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238

What You’ll Learn• How large amounts

of water are storedunderground.

• How groundwater dissolves limestone andforms caves and othernatural features.

• How groundwater isremoved from theground by humans andwhat problems endangerour groundwater supply.

Why It’s ImportantGroundwater providesdrinking water for halfof the world’s populationand is a major source ofthe water used by agri-culture and industry.However, groundwatersupplies are threatened by overuse and pollution.

GroundwaterGroundwater1010

Lechuguilla Cave,New MexicoLechuguilla Cave,New Mexico

To find out more aboutgroundwater, visit the Earth Science Web Site at earthgeu.com

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10.1 Movement and Storage of Groundwater 239

Beneath your feet, there are vastamounts of water. This water fills inthe pore spaces of sediments androcks deep in the ground. In thisactivity, you will discover how muchwater can be stored in sand.

1. Fill a 250-mL graduated cylinderwith dry sand.

2. Fill another 250-mL graduatedcylinder with water.

3. Pour water from the second cylin-der into the sand-filled cylinderuntil the water level is flush withthe surface of the sand. Measure

and record the volume of saturatedsand in the cylinder.

4. Measure and record how muchwater is left in the second cylinder.

CAUTION: Always wearsafety goggles and an apron in thelab.

Observe In your science journal,describe how much water is present inthe saturated sand. Calculate the ratioof water volume to the volume ofsand. Infer how many litersof water could be stored ina cubic meter of sand.

Model UndergroundWater Storage

Discovery LabDiscovery Lab

OBJECTIVES

• Describe how ground-water is stored and moves underground.

• Explain what an aquifer is.

VOCABULARY

infiltrationporosityzone of saturationwater tablepermeabilityaquifer

If you drill a deep enough hole anywhere on Earth, it will partially fillwith groundwater, even in the desert! Groundwater is present every-where beneath the surface of the land, but nevertheless is a smallfraction of all the water on Earth.

THE HYDROSPHEREThe water on and in Earth’s crust makes up the hydrosphere,named after hydros, the Greek word for “water.” About 97 percentof the hydrosphere is contained in the oceans. The water containedby 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 renew-able resources. However, of all the freshwater, more than 90 percent isin the form of polar ice caps and glaciers. You may be surprised to

Movement and Storage of Groundwater

10.110.1

learn that most of the remaining freshwater is groundwater. All therivers, streams, and lakes on Earth represent only a small fraction ofEarth’s liquid freshwater, as shown in Table 10-1.

PRECIPITATION AND GROUNDWATERThe ultimate source of all water on land is the oceans. Evaporationof seawater introduces water into the atmosphere in the form ofinvisible water vapor and visible clouds. Winds and weather systemsmove this atmospheric moisture all over Earth, much of it over thecontinents. Precipitation brings atmospheric moisture back toEarth’s surface, mostly in the form of rain and snow. Some of thisprecipitation falls directly into the oceans, and some falls on land.

Much of the precipitation that falls on land enters the groundthrough the process of infiltration and becomes groundwater. Onlya small portion of precipitation becomes runoff and is returneddirectly to the oceans through streams and rivers. Solid precipitation,such as snow, may cover the ground for long periods of time beforeit melts and becomes runoff or infiltrates to become groundwater.Groundwater slowly moves through the ground, eventually returnsto the surface through springs, and then flows back to the oceans.

GROUNDWATER STORAGEPuddles of water that are left after a rain quickly disappear, partly byevaporating and partly by percolating into the ground. On sandysoils, rain soaks into the ground almost immediately. Where doesthat water go? Subsurface Earth materials are not totally solid, butinstead contain countless small openings, or pores, which make up alarge portion of some of these materials, as you see in Figure 10-1.

240 CHAPTER 10 Groundwater

Table 10-1 World’s Water Supply

Percentage EstimatedSurface Area Water Volume of Total Average Residence

Location (km2) (km3) Water Time of Water

Oceans 361 000 000 1 230 000 000 97.2 Thousands of years

Atmosphere 510 000 000 12 700 0.001 Nine days

Rivers and streams — 1200 0.0001 Two weeks

Groundwater: shallow, 130 000 000 4 000 000 0.31 Hundreds to many to a depth of 0.8 km thousands of years

Lakes (freshwater) 855 000 123 000 0.009 Tens of years

Ice caps and glaciers 28 200 000 28 600 000 2.15 Tens of thousandsof years and longer

The percentage of pore space in a material is called its porosity.Subsurface materials have porosities ranging from 2 or 3 percent tomore than 50 percent. For example, the porosity of well-sorted sandis typically around 30 percent. In poorly sorted sediments, however,smaller particles of sediment occupy some of the pore spaces andreduce the overall porosity of these sediments. Similarly, the cementthat binds the grains of sedimentary rocks together reduces therocks’ porosity. Nevertheless, enormous quantities of groundwaterare stored in the pore spaces of rocks and sediments.

THE ZONE OF SATURATIONThe depth below Earth’s surface at which groundwater completelyfills all the pores of a material is called the zone of saturation. Theupper boundary of the zone of saturation is the water table, asshown in Figure 10-2. Strictly speaking, only the water in the zoneof saturation is called groundwater. In the zone of aeration, which

Figure 10-1 Pore spaces in sediments: the highestpercentage of porosity isfound in well-sorted sedi-ments (A) while poorlysorted sediments (B) havea lower percentage.


A B

Stream

Normal water table

Soil moisture

Water table during drought

Zone of aeration

Water table

Zone of saturation

Figure 10-2 Groundwaterflows toward valleys wherethe water table is close tothe surface. During dry peri-ods the level of the watertable falls.

is above the water table, materials are moist, but the pores containmostly air. Water in the zone of saturation can be classified as eithergravitational water or capillary water. Gravitational water is waterthat trickles downward as a result of gravity. Capillary water iswater that is drawn upward from the water table and is held in thepore spaces of rocks and sediments as a result of surface tension.Materials that are directly above the water table, especially fine-grained materials, are nearly saturated with capillary water.Capillary action is similar to the action of water that is drawnupward through the pore spaces of a paper towel when the end of itis dipped into water.

The Water Table The depth of the water table varies dependingon local conditions. For example, in stream valleys, groundwater isclose to Earth’s surface, and thus the water table is a few meters deepat most. In swampy areas, the water table is almost at Earth’s surface,whereas on hilltops or in arid regions, the water table can be tens tohundreds of meters or more beneath the surface. As shown in Figure10-2, the topography of the water table follows the topography of theland above it. For example, the water table slopes toward valleys andforms hills under topographic hills. Water table topography forms inthis way because water underground moves slowly and conforms tosurface contours.

Because of its dependence on precipitation, the water table fluc-tuates with seasonal and other weather conditions. It rises duringwet seasons, usually in spring, and drops during dry seasons, oftenin late summer.

GROUNDWATER MOVEMENTGroundwater flows downhill in the direction of the slope of the watertable. In most cases, this downhill movement is slow because the waterhas to squeeze through numerous tiny pores in the subsurface mater-ial. In fact, if the pores are small, not even individual water moleculescan squeeze through. The ability of a material to let water pass throughit is called permeability. Materials with large, connected pores, such assand and gravel, as shown in Figure 10-3A, have high permeabilitiesand permit relatively high flow velocities, up to 1 m/h or more. Otherpermeable subsurface materials include sandstone, limestone, and allhighly fractured bedrock.

Fine-grained materials typically have low permeabilities becausetheir pores are so tiny, as shown in Figure 10-3B. These materials aresaid to be impermeable. Flow velocities in impermeable materialsare so low that they are often measured in meters per year. Someexamples of impermeable materials are silt, clay, and shale. Clay is so


1 mm

0.1 mm

A

B

Figure 10-3 In a saturatedmaterial, all grains arecoated with a thin film ofmotionless water. In coarse-grained material like sand(A) this film occupies a rela-tively small fraction of thepore space, and movingwater can pass freelythrough the pores. In fine-grained material like silt (B)this film occupies most ofthe pore space and blocksthe movement of water. As a result, sand has a muchhigher permeability thansilt.

impermeable that a clay-lined depression will hold water. For thisreason, clay is often used to line artificial ponds and landfills.

The flow velocity of groundwater primarily depends on the slopeof the water table, because the force of gravity pulling the water down-ward is greater when the slope of the water table surface is steeper. Youhave experienced a similar effect if you have ever ridden a bicycle downa steep street and a gently sloping street. Although the flow velocity ofgroundwater is proportional to both the slope of the water table andthe permeability of the material through which the water flows, per-meability is the major factor. Thus, flow velocities through permeablematerials are always higher than those through impermeable materi-als, regardless of the slope of the water table. Most groundwater flowtakes place through permeable layers, called aquifers, such as the oneshown in Figure 10-4. Impermeable layers, called aquicludes, are bar-riers to groundwater flow. In the next section, you’ll discover whathappens when groundwater moves slowly through materials.


Precipitation

WellImpermeable layer

Aquifer

1. What is the greatest source of freshwateron Earth?

2. Where is the water table closest to Earth’ssurface: in the floodplain of a river, in a swamp, or on a hilltop?

3. What two factors determine the flowvelocity of groundwater?

4. What is an aquifer?

5. Thinking Critically What is the differencebetween porosity and permeability insubsurface materials?

SKILL REVIEW

6. Making and Using Tables Design a datatable that compares and contrasts theporosity and permeability of sand and a mixture of sand and gravel. Whichmaterial has the greater porosity? Thegreater permeability? For more help,refer to the Skill Handbook.

Figure 10-4 The aquifer is locatedin a permeable sandstone layerthat is sealed beneath a cappinglayer of impermeable rock.

earthgeu.com/self_check_quiz

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10.210.2 Groundwater Erosion and Deposition


OBJECTIVES

• Explain how ground-water dissolves anddeposits rocks and minerals.

• Describe how caves form and how karsttopography develops onEarth’s surface.

VOCABULARY

cavesinkholekarst topographystalactitestalagmitetravertine

In Chapter 3, you learned about the corrosive properties of acids.Acids are solutions that contain hydrogen ions. Most groundwatercontains some acid, in most cases carbonic acid. Carbonic acidforms when carbon dioxide dissolves in water and combines withwater molecules. This happens when rain falls through the atmos-phere and interacts with carbon-dioxide gas or when groundwaterpercolates through carbon-rich, decaying organic material in soil.As a result of these processes, groundwater is usually slightly acidic and attacks carbonate rocks, especially limestone. Limestoneconsists mostly of calcium carbonate (CaCO3), which dissolvesreadily in any kind of acid, the results of which are shown inFigure 10-5.

DISSOLUTION BY GROUNDWATERThe process by which carbonic acid forms and dissolves calcium car-bonate can be described by three simple chemical equations.

In the first process, carbon dioxide and water combine to formcarbonic acid, as represented by the following equation.

CO2 + H2O ——> H2CO3

In the second process, the carbonic acid (H2CO3) molecules in thewater split into hydrogen ions (H+) and bicarbonate ions (HCO3

–).This process is represented by the following equation.

H2CO3 ——> H+ + HCO3–

In the third process, the hydrogen ions react with calcium car-bonate and dissolve it, as represented by the following equation.

CaCO3 + H+ ——> Ca2+ + HCO3–

For every carbon dioxide molecule dissolved ingroundwater, one hydrogen ion is produced and onecalcium carbonate molecule is dissolved. The result-ing calcium (Ca2+) and bicarbonate (HCO3

–) ionsare then flushed away by the groundwater.Eventually, they precipitate out somewhere else.Precipitation of calcium carbonate occurs when thegroundwater evaporates or when the gas carbondioxide diffuses out of the water. Both the dissolu-tion and formation of calcium carbonate play amajor role in the formation of limestone caves.

Figure 10-5 A viewingpagoda is standing amongthe massive limestone pillars of the stone forest in China. Carbonic acid isslowly dissolving the cal-cium carbonate in the limestone pillars.

10.2 Groundwater Erosion and Deposition 245

Figure 10-6 In MammothCave, Kentucky, there are a series of undergroundpassages that extend to a length of more than 500 km.

Caves A natural underground opening with a connection to Earth’ssurface is called a cave. Some caves form three-dimensional mazes ofpassages, shafts, and great chambers that stretch for many kilometers.Many caves have structures that hang from the caves’ ceilings. Somecaves are dry, while others contain underground streams or lakes. Stillothers are totally flooded and can be explored only by cave divers. Oneof the most spectacular caves is the recently discovered LechuguillaCave of New Mexico, shown in the photograph at the beginning ofthis chapter. Another cave system in New Mexico, Carlsbad Caverns,includes a huge subterranean chamber over 1 km long and 100 mhigh. Mammoth Cave, in Kentucky, as shown in Figure 10-6, is com-posed of a series of connected underground passages.

Practically all caves of significant size are formed when ground-water dissolves limestone. Most caves develop in the zone of satura-tion just below the water table. As groundwater percolates throughthe cracks and joints of limestone formations, it gradually dissolvesthe adjacent rock and enlarges these passages to form an intercon-nected network of openings. Thus, the limestone formation becomesmore permeable. The resulting increased downhill flow of ground-water gradually lowers the water table until much of the cave systemis filled with air. New caves then form beneath the lowered watertable. If the water table continues to drop, the thick limestone for-mations eventually become honeycombed with caves and caverns.This is a common occurrence in limestone regions that have beenuplifted by tectonic forces.

Karst Topography Figure 10-7 shows some of the characteristicsurface features produced by the dissolution of limestone. The mainfeature is a sinkhole, as shown in Figure 10-8. A sinkhole is a depres-sion in the ground caused by the collapse of a cave or by the directdissolution of bedrock by acidic rain or moist soil. Another type offeature forms when a surface stream drains into a cave system, con-tinues underground, and leaves a dry valley above. Such a stream,called a sinking stream, sometimes reemerges abruptly on Earth’ssurface as a karst spring.

Limestone regions that have sinkholes, sinks, and sinking streamsare said to have karst topography. The word karst comes from the name of a limestone region in Croatia where these features are


Water-filled caves

Sinking streamSinkholes

Water table

Figure 10-7 In the development of karst topography, caves form near or below the watertable. Streams deepen their valleys. The water table drops and new caves form below thewater table. Collapsing caves or dissolution of bedrock at the surface produce sinkholes.

Figure 10-8 Sinkholesdeveloped near Roswell,New Mexico.

10.2 Groundwater Erosion and Deposition 247

Figure 10-9 Dissolved mineralscan build up thick deposits inplumbing pipes.

Using NumbersEach drop of waterdeposits a 1-nm-thicklayer of calcium car-bonate at the tip of a stalactite growingin a cave. Given that1 nm = 10 –9 m, if adrop falls every 30seconds, what length,in centimeters, willthe stalactite be in100 years? How manyyears will it take forthe stalactite to reacha length of 5 m?

especially well developed. Prominent karst regions in the UnitedStates are located in Kentucky. The Mammoth Cave region inKentucky has karst topography that contains tens of thousands ofsinkholes. Most of the lakes in Central Florida are sinkholes.

GROUNDWATER DEPOSITSYou are probably aware that your tap water contains various dis-solved materials. Some water contains sulfur compounds, and somecontains dissolved iron compounds. Water that contains iron com-pounds typically leaves brownish or red stains on kitchen and bath-room fixtures.

Hard Water Water that contains high concentrations of calcium,magnesium, or iron is called hard water. Hard water is common inlimestone areas where the groundwater is nearly saturated with cal-cium carbonate. Household use of hard water usually can cause aproblem: deposits of calcium bicarbonate eventually clog waterpipes, as shown in Figure 10-9. These problems can be controlledwith a water softener, which removes dissolved ions from hard water.Water that contains few dissolved ions is called soft water.

Natural Deposits The most remarkable deposits produced bygroundwater are the dripstone formations that decorate many cavesabove the water table. As their name indicates, these formations arebuilt slowly as water drips through caves. Each drop of water hang-ing on the ceiling of a cave loses some of its carbon dioxide anddeposits a tiny amount of calcium carbonate. Over many years, thesedeposits gradually form cone-shaped or cylindrical structures called

stalactites that hang from the cave’s ceiling like icicles. As the waterdrops splash to the floor of the cave, they gradually build mound-shaped dripstone deposits, called stalagmites, underneath the sta-lactites. In time, stalactites and stalagmites may grow together toform dripstone columns, such as the ones shown in Figure 10-10.These and other types of dripstone formations are composed of atype of limestone called travertine.


Figure 10-10 Stalactites,stalagmites, and dripstonecolumns are found in theCarlsbad Caverns of NewMexico.

1. What acid is most commonly present in groundwater?

2. How do caves form?

3. Compare the formation of stalactites and stalagmites.

4. What is karst topography?

5. Thinking Critically If you visited a regionthat consisted mostly of igneous rocks,would you expect to find karst topog-raphy? Explain.

SKILL REVIEW

6. Concept Mapping Use the followingterms to construct a concept map to orga-nize the major ideas in this section. Formore help, refer to the Skill Handbook.

stalactites

stalagmites

Croatia

sinkholes

calciumcarbonate

hardwater

karsttopography

cloggedwater pipes

caves



Environmental ConnectionEnvironmental Connection

10.310.3

10.3 Groundwater Systems 249

Groundwater Systems

The average length of time that groundwater remains under-ground is several hundred years. Groundwater moves slowly butcontinuously through aquifers and eventually returns to Earth’s sur-face. You may wonder how this can happen. Can groundwater flowupward against gravity? In some cases, it can, as you will learn in thissection. In most cases, however, groundwater emerges wherever thewater table intersects Earth’s surface. Such intersections commonlyoccur in areas that have sloping surface topography. The exact placeswhere groundwater emerges depend on the arrangement of aquifersand aquicludes in an area.

SPRINGSYou have learned that aquifers are permeable underground layersthrough which groundwater moves with relative ease, whileaquicludes are impermeable layers. Aquifers are commonly com-posed of layers of sand and gravel, sandstone, and limestone. Becauseof its many solution cavities, limestone is usually highly permeableand permits high flow velocities. Underground streams in cavernouslimestone formations may transport groundwater at a rate of severalkilometers per day. In contrast,aquicludes, such as layers ofclay or shale, block ground-water movement. As a result,groundwater tends to dis-charge at Earth’s surface wherean aquifer and an aquicludecome in contact, as shown inFigure 10-11. These naturaldischarges of groundwater arecalled springs.

OBJECTIVES

• Relate the different typesof springs to common systems of aquifers.

• Explain how ground-water is withdrawn fromaquifer systems by wells.

• Describe the majorproblems that threatengroundwater supplies.

VOCABULARY

springhot springgeyserwell

drawdownrecharge artesian well

Figure 10-11 A springoccurs where an aquiferand an aquiclude come incontact at Earth’s surface.At this point, the waterflows out of the rock.


Emergence of Springs The volume of water that is dischargedby a spring may be a mere trickle, or, in karst regions, an entire rivermay emerge from the ground. Such a superspring is called a karstspring. Many of Florida’s lakes are flooded sinkholes that are fed bykarst springs whose discharge causes full-sized rivers to flow out ofthese lakes. In regions of near-horizontal sedimentary rocks, springsoften emerge on the sides of valleys at about the same elevation, atthe bases of aquifers, as shown in Figure 10-12A. Springs may alsoemerge at the edges of perched water tables. A perched water table,as shown in Figure 10-12B, is a zone of saturation that overlies anaquiclude that separates it from the main water table below. Otherareas where springs tend to emerge are along faults, which are hugefractures that offset rock formations and sometimes block aquifers,as shown in Figure 10-12C. In limestone regions, springs dischargewater from underground pathways, as shown in Figure 10-12D.

Temperature of Springs Spring water is usually thought of asbeing cool and refreshing. Actually, the temperature of groundwaterthat is discharged through a spring is generally the average annualtemperature of the region in which it is located. Thus, springs in New

Figure 10-12 Springs occurin various places, such as atthe sides of valleys and atthe edges of perched watertables. The volume of watervaries from the flow of asmall spring to the rush of a large river.

Perched water table

Spring

Layer of impermeable clay

Main watertable

Cavern

Spring

Water table

Sandstone

Spring

Water table

Sandstone

Spring

Water table

Shale

Fault

A B

C D


England have year-round temperatures of about 10°C, while those inthe Gulf states have temperatures of about 20°C.

Compared to air temperatures, groundwater is colder in the sum-mer and warmer in the winter. However, in some regions of theUnited States, certain springs discharge water that is much warmerthan the average annual temperature. These springs are called warmsprings or hot springs, depending on their temperatures. Hot springshave temperatures higher than that of the human body. There arethousands of hot springs in the United States alone, as shown inFigure 10-13. Most of these are located in the westernUnited States in areas where the subsurface is still quite hotfrom relatively recent igneous activity. A number of hotsprings also occur in some eastern states. These eastern hotsprings emerge from aquifers that descend to great depthsin Earth’s crust and allow deep, hot water to rise. Theunderground water is hot because temperatures in Earth’scrust increase with depth by about 25°C for every kilome-ter. Among the most spectacular features produced byEarth’s underground heat in volcanic regions are geysers, asshown in Figure 10-14. Geysers are explosive hot springsthat erupt at regular intervals. One of the world’s mostfamous geysers, Old Faithful, is located in YellowstoneNational Park, Wyoming. Old Faithful erupts approxi-mately every hour with a 40-m high column of boilingwater and steam.

400 Km

Hot springs

0

Hot Springs in the United States Figure 10-13 Hot springsoccur in many areas of theUnited States.

Figure 10-14 The BlackRock Desert geyser is a hotspring that erupts at regularintervals in Nevada.

Sandstone

Well 1

Pressure surface

Water table

Well 2

Shale

Recharge area


WELLSWells are holes dug or drilled deep into the ground to reach a reser-voir of groundwater. To produce water, a well must tap into anaquifer. The simplest wells are those that are dug or drilled below thewater table, into the zone of saturation, and into what is called awater-table aquifer, as shown in Figure 10-15A. Initially, the waterlevel in such a well is the same as the level of the water table. However,overpumping of the well lowers the water level in it and produces acone of depression in the water table around the well, as shown inFigure 10-15B. The difference between the original water-table leveland the water level in the pumped well is called the drawdown. Ifmany wells withdraw water from a water-table aquifer, their cones ofdepression may overlap and cause an overall lowering of the watertable, which can cause shallow wells to go dry. Water from precipita-tion and runoff is added back to the zone of saturation in the processof recharge. Groundwater recharge from precipitation and runoffsometimes replenishes the water withdrawn from wells. However, ifrecharge does not keep pace with groundwater withdrawal, the watertable continues to drop until all wells in the area go dry.

Figure 10-16 The level to which the water in anartesian well can rise iscalled the pressure surface.

WellWell WellDrywell

Drywell

Former water table

Before heavy pumping After heavy pumping

Lowered water table

Cone of depression

Figure 10-15 Wells mustbe drilled far enough belowthe water table so that theyare not affected by seasonal water table fluctuations (A).Overpumping of wellscauses a lowering of theentire water table (B).

A B


CONFINED AQUIFERSWater-table aquifers are unconfined and unprotected, and thus, theyare easily polluted. Surface spills of pollutants often reach the watertable and spread throughout aquifers. More reliable and less easily pol-luted water supplies can be found in deeper aquifers, called confinedaquifers, which are generally sandwiched between aquicludes. Theaquicludes form barriers that prevent pollutants from reaching suchaquifers.

Artesian Wells Because the area of recharge is usually at a higherelevation than the rest of an aquifer, a confined aquifer contains waterunder pressure, as you can see by doing the Problem-Solving Lab onthis page. The aquifer is called an artesian aquifer. When the rate ofrecharge is high enough, the pressurized water in a well drilled into aconfined aquifer may spurt above the land surface in the form of afountain called an artesian well, as shown in Figure 10-16. Similarly,a spring that discharges pressurized water is called an artesian spring.

Make inferences about thewater levels of an artesianaquifer Artesian aquifers containwater under pressure. The tableprovides the following data aboutan artesian aquifer for three sites,spaced 100 m apart, along a survey line:elevations of the land surface, the watertable, and the upper surface of theaquiclude on top of the artesian aquifer;and the artesian pressure surface, which is the level to which the artesian water can rise.

Analysis1. Plot the elevation data on a graph

with the sites on the x-axis and theelevations on the y-axis. Make a crosssection of the survey line from site 1 to site 3. Use a heavy line to indicatethe land surface.

2. A well has been drilled at each site.The wells at sites 1 and 3 are 7 mdeep. The well at site 2 was drilledinto the artesian aquifer at a depth of14 m. Sketch the wells at their properdepths on your cross section.

Thinking Critically 3. At what depth below the ground sur-

face are the water levels in the threewells before they are pumped?

4. What would happen if a well wasdrilled into the confined aquifer at site 3?

5. At what sites could there be an artesian spring?

Using Tables and Making Graphs

Surface Water Table Aquiclude PressureSite Elevation Elevation Elevation Surface

1 396 m 392 m 388 m 394 m2 394 390 386 3933 390 388 381 392

How does an artesian well work?Model the changes that an artesian aquiferundergoes when a well is dug into it. Whatcauses the water to rise above the groundsurface?

Procedure CAUTION: Always wearsafety goggles and an apron in the lab.

1. Half-fill a plastic shoe box or other con-tainer with sand. Add enough water tosaturate the sand. Cover the sand com-pletely with a 1-or 2-cm layer of clay or a similar impermeable material.

2. Tilt the box at an angle of about 10°. Use a book for a prop.

3. Punch three holes through the clay, oneeach near the low end, the middle, andthe high end of the box. Insert a clearstraw through each hole into the sandbelow. Seal the holes around the straws.

Analyze and Conclude1. Observe the water levels in the straws.

Where is the water level the highest? The lowest?

2. Where is the water table in the box? 3. Where is the water under greatest

pressure? Explain. 4. Predict what will happen to the water

table and the surface if the water flowsfrom one of the straws.

Agriculture41%

Domestic10%

Industry11%

Cooling of electric powerplants38%


The name artesian is derived from theFrench province of Artois, where such wellswere first drilled almost 900 years ago. Todiscover how an artesian well works, refer tothe MiniLab on this page.

An important artesian aquifer in theUnited States is the Ogallala Aquifer, whichis located in the Great Plains. This aquiferdelivers water to a huge area stretching fromSouth Dakota to Texas. The recharge areasof the Ogallala Aquifer are located in theBlack Hills and the Rocky Mountains.

THREATS TO OUR WATER SUPPLYFreshwater is Earth’s most precious naturalresource. Think about it! You can survivewithout gasoline, without electricity, andwithout most of the other materials thatmay seem to be essential, but you can’t livewithout water. Human demands for fresh-water are enormous. Not only is water usedin households, but it is also used extensivelyin agriculture and industry, as shown inFigure 10-17. Groundwater supplies muchof this water.

U.S. Water Use

Figure 10-17 In the United States,the greatest amount of water usageis for agricultural activities, mostly for irrigation.

Overuse Groundwater supplies can be depleted. If groundwater ispumped out at a rate greater than the recharge rate, the groundwatersupply will inevitably decrease, and the water table will drop. This iswhat is happening to the Ogallala Aquifer. Its water, used mostly forirrigation, is being withdrawn at a rate much higher than the rechargerate. You will learn more about the Ogallala Aquifer in the Science &Environment feature at the end of the chapter.

Subsidence Another problem caused by the excessive withdrawalof groundwater is ground subsidence, the sinking of land. The weightof the material overlying an aquifer is partly borne by water pressure.If that pressure is reduced, the weight of the overlying material isincreasingly transferred to the aquifer’s mineral grains, which thensqueeze together more tightly. As a result, the land surface above theaquifer sinks.

Pollution in Groundwater In general, the most easily pollutedgroundwater reservoirs are water-table unconfined aquifers. Con-fined aquifers are affected less frequently by local pollution becausethey are protected by impermeable barriers. When the recharge areasof confined aquifers are polluted, however, those aquifers becomecontaminated as well.

The most common sources of groundwater pollution aresewage, industrial waste, landfills, and agricultural chemicals. Thesepollutants enter the ground above the water table, but they areeventually flushed downward by infiltrating precipitation andbecome mixed with the groundwater. Most sewage enters ground-water from sources such as faulty septic tanks. In highly permeableaquifers, all pollutants, including raw sewage, can spread quickly, asshown in Figure 10-18.


Faultyseptictank

Coarse sandy gravel

Contaminated wells

Wellproducingpure water

Sand

Watertable

Polluted water

Figure 10-18 Pollutantsfrom sewage are travelingthrough a coarse-grainedaquifer that will contami-nate downslope wells.


Chemicals Chemicals dissolved or transported with groundwaterare in the form of ions and molecules, so they cannot be filtered outin fine-grained sediments. For this reason, chemicals such as arseniccan contaminate any type of aquifer. They generally move downslopefrom a source in the form of a pollution plume, a mass of contami-nants that spreads through the environment. Once chemical conta-minants have entered groundwater, they cannot be easily removed.

Salt Not all pollutants are toxic or unhealthful in and of themselves.For example, ordinary table salt is widely used to season food.However, water is undrinkable when its salt content is too high. Infact, salt pollution is one of the major threats to groundwater sup-plies. In many coastal areas, the contamination of freshwater by saltwater is the major problem. In such areas, the fresh groundwater nearEarth’s surface is underlain by denser, salty seawater as shown inFigure 10-19A. The overpumping of wells can cause the underlyingsalt water to rise into the wells and contaminate the freshwateraquifer, as shown in Figure 10-19B.

Radon Another source of natural pollution is radioactive radongas, which is one of the leading causes of cancer in the United States.This form of radon is generated by the radioactive decay of uraniumin rocks and sediments, and it usually occurs in very low concentra-tions in all groundwater. However, some rocks, especially granite andshale, contain more uranium than others. The groundwater in areaswhere these rocks are present therefore contains more radioactiveradon than normal. Some of this radon may seep into houses, and,because it is heavier than air, it can accumulate in poorly ventilatedbasements. The U. S. Environmental Protection Agency (EPA)advises homeowners in radon-prone regions to regularly have theirhomes tested for radon gas.

Before pumping After pumping

Ocean

Water table

Fresh groundwaterSalt water

Ocean

Pumpedwell

Freshgroundwater

Salt water

Figure 10-19 Saltwaterincursion into groundwatercan occur in coastal areas(A). Freshwater floats ontop of denser seawaterwithin the aquifer (B).Overpumping of wellsdraws the underlying salt-water into the wells and the freshwater zone.

A B

Topic: GroundwaterTo find out more about the ways to protectgroundwater, visit theEarth Science Web Site atearthgeu.com

Activity: Write a TV addescribing measures beingtaken in your communityto protect the groundwater.

http://earthgeu.com


PROTECTING OUR WATER SUPPLYThere are a number of ways in which groundwater resources can beprotected and restored. First, all major pollution sources, which arelisted in Table 10-2, need to be identified and eliminated. Pollutionplumes that are already in the ground can be monitored with obser-vation wells and other techniques. You will learn more about pollu-tion plumes in the Mapping GeoLab at the end of this chapter. Mostpollution plumes spread slowly. Thus, there is often time for alter-nate water supplies to be found. In some cases, pollution plumes canbe stopped by the building of impermeable underground barriers.Polluted groundwater can be pumped out for chemical treatment onthe surface.

While these measures can have limited success, they alone cannotsave Earth’s water supply. An important part of the solution is forhumans to become more aware of how their activities impact thegroundwater system.

1. How are springs related to the watertable?

2. What is the basic characteristic of an artesian well?

3. List four common sources of groundwaterpollution.

4. Why are chemical contaminants a seriouspollution problem in groundwater?

5. Artesian aquifers contain water underpressure. Explain why.

6. Thinking Critically What can you do toconserve and protect groundwater so thatthere will be safe and abundant watersupplies in the future?

SKILL REVIEW

7. Comparing and Contrasting Compareand contrast different uses of water inthe United States. For more help, refer tothe Skill Handbook.

Table 10-2 Groundwater Pollution Sources

Accidental spills from vehicles

Leaks from storage tanks

Seepage from acid mine drainage

Seepage from faulty septic systems

Saltwater intrusion into aquifers near shorelines

Leaks from waste disposal sites



ProblemA major gasoline spill occurred at Jim’sGas Station near Riverside Acres, Florida.How can you determine the movementof the resulting pollution plume?

MaterialsUSGS topographic map of Forest City,

Floridatransparent paper rulergraph paper calculator

Mapping Pollution

You can use a map to estimate the direction of groundwa-ter flow and the movement of a pollution plume from its

source, such as a leaking underground gasoline storage tank.

1. What is the slope of the water table atJim’s Gas Station?

2. What is the approximate direction of the water table slope at Jim’s Gas Station?

3. In which direction will the pollutionplume move?

4. Which settlements or houses arethreatened by this pollution plume?

1. Identify the lakes and swamps in thesoutheast corner of this map, and list their names or numbers and ele-vations in a data table. Note: The ele-vations are given or can be estimatedfrom the contour lines.

2. Place the transparent paper over thesoutheast part of the map and tracethe approximate outlines of theselakes or swamps, as well as the majorroads. Enter lake or swamp elevations

on your overlay, and indicate thelocation of Jim’s gas station on ForestCity Rd., about 1400 feet north of theSeminole County line (at the 96 footelevation mark).

3. Add contour lines to your overlayusing a contour interval of 10 feet.

4. Construct a cross section of the surface topography and the watertable from Lake Lotus to Lake Lucien (through Jim’s Gas Station).

1. How far below the surface is thewater table in the highest area?

2. What is the relationship of the watertable to the surface topography?

Preparation

Procedure

Analyze

Conclude & Apply


259259

Demand placed upon the water supply overthe years increased rapidly as more wells weredrilled. In 1950, an estimated 8.6 trillion L ofwater were pumped from the aquifer. By 1980,the estimated amount had increased to 24 trillion L. The amount of water currently beingpumped from the aquifer far exceeds the amountof water naturally replenished. Presently, 89 percent of the aquifer’s water supply is still available despite the 170 000 wells drawingwater. However, it has been predicted that by the year 2020, 25 percent of the water supplycould be exhausted.

What Needs to be DoneWhile scientists cannot be sure that the

aquifer will ever be completely depleted, there is reason to be concerned about the continuouslowering of water levels in the region. Wells insome areas overlying the aquifer have gone dry.Concerns about the depletion of the water supplyhave prompted several states to develop regula-tory policies to protect this natural resource,which has often been taken for granted.


Long Term Effects of Water UseBuried beneath 106 250 km2 of sand and

rock lies the largest groundwater system in theUnited States, the High Plains Aquifer, alsoknown as the Ogallala Aquifer. The High PlainsAquifer, which contains the Ogallala Formation, is made of clay, gravel, sand, and silt. The resultof deposition from an ancient snow melt in theRocky Mountains region, the formation is dividedinto two sections. The upper section is known asthe unsaturated zone. The lower section containsthe Ogallala Aquifer, the water-bearing area. TheOgallala makes up approximately 80 percent ofthe High Plains Aquifer.

The aquifer’s freshwater supply has beenused for many years. Shortly after World War I,farmers began tapping into the groundwaterreserves in Texas. By 1940, the aquifer hadbecome a widely used source of irrigation in theGreat Plains region as more and more farmersbegan to pump water from the aquifer.

By 1980, it was estimated that parts of theaquifer’s water level were down as much as 30 meters. Because of concern about the declinein water levels, the U.S. Geological Survey begana monitoring program in 1988. The programrevealed that from 1980 to 1994, water levelscontinued to decline in certain areas of the TexasHigh Plains, the Oklahoma Panhandle, and south-western Kansas. In the eastern High Plains ofNebraska, higher than normal amounts of rainfallsince 1980 have caused either a rise in the waterlevel or a slower decline.

The High Plains AquiferThe High Plains Aquifer, also known as the Ogallala Aquifer, islocated beneath parts of Colorado, Kansas, Nebraska, New Mexico,South Dakota, Texas, Oklahoma, and Wyoming. Environmentalists areconcerned about the future of this source of fresh groundwater.

Research and construct a 3-dimensionalmodel of the Ogallala Aquifer. Include thestates that are part of the groundwater sys-tem. Label the aquifer’s zone of saturationand water table.

Activity

Summary

Vocabularyaquifer (p. 243)infiltration (p. 240)permeability (p. 242)porosity (p. 241)water table (p. 241)zone of saturation

(p. 241)

Vocabularycave (p. 245)karst topography

(p. 246)sinkhole (p. 246)stalactite (p.248)stalagmite (p. 248)travertine (p. 248)

Vocabularyartesian well

(p. 253)drawdown (p. 252)geyser (p. 251)hot spring (p. 251)recharge (p. 252)spring (p. 249)well (p. 252)

Main Ideas• Some precipitation infiltrates the ground to become groundwater.• Groundwater is stored below the water table in the pore spaces

of rocks and moves through permeable layers called aquifers.Impermeable layers are called aquicludes.

Main Ideas• Groundwater dissolves limestone and forms underground cav-

erns. Sinkholes form at Earth’s surface when bedrock is dissolvedor when caves collapse. Irregular topography caused by ground-water dissolution is called karst topography.

• The precipitation of dissolved calcium carbonate forms stalac-tites, stalagmites, and travertine deposits, including dripstone columns, in caves.

Main Ideas• The natural discharge of groundwater takes place through

springs. Springs emerge where the water table intersects Earth’s surface.

• Wells are drilled into the zone of saturation to provide water forhuman needs. The pumping of shallow wells produces cones ofdepression in the water table. Artesian wells tap deep, confinedaquifers that contain water under pressure.

• In many regions, groundwater withdrawal exceeds groundwaterrecharge and causes considerable lowering of the water table aswell as ground subsidence.

• The most common sources of groundwater pollution are sewage,industrial waste, landfills, and agricultural chemicals.

SECTION 10.1

Movement andStorage ofGroundwater

SECTION 10.2

GroundwaterErosion andDeposition

SECTION 10.3

GroundwaterSystems

Study Guide 261earthgeu.com/vocabulary_puzzlemaker

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1. Where is most freshwater found on Earth?a. the oceansb. the atmospherec. polar ice caps and glaciersd. lakes and rivers

2. What is a major source of freshwater in theUnited States?a. the Rocky Mountain snowpackb. the Mississippi Riverc. groundwaterd. the Great Lakes

3. What happens to most of the precipitation thatfalls on land?a. It evaporates. c. It seeps into the ground.b. It becomes runoff. d. It becomes glacial ice.

4. What source usually replenishes groundwater?a. precipitation c. underground streamsb. surface water d. municipal wastewater

5. Of the following materials, which is the mostporous?a. a well-sorted sand c. sandstoneb. a poorly sorted sand d. granite

6. Of the following materials, which is the most permeable?a. sandstone c. siltb. shale d. clay

7. What is the main characteristic of an aquifer?a. surface topography c. subsidenceb. permeability d. dissolution

8. Which rock type is most easily dissolved bygroundwater?a. sandstone c. limestoneb. granite d. shale

Understanding Main Ideas 9. What are the cone-shaped dripstone depositsthat are found on the floor of caves?a. icicles c. stalactitesb. rocksicles d. stalagmites

10. Which of the following are typical features ofkarst topography?a. moraines c. sinkholesb. dunes d. landslides

11. Where is groundwater closest to Earth’s surface?a. in stream valleysb. on hilltopsc. on mountaintopsd. in arid regions

12. What does hard water usually contain?a. fluorine c. carbonic acidb. chloride d. calcium

13. What do artesian aquifers always contain?a. hot water c. salt waterb. water under pressure d. steam

14. Which of the following is a common groundwaterproblem in coastal areas?a. saltwater contaminationb. contamination by crude oilc. high sulfur contentd. excessive recharge

BREATHE Oxygen helps to calm the anxietyassociated with test-taking. When you start tofeel your stomach lurch, take a deep breath andexhale slowly.

Test-Taking Tip

earthgeu.com/chapter_test

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Assessment 263

1. Which of the following materials would bebest suited for lining a pond?a. gravel c. clayb. limestone d. sand

2. What are natural structures hanging from acave’s ceiling?a. geyserites c. stalagmitesb. travertines d. stalactites

3. Which of the following usually describes thetemperature of groundwater flowing througha natural spring?a. hotter than the region’s average

temperatureb. cooler than the region’s average

temperaturec. the same temperature no matter where

the spring is locatedd. the same temperature as the region’s

average temperature

4. Which of the following water sources are themost easily polluted?a. water-table aquifers c. artesian wellsb. confined aquifers d. hot springs

5. What is the composition of dripstone formations?a. carbonic acidb. carbon dioxidec. iron oxided. calcium carbonate

15. What is the difference between soft water andhard water?

16. What type of bedrock most likely exists in an areathat has numerous sinkholes?

17. Subsurface temperatures increase with depth byabout 25°C/km. What is the subsurface tempera-ture 2 km below the surface in a region wherethe average annual surface temperature is 10°C?

18. A dripping water faucet in your home has pro-duced brownish-red stains in the sink. What could have caused these stains?

19. Describe the processes involved in the formationof caves.

20. If the withdrawal of groundwater from an arte-sian aquifer exceeds the groundwater recharge,what consequence can be expected?

21. A well drilled into a water-table aquifer produceswater only during springtime. Why?

Use the diagram below to answer question 22.

22. Make an inference on what can happen to thespring if the water table drops.

Thinking Critically

Applying Main Ideas Standardized Test Practice

Sandstone

Spring

Water table

earthgeu.com/standardized_test

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Weathering, Erosion, and SoilChemical and mechanical weathering break downEarth materials. Chemical weathering causes achange in the composition of a rock. Agents ofchemical weathering include hydrolysis, oxidation,acids from decaying organic matter, and acid pre-cipitation. Each of these processes or substancescombines with Earth materials, resulting in newcombinations of minerals or in other substances.Mechanical weathering causes a change only in arock’s size and shape. Temperature and pressure arethe major factors in mechanical weathering. Temp-erature changes can cause rocks to split. Pressurechanges can cause rocks to crack or break apart.

Erosion and Deposition Gravity is the driv-ing force behind all agents of erosion, the processby which weathered pieces of rock are moved tonew locations. Other agents of erosion include mov-ing water, wind, and glaciers. Deposition occurswhen the movement of transported materials slowsdown and they are dropped in a new location.

Formation of Soil Soils vary with climateand are classified as polar, temperate, desert, ortropical. A single centimeter of soil takes hundred ofyears to develop, but can erode away in just sec-onds. Soil is made of weathered rock and decayedorganic matter called humus. Residual soil remainson top of its parent bedrock. Transported soil ismoved away from its parent bedrock by weatheringagents. A cross section of layers of soil is called asoil profile. The top layer, called horizon A, is topsoil.

264 UNIT 3

Horizons B and C are subsoil. Below horizon C issolid bedrock. Parent rock and environmental condi-tions determine a soil’s composition. Soil texture isdetermined by the relative amounts of clay, sand,and silt the soil contains. Soil fertility is a soil’s abil-ity to grow crops. Farmers conserve soil throughmethods that include wind barriers.

Mass Movements, Wind, and GlaciersThe landscape is changed by mass movements,wind, and glaciation. Mass movement refers to themovement of Earth materials downslope as a resultof gravity. Almost all of Earth’s surface undergoesmass movements, which may be slow, as in creep,or rapid, as in landslides, mudflows, rock slides, rockfalls, and avalanches. Mass movements are affectedby the weight of the material involved, its level of

Surface Processes on Earth

For a preview of Earth’s surface processes, study this GeoDigest before you read the chapters.After you have studied the chapters, you can use the GeoDigest to review the unit.

Yosemite National Park, California

saturation, its resistance to sliding, and sometimes,a trigger such as an earthquake. Mass movementscan cause great damage and loss of life.

Wind Limited precipitation and scarce vegetation,conditions common to arid, semi-arid, and seashoreenvironments, contribute to wind erosion. Wind-carried sediment causes abrasive action whichwears down or polishes the sides of rocks that facethe wind. Wind-formed Earth features include defla-tion blowouts, desert pavement, and sand dunes.Dunes are classified by shape as barchan, trans-verse, longitudinal, or parabolic. Wind-depositedsoils called loess contain minerals and nutrients andare highly fertile.

Glaciers Large, moving masses of ice calledglaciers form near Earth’s poles and high in moun-tains, where cold temperatures keep fallen snowfrom completely melting. Over time, the weight ofthe snow exerts enough downward pressure tocause the accumulated snow to recrystallize intoice. Glacial features include U-shaped valleys, hang-ing valleys, and waterfalls in the mountains;moraines, drumlins, and kettles in outwash plains;and a variety of glacially formed lakes. Valley glaciers

form in mountains and move downslope. Valleyglaciers are much smaller than continental glaciers,which form over broad regions and spread out fromtheir centers.

Surface Water Many landscape features on Earth are producedand changed by surface water. The amount of waterin the ground depends on the number and sizes ofpores in a particular Earth material and the amountof vegetation. A watershed or drainage basin is theland area drained by a stream system. Divides,which are raised areas of land, separate water-sheds. All of the material carried by the stream,including material in solution, in suspension, and asbed load, is called the stream’s load. Throughouthistory, humans built communities near watersources for survival and economic reasons.However, this practice has left humans vulnerableto dangerous floods. Weather and stream monitor-ing provides warnings of flooding.

Stream Development At a stream’s source,or headwaters, water from precipitation begins itsflow in channels confined by the stream’s banks.Mountain streams have rapidly flowing water andoften form waterfalls. When stream velocity

Surface Processeson Earth

GeoDigest 265

Tahquamenon River, Michigan

Sand overwhelms building, Namibia

decreases, the stream’s load is deposited intriangle-shaped alluvial fans or deltas. Alluvial fansform when streams flow out onto plains. Deltasform when streams enter large bodies of water.Uplifting of the land or lowering of the base levelcauses a stream to undergo rejuvenation and againbegin to cut a V-shaped valley.

Lakes and Freshwater Wetlands Lakesform when depressions on land fill with water.Some lakes are human-made. When nutrients fromfertilizers, detergents, or sewage enter a lake,eutrophication may be accelerated. The nutrientslead to an overabundance of some organisms andthen a depletion of oxygen. Wetlands such as bogs,marshes, and swamps are low areas that regularlyfill with water and support specific plants. Wetlandsfilter and clean water and are protected by law.

GroundwaterGroundwater is the largest source of freshwateravailable for human use. Groundwater is the por-tion of precipitation that infiltrates into the groundand is stored below the water table in the porespaces of rocks. Groundwater moves through per-meable layers called aquifers. Most groundwatercontains carbonic acid which attacks carbonate

rocks such as limestone which consists of calciumcarbonate. The dissolution and precipitation of cal-cium carbonate plays a role in the formation oflimestone caves. Caverns, sinkholes, karst topog-raphy, and travertine deposits are formed fromgroundwater action.

Groundwater Systems Springs, which arenatural discharges of groundwater, emerge wherethe water table intersects Earth’s surface. Wellsdrilled into the zone of saturation provide water forhumans, but pumping of these wells may causecones of depression in the water table. Artesianwells contain water under pressure from confinedaquifers. When groundwater withdrawal exceedsgroundwater recharge, it causes considerable lower-ing of the water table and ground subsidence.Pollution of aquifers may come from sewage, indus-trial waste, agricultural chemicals, and landfills.

266 UNIT 3

Surface Processes on Earth

Earth’s Eight Longest RiversNile (Africa) 6650 kmAmazon (South America) 6400 kmYangtze (Asia) 6300 kmMississippi-Missouri 5971 km

(North America)Yenisey (Asia) 5540 kmOb-Irtysh (Asia) 5410 kmParaná (South America) 4880 kmCongo (Africa) 4700 km

Vital Statistics

F O C U S O N C A R E E R S

LandscaperA landscaper makes a plan foroutdoor scenery in an area andthen follows the plan by plantingthe gardens and grounds. In addi-tion to knowing about plants, alandscaper needs to know thesoil characteristics and waterdrainage patterns in the area.Landscape architects have collegedegrees, but many others in thefield start out with a high schooldegree and then gather the training they need on the job.

Understanding Main Ideas1. What is the most powerful agent of erosion?

a. water c. glaciersb. wind d. living things

2. What material makes up horizon B in a soilprofile?a. topsoil c. subsoilb. loess d. bedrock

3. What is the underlying force of all agents oferosion?a. suspension c. magnetismb. friction d. gravity

4. Which of the following types of mass move-ment is not rapid?a. a landslide c. an avalancheb. a mudflow d. creep

5. What erosional agent causes deflationblowouts, desert pavement, and dunes?a. wind c. earthquakesb. water d. groundwater

6. Which of the following is a characteristic ofvalley glaciers?a. They form over broad regions.b. They move downslope.c. They are larger than continental glaciers.d. They completely melt each summer.

7. Which of the following occurs when a stream’s velocity decreases?a. The stream load increases.b. The stream cuts a V-shaped valley.c. Deposition occurs.d. A drainage basin is formed.

8. Where do alluvial fans mostly occur?a. near lakesb. along the bases of mountainsc. on the outside of meandersd. where streams enter the ocean

9. Of the following materials, which one ismost permeable?a. mud c. siltb. clay d. gravel

10. What is the largest source of freshwater onEarth?a. lakes c. oceansb. groundwater d. glaciers

Thinking Critically1. Describe the agents of erosion involved in

chemical weathering.2. Explain how groundwater works to form sedi-

mentary rock.3. How would you describe a soil’s texture?4. Discuss at least two factors that affect mass

movement.5. List at least two reasons why wetlands should

be protected.

GeoDigest 267

A S S E S S M E N T

Surface Processeson Earth

Himalayas

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