeable clay

Main water table

Sandstone

Spring Water table

Self-Check Quiz glencoe.com

Hot water

Eruption of geyser

■ Figure 10.6 A geyser is a type of hot

spring from which very hot water and vapor

erupt at the surface.

Identify the origin of a geyser.

Section 10.1 Assessment

Section Summary

◗◗ Some precipitation infiltrates the

ground to become groundwater.

◗

◗ Groundwater is stored below the

water table in pore spaces of rocks

and sediment.

◗

◗ Groundwater moves through

perme-able layers called aquifers and is

trapped by impermeable layers

called aquicludes.

◗

◗ Groundwater emerges from the

ground where the water table

inter-sects Earth’s surface.

Understand Main Ideas

1 MAIN Idea Explain how the movement of groundwater is related to the water

cycle.

2 Illustrate how the relative positions of an aquifer and aquiclude can result in the presence of a spring.

3 Describe how the water in hot springs gets hot.

4 Analyze the factors that determine flow velocity.

Think Critically

5 Differentiate between porosity and permeability in subsurface materials.

6 Infer why it is beneficial for a community to have an aquiclude located beneath

the aquifer from which it draw its water supply.

Compared to air temperatures, groundwater is generally colder

in the summer and warmer in the winter However, in some regions around the world, springs discharge water that is much warmer than the average annual temperature These springs are

called warm springs or hot springs, depending on their

tempera-tures Hot springs are springs that have a temperature higher than that of the human body, which is 37°C

There are thousands of hot springs in the United States Most of them are located in the western United States in areas where the subsurface is still hot from nearby igneous activity A number of hot springs also occur in some eastern states These hot springs emerge from aquifers that descend to tremendous depths in Earth’s crust and through which deep, hot water rises The water

is hot because temperatures in Earth’s upper crust increase by an average of 25°C for every km of depth

Among the most spectacular features produced by Earth’s underground thermal energy in volcanic regions are geysers, shown in Figure 10.6 Geysers are explosive hot springs In a gey-

ser, water is heated past its boiling point, causing it to vaporize The resulting water vapor builds up tremendous pressure This pressure

is what fuels the eruptions One of the world’s most famous geysers, Old Faithful, is located in Yellowstone National Park, Wyoming

Section 1 10 0 2 2

Objectives

◗ Explain how groundwater dissolves

and deposits rocks and minerals.

◗ Illustrate how caves form.

◗ Describe how the features of karst

topography shape the landscape.

Review Vocabulary

hydrolysis: chemical reaction of

water with other substances

■ Figure 10.7 Carbonic acid has

dissolved large portions of this

lime-stone This resulting formation is the

Stone Forest in China.

Section 2 • Groundwater Weathering and Deposition 259

Groundwater Weathering and Deposition

MAIN Idea Chemical weathering of limestone by water causes the characteristic topography of karst areas.

sculpture that has been weathered by acidic water Similar processes form stone caves underground.

lime-Carbonic Acid

Acids are aqueous solutions that contain hydrogen ions Most groundwater is slightly acidic due to carbonic acid Carbonic acid forms when carbon dioxide gas dissolves in water and combines with water molecules This happens when precipitation falls through the atmosphere and interacts with carbon dioxide gas or when groundwater infiltrates the products of decaying organic matter in soil As a result of these processes, groundwater is usually slightly acidic and attacks carbonate rocks, especially limestone

Limestone mostly consists of calcite, also called calcium carbonate, which reacts with any kind of acid The results of this reaction over time are shown in Figure 10.7. This process occurs above ground and below ground

CO2 + H2O → H2CO3

In the second reaction, carbonic acid splits into hydrogen ions (H+) and bicarbonate ions (HCO3–) This process is represented by the following equation

H2CO3 → H+ + HCO3–

In the third reaction, the hydrogen ions (H+) react with calcite (CaCO3) and form calcium ions (Ca2+) and bicarbonate ions (HCO3–)

CaCO3 + H+ → Ca2+ + HCO3–

Michele Burgess/Index Stock

260 Chapter 10 • Groundwater

■ Figure 10.8 Groundwater dissolution

and precipitation result in a variety of features

Caves A natural underground opening with a connection to Earth’s

surface is called a cave or a cavern Some caves form

three-dimen-sional mazes of passages, shafts, and chambers that stretch for many kilometers Some caves are dry, while some contain underground streams or lakes Others are totally flooded and can be explored only

by cave divers Mammoth Cave in Kentucky, shown in Figure 10.8,

is composed of a series of connected underground passages

Most caves are formed when groundwater dissolves limestone

The development of most caves begins in the zone of saturation just below the water table As groundwater infiltrates the cracks and joints of limestone formations, it gradually dissolves the adjacent rock and enlarges these passages to form an interconnected network

of openings As the water table is lowered, the cave system becomes filled with air New caves then form beneath the lowered water table

If the water table continues to drop, the thick limestone formations eventually become honeycombed with caves This is a common occurrence in limestone regions that have been uplifted by tectonic forces

Carlsbad Caverns, New Mexico Mammoth Cave, Kentucky

(l)Fritz Polking/Visuals Unlimited, (r)Adam Jones/Visuals Unlimited

Section 2 • Groundwater Weathering and Deposition 261

■ Figure 10.9 Karst topography is characterized by a landscape of sinkholes formed by dissolution of limestone.

Identify what controls the rate

of dissolution of bedrock in karst topography.

■ Figure 10.10 Stalactites are stones produced by a buildup of minerals precipitated from groundwater.

drip-Karst topography Figure 10.9 shows some of the surface

fea-tures produced by the dissolution of limestone bedrock One of the

main features is a sinkhole — a depression in the ground caused by the

collapse of a cave or by the direct dissolution of limestone by acidic

water Another type of feature, called a disappearing stream, forms

when a surface stream drains into a cave system and continues flowing

underground, leaving a dry valley above Disappearing streams

some-times reemerge on Earth’s surface as karst springs

Limestone regions that have sinkholes and disappearing streams

are said to have karst topography The word karst comes from the

name of a region in Croatia where these features are especially well

developed Prominent karst regions in the United States are located

in Kentucky, Indiana, Florida, and Missouri The Mammoth Cave

region in Kentucky has karst topography that contains tens of

thousands of sinkholes

In karst areas, sinkholes proliferate, grow, and eventually join to

form wide valleys Most of the original surface has been dissolved,

with the exception of scattered mesas and small buttes The rate of

the dissolution process varies greatly among locations, depending

on factors such as humidity and soil composition In humid areas,

where there is more precipitation, more water infiltrates areas of

porous soil, and dissolves the limestone in the subsurface

Groundwater Deposits

Calcium ions eventually precipitate from groundwater and form new

calcite minerals These minerals create spectacular natural features

Dripstones The most remarkable features produced by

ground-water are the rock formations called dripstone that decorate many

caves above the water table, as shown in Figure 10.10 These

for-mations are built over time as water drips through caves Each

drop of water hanging on the ceiling of a cave loses some carbon

dioxide and precipitates some calcite A form of dripstone, called a

grad-ually As the water drips to the floor of the cave, it may also slowly

build mound-shaped dripstone called stalagmites.

(t)Lloyd Homer/GNS Science , (b)Albert J Copley/Visuals Unlimited

Self-Check Quiz glencoe.com

■ Figure 10.11 Hard water contains high concentrations of minerals., which leave precipi- tates in household water pipes such as this one.

Identify one of the likely precipitates in this pipe.

Section 10.2 Assessment

Section Summary

◗◗ Groundwater dissolves limestone

and forms underground caves.

◗

◗ Sinkholes form at Earth’s surface

when bedrock is dissolved or when

caves collapse.

◗

◗ Irregular topography caused by

groundwater dissolution is called

karst topography.

◗

◗ The precipitation of dissolved

calcite forms stalactites and

stalag-mites in caves.

Understand Main Ideas

1 MAIN Idea Analyze how limestone is weathered, and identify the features that

are formed as a result of this dissolution.

2 Identify the acid that is most common in groundwater.

3 Illustrate in a series of pictures how caves are formed.

4 Examine Why is hard water more common in some areas than others?

Think Critically

5 Compare and contrast the formation of stalactites and stalagmites.

6 Analyze how you might be able to tell an area of karst topography on a

drip-It is possible that these organisms play an important role in the tion of at least some of the materials found in caves

deposi-Hard water You are probably aware that tap water contains ous dissolved solids While some of these materials are added by water treatment facilities, others come from the dissolution of min-erals in soils and subsurface rock and sediment Water that contains high concentrations of calcium, magnesium, or iron is called hard water Hard water is common in areas where the subsurface rock is limestone Because limestone is made of mostly calcite, the ground-water in these areas contain significant amounts of dissolved calcite

vari-Hard water used in households can sometimes cause problems Just

as calcite precipitates in caves, it can also precipitate in water pipes,

as shown in Figure 10.11, and on the heating elements of ances Over time, deposits of calcite can clog water pipes and render some electrical appliances useless

Section 1 10 0 3 3

Objectives

◗ Explain how groundwater is

with-drawn from aquifers by wells.

◗ Describe the major problems that

threaten groundwater supplies.

Review Vocabulary

runoff: water flowing downslope

along Earth’s surface

MAIN Idea Water is not always available in the quantities and

in the locations where it is needed and might be compromised

by pollution.

as much money as you want? Of course not Like a bank account, groundwater can be withdrawn, but only in the amount that has been deposited there.

Wells Wells are holes dug or drilled into the ground to reach an aquifer

There are two main types of wells: ordinary wells and artesian wells

Ordinary wells The simplest wells are those that are dug or drilled below the water table, into what is called a water-table aquifer, as shown in Figure 10.12. In a water-table aquifer, the level of the water in the well is the same as the level of the surrounding water-table As water is drawn out of a well, it is replaced by surrounding water in the aquifer

Overpumping occurs when water is drawn out of the well at a rate that is faster than that at which it is replaced Overpumping of the well lowers the local water level and results in a cone of depres-sion around the well, as shown in Figure 10.12. The difference between the original water-table level and the water level in the

pumped well is called the drawdown If many wells withdraw

water from a water-table aquifer, the cones of depression can lap and cause an overall lowering of the water table, causing shal-low wells to become dry Water from precipitation replenishes the

over-water content of an aquifer in the process of recharge

Ground-water recharge from precipitation and runoff sometimes replaces the water withdrawn from wells However, if withdrawal of ground-water exceeds the aquifer’s recharge rate, the drawdown increases until all wells in the area become dry

■ Figure 10.12 Overpumping from one well

or multiple wells can result in a cone of depression

and a general lowering of the water table.

Section 3 • Groundwater Supply 263

Well

Dry well

Water table

Lowered water table

Cone of depression