Ocaan Glaciers and other ice Ground water Lakes-Fresh

-saline Soll moimre Atmosphere Rbars

Porosity and Permeability Po-% the percentage of rock or sedimcnt that consists of voids or openings, is a measurement of a rodis ability to hold water. Most rocks can hold some water. Some sedimentary rocks, such as sandstone, conglomerate, and many limestones, tend to have a high porosity and therefore can hold a consider- able amount of water. A deposit of loose sand may have a

Chapter 17

The Water Table Responding to the pull of graviry, water percolates down in ihe ground through the soil and through cracks and pores the rock. Scvcral kilometers down in the crust percolati stops. With increasing depth, sedimentary rock pores tend be closed by increaing amounts of cement and by the weight of the overlying rock. Moreover, sedimentary r o d overlia igneous and metamorphic crystalline basement rock, which

Perched byater table

Fbum 17.2 Perched water tables above lenses of less permeable shale within a large body of sandstone. Downward percolatlon of water Is impeded by the less permeable shale.

The subsurface zone in which all rock openings are filled with water is called the uturated zonc (figure 17.1A). If a well were drilled downward into this zone, ground water would fill the lower part of rhe well. The water level inside the well marks the upper surface of the saturated zone: this surface is the water table.

Most riven and lakes intersect the saturated zone. Rivers and lakw occupy low places on the land surface, and ground water Bows out of the saturated zone into these surface depres- sions. The water level at the surface of most lakes and rivers coincides with the water table. Ground water also flows into mines and quarries cut below the water table ( f i r e 17.18).

Above the water table there is a zone that is g c n e d y unsaturated and is referred to w the d a r zone (figure 17.1A). Within the vadose zone, capillary action causes watcr to be held a b m the water table. The capilhyfingc is a transi- tion zone with higher moisture wntent at the blsc of the vadose zone just above the water table. Some of the water in the capillary fringe has been drawn or wi&d upward from the water table (much like water rising up a paper towel if the cor- ner is dipped in water), whereas most of the capillary fringe water is due to fluctuations in the level of the water table. The capillary fringe is generally iws than a meter hi&, but may be much thidkct in fine-grained sediments and thinner in -- grained sediments such as sand and gravel.

Plant roots generally obtain their water from the bdt of soil moisture near the top of the Mdose zone, where fine- grained day minerals hold water and mnke it available for plant growth. Most plants *drownn if their roots are covered by water in the saturated zone: plants need both water and air in soil pores to su:.vivc. (The water-loving plants of swamps and marshes are an exception.)

A puchsd rsacs &Ie is the top of a body of ground mrer separated from the main water table beneath it by a zone that L not satunted (figure 17.2). It may form as ground watcr & above alms of less p c d e shale within a more purrnabs Pod4- such as sandstone. If the perched wzar table in- rhr l.d

* I

Cmund wnrrr 4 s

Vadose zone I The Movement of

Ground Water Cornparcd to the rapid flow of water in surface streams, most ground water rnovcs relatively slowly through rock underground. Because it moves in response to differences in water pressure and elevation, water within the upper part of the saturated zone tends to move downward follow- ing the slope of the water table (figure 17.3). See

Figure 17.3 box 17.1 for Darcy's Law.

The circulation of ground water in the satu- Movement of ground water beneath a sloping water table in unlformly permeable rated zone is not confined to a shallow layer rock. Near the surface the ground water tends to flow parallel to the sloping water bcncath the water table. ~~~~~d .., , rauie. move hundreds of fcct vcrtically downwatd surface, a line of springs an form along the upper contact of the before rising again to discharge as a spring or to seep into the shale lens. The water perched above a shale lens an provide a lim- beds of rivcrs and lakes at the surface (figure 17.3) due to the i t 4 water supply to a well; it is an unrdiablc long-term supply combined effects of gravity and the slopc of thc water table.

426 Chapter I7

n, so F haia higher head than G , and water moves to G. Note that underground water may move

. ...

awd well Dry well

- w d

I

A w o l l m u M b e ~ l n a n a q u k t o ~ w P t w . M a ~ p r t a f t h e h @ h l y ~ ~ k m r q u H r , b r i t ~ W ~ m e l l b l s h b h n o l . A ~ t h . r h d s l ~ w t ~ , I withmrcr,Pnda W r# m d l l y hmmn waftn: "

&ve a high roliy of 30%. Yet che emWcly srnd 8 k ofthe porn# & with the -tic mner ion h y min- d s haw for wrer molecule6 (see chapter 12), prrvmts ~ p t n

t h q d c h c f l o w o f @ ~ i l ~ . ) ~ e & r h a r ~ ~ f R c a r n d h w s e u a , m a y b e ~ a d p c r m & ~ Wlh m ~ a f a i r l y d c p c r P d r b t c ~ mndk(figun17.5). A well is a deep hole, +y cylindrical, that i % an unconhd drilled into the ground to penetrate an aquikr within Flgurc 17.6 s h m the &&ilnce aqa& whkh has a water & h u h it in only pvrly filled rated ZOQC (@art 17.4). U s d y wr*ter d r ~ t ff ona inm chc

I Flgum 17.6 .;t '1 An unconfined aquifer is exposed to the sulface and Is only partly filled with water; water In a shallow well will rise to the level of the water table. A conflned aquifer is separated from the surface by a conflnlng bed, and is completely fllled wlth water under pressure; water In wells rlros above the aqulfer. Flow jlnes show direction of ground-water flow. Days, years, decades, centuries, and millennia refer to the time required for ground water to flow from the recharge area to the discharge area. Water enters aquifers in recharge areas, and flows out of aquifers in discharge areas.

called an artesian well (and confined aquifers are also called arrerian aquif;r).

In some artesian wells the water rises above the land sur- face, producing a flowing well that spouts continuously into the air unless it is capped (figure 17.9). Flowing wells used to occur in South Dakota, when the extensive Dakota Sandstone aquifer was first tapped (figure 17.10), but continued use has lowered the water pressure surface below the ground surface in most parts of the state. Water still rises above the aquifer, but does not reach the land surface.

Well (not pumped)

. - . ~.

Nkdw6~ijlow nidily. (8) Ov m : m t s r t a b l s a;ld&&erd #; somawha@ ad wells dry up.

:$bki ia" U W @ ! S & m f ill w 8 h S @fld ~ W i ~ 0 t t h l ~ I O r H k A O : titit ~gddhn am hiah: WIW

. - , . . , ..

. ,

chCuru1pt6d ~ ~ ~ ( ~ b e i i ' o r ~ ~ ~ m . ~ ~ . As f ihre 17.7 s h m , a will di ip ini Mnni w d v has to

table during rhc nnt rainy &n nor- o the drv'+ The addition of

water is pumped from a well, the wa& table the well into a dcprer-

€one k n m @# a a n e of. , ., I d he l ing aE,the

~ r i m p l a r d d w i t $ a , rheEndofarapki+ateicrnnottic Oudvtorignifieanrhr h c s the Waw

. .

4 1 4 w v r r l v-m II a i-8-y unumum cat

B l k M 1 W # d a n d ~ - t 0 t h 0 ~ W b y * o l under pressure and rk in wells-to a level water in & welie mw a h the land surface when the mrr

the top of the aq&r (figure 17.6). Such a wen is tapped In the 1800s.

G d W

p down shorter distvlce to hit &r than ; well d;$ on a rilltop. During dry seasons the water able fall, aa water aajrs # kt of the s a d urnc into rpringp and h. Wctls not Flmm 17.8 k p enough to inpersect the lawered mter w dry, but ~ I ~ I M I-* iha m+.r t a ~ . inm s rrnm A+ bnv.-slnn

k a r to the saturated z&e is called redwee. h a of e a r g .

m#um 17.e ,.In unconfinwl aquifers, war risar in &dmv wells oakta S,,,- L. 4 ~ ~ 1 . . ." ..... + &-- 4

the l m l of the water table. In eontined aquifar, rho d i n d muihr bscaw

F m 17.10 Artesian well rpouts water abow W eurlace In South Dakota, early I-. H ~ w y use of UUs aquWh6 reduced water pressure 80 much that 6wBpouts 60 not occur m. Photo by N. H. Danofl, U 9. OWbglCd $ U N ~ Y

Springs and Streams - A bring is a p k a whm worn &ow# Lwadky from rock onto the land sutface (6gure 17.1 1). Sbinc ap&g d k h q c where thc wzru nble intemm the land s&, bttt they ah ~ z u r where warv flows out from cpverns or along fr~cnuer, fpulci, or rock contam that come to the surfice (figure 17:12).

Climate determines the relationship bnmwn s u c ~ m flow and the water table. In rainy regions most streams are mmmj that is, they receive water from the saturated zone (figure 17.13). The surface of these srrroms urincidcs with the water table. Water from the sotunad zone flowqinro the stream through the stream bed ad hrh rshu %.$day & water table, Because of the addd gmvnri b u r , the hew of these stre~ma increases domrstnam. Whw the warn table intersem the land surfaec ov+r a bmad area, ponds, l ab , and swamps am fbund.

In drier climates rivers t a d to be I+ rtnrrm; that is, they arc loding water to the maturated mnc (figure 17-13), The channels of losing streams lie above h e water table. The water percolating into the ground b e n d a losing stream may cause the water cable below the stream to rise. This ground-water mound remains beneath the stream even wfim

.. . . .. . and in a deberr rhe nnrcst source of htance under a dry stream bed.

.q;sgl-J<,:; . . . ..

(A) A k r ~ 8 q i n g Issuing fmm a m r n in limestons, Jarper 8

National Pvk, AUlerta, Canadaa (6) A line of oprlngs seep^ from the gmund at the wn(rot k M * n lees permeae shale and ihr owlying per- 8aWtow. Sauthern Utah.

Pollution of Ground Water Ground water in its nand etatc~knb to be nla.tivdy concaminma in marcame.. &aurc it is a widely used m ofdrinking wpm, pollution of gmund watw can be a very - oua problck.

&I& and hnbiri&I h c b as DDT and 2,4-D) av ' to @&d crops (figure 17.144) can find their mi-intd ground water when rain or irrigation rnw leaches the p o w 1 downward into the soil F M h art also a concern. Nimce, - o n e o f r h c m o ~ w i r W y U M C L ~ , i s h p r m z l i n ~ c n d "

Chapter 17

Land stniace KWiWS Lostno stream F 4

FIsum 17.1 3 Qaining and losing streams. (A) Stream gaining water from saturated zone. (B) Stream losing water through stream bed to saturated zone. (C) Water table can be close to the land surface beneath a dry stream bed.

Rain can also leach pollutants from city dumps into ground-water supplies (figure 17.14B). Consider for a moment some of the things you threw away last year. A par- tially empty aerosol can of ant poison? The can will rust through in the dump, releasing the poison into the ground and into the saturated zone below. A broken thermometer? The toxic mercury may eventually find its way to the ground-water supply. A half-used can of oven cleaner? The dried-out remains of a can of lead-base paint? Heavy metalr such as mercury, lead, chromium, copper, and cadmium, together with household chemicals and poisons, can all be concentrated in ground- water supplies beneath dumps (figure 17.15).

Liquid and solid wastes from septic tanks, sewage plants, and animal feedlots and slaughterhouses may contain bacteria, vimes, andparasites that can contaminate ground water (figure 17.14C). Liquid wastes from industries (figure 17.140) and military bases can be highly toxic, containing high concentra- tions of heavy metals and compounds such as cyanide and PCBs (polychlorinated biphenyls), which are widely used in industry. A degreaser called TCE (trichloroethylene) has been increasingly found to pollute both surface and underground water in numerous regions. Toxic liquid wastes are often held in surface ponds or pumped down deep disposal wells. If the

Ground Waw a

Chapter 17

C D

Figure 17.14 Some sources of ground-water pollution. ( A ) Pesticides. (8) Household garbage. (C) Animal waste. ( D ) Industrial toxic waste. Photo A by Michael Stimrnann; photo B by Frank M. Hanna; photos C and Dfrorn USDA-Soil Conservation Service

ponds leak, ground water can become polluted. Deep wells into the saturated zone and as the seasonal rise a may be safe for liquid waste disposal if they are deep enough, water table at some sites periodically covers the waste but contamination of drinking water supplies and even surface ground water. The search for a permanent disposal sit water has resulted in some localities from improper design of solid, high-level radioactive waste (now stored tem the disposal wells. on the surface) is a major national concern for the

Acid mine dminage from coal and metal mines can con- States. The permanent site will be deep undergrou taminate both surface and ground water. It is usually caused by must be isolated from ground-water circulation sulfuric acid formed by the oxidation of sulfur in pyrite and sands of years. Salt beds, shale, glassy tuffs, and other sulfide minerals when they are exposed to air by mining rock deep beneath the surface have all been studie activity. Fish and plants are often killed by the acid waters larly in arid regions where the water table is hundreds of draining from long-abandoned mines. below the land surface. The likely site for disposal of hi

Radioactive waste is both an existing and a very serious ' level waste, primarily spent fuel from nuclear reactors, potential source of ground-water pollution. The shallow Yucca Mountain, Nevada, 180 krn (1 10 miles) n burial of low-level solid and liquid radioactive wastes from Las Vegas. The site would be deep underground in vo the nuclear power industry has caused contamination of tuff well above the current (or predicted future) water ground water, particularly as liquid waste containers leak and in a region of very low rainfall. The U.S

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Dump waste piled on the land surface creates a ground-water mound beneath it because the dump forms a hill, and because the waste material is more porous and permeable than the surrounding soil and rock. Rain leaches pollutants into the saturated zone. A plume of polluted water will spread out in the dlrectlon of ground-water flow.

under intense political pressure from other candidate states Not all ground-water pollutants form plumes within the who did not want the site, essentially chose Nevada in late saturated zone as shown in figure 17.15. Garoline, which leaks 1988 by eliminating the funding for the study of all alterna- from gas station storage tanks at tens of thousands of U.S. rive sites, but the final decision regarding the safety of Yucca locations, is less dense than water, and floats upon the water Mountain will not be made until after much additional table (figure 17.16). Some liquids such as TCE are heavier study. Even if the site is deemed safe, it could not open than water and sink to the bottom of the saturated zone, per- before the year 2010. It could be delayed much later than haps traveling in unpredicted directions upon the surfacc of an this: in 1992, a 5.G-magnitude aftershock of the Landers, impermeable layer (figure 17.16). Determining the extent and California, earthquake occurred 19 kilometers (12 miles) flow direction of ground-water pollution is a lengthy process from the proposed disposal site. The quake caused $1 mil- requiring the drilling of tens, or even hundreds, of costly wells lion damage to a U.S. Energy Department office building for each pollution site. near the sire, and may indicate that the region is too seismi- Not a11 sources of ground-water pollution are rnan- cdly active for the site to be built here at all. made. Naturally occurring minerah within rock and roil may

Ground Warrr iBd

Chapter 17

contain elements such as arsenic, selenium, mercury, and other toxic metals. Circulating ground water can leach these elements out of the minerals and raise their concentrations to harmful levels within the water. Not all spring water is safe to drink. Like a "bad watcrhole" depicted in a Western movie, some springs contain such high levels of toxic ele- ments that the water can sicken or kill humans and animals

.Figurn 17.16 Not all pollutants move within the saturated zone as shown in figure 17.15. Gasoline floats on water; many dense chemicals move along impermeable rock surfaces below the saturated zone.

A

B

Flgun 17.1 7 Rock type and distance control possible sewage contamination of neighboring wells. (A) As little as 30 meters (1 00 feet) of movement effectively filter human sewage in sandstone and some other rocks and sediments. (6) If the rock has large open fractures, contamina can occur many hundreds of meters away.

water, the clean-up process for a large region can take decades Bdancing Withdrawal and tens o f millions o f dollars to complete.

Chapter 17

und-water pollutlon problems caused or aggravated by ing wells. ( A ) Water table steepens near a dump, lmar ing locity of ground-water How and drawing pollutants into a

er-table slope Is reversed by pumping, changing f the ground-water flow, and polluting the well. (C) Well st (before pumping). Fresh water floats on salt water. C beains ~ u m ~ i n a , thinning the freshwater lens and

Ing salt water into the well. -

I

Flgum 17.19 Subsidence of the land surface caused by the extraction of ground water, near Mendota, San Joaquin Valley, California. Signs on the pole indicate the positions of the land surface in 1925, 1955, and 1977. The land sank 9 meters (30 feet) in 52 years. Photo by Richard 0. Ireland, U.S. Geological Survey

pipelines. Overpumping of ground water also causes corn. paction and porosity loss in rock and soil, and can perma. nently ruin good aquifers.

To avoid the problems of falling water tables, subsidence and compaction, many towns use amycial recharge to incrcasc the natural rate of recharge. Natural floodwaters or treated industrial or domestic wastewaters are stored in infiltration ponds on the surface to increase the rate of water percolation into the ground. Reclaimed, clean water from sewage tmt . ment plants is commonly used for this purpose. In some cases especially in areas where ground water is under confined con. ditions, water is actively pumped down into the ground tc replenish the ground-water supply This is more cxpensivc than filling surface ponds, but it rcduas the amount of watel lost through evaporation.

Ground Water

Effects of Ground-Water Action the cave, some of the dissolved carbon dioxide ( ~ 2 0 ~ ) lost into the cave's atmosphere. The CO, loss causes amount of calcite to precipitate out of the water onto Sinkholes% and Karst Topography ceiling. When the water drop falls to the cave floor, the i

Cam (or CIVUIU) are naturally formed underground cham- may cause more C O loss, and another small amount of bers. Most caves develop when slightly acidic ground water may precipitate on t i e cave floor. A falling water dissolves limestone along joints and bedding planes, opening fore, can precipitate small amounts of calcite on both up cavern Wtems as calcite is carried away in solution (figure ceiling and the cave floor and each subsequent drop ad 17.20). Natural ground water is commonly slightly acidic calcite to the first deposits. because of dissolved carbon dioxide (CO,) from the atmo- Deposits of calcite (and, rarely, other minerals) built sphere or from soil gases (see chapter 12). caves by dripping water are d c d dr)stone.

Geologists disagree whether limestone caves form above, like pendants of dripstone hanging from below, or at the water table. Most caves roba ably are formed by 17.208). They are generally slender p u n d water circulating below the water table, as shown in aligned along cracks in the ceiling, which act as c figure 17.20. If the water table drops or the land is elevated ground water. Sdogrnita are cone-shaped masses o abwe the water table, the cave may begin to fill in again by cal- stone formed on cave floors, generally cite precipitation. Read the equation below from left to tight rites. Splashing water precipitates calci for calcite solution, and from right to left for the calcite precip- the cave floor, so stalagmites are usually t itation reaction (see also table 12.1). Iactites above them. As a stalactite grows downward

Ground water with a high concentration of calcium (CaT*) stalagmite grows upward, they may eventually join to and bicarbonate (HCO;) ions may drip slowly from the ceil- column (figure 17.20B). Figure 17.21 ing of an air-filled can. As a water drop hangs on the ceiling of intriguing features formed in caves.

H z 0 + CO2 t CaCO, Ca* t water carbon calcite in 2 calcium

dioxide limestone ion b

development of caves (solution) <

development of flowstone and dripstone (precipitation)

A In parts of some caves, water flows in a thin film over the cave surfaces rather than dripping from the ceiling. Sheetlike or ribbonlike fiwstone deposits develop from calcite that ir precipitated by flowing water on cavc walls and floors.

The floors of most caves are covered ofwhich is wsidualchj the fine-grained particles left behind as insoluble residue when a limestone containing day dissolves. (Some limestone contains only about 50% calcite.) Other sed- iment, including most of the coarse-grained material found on cave floors, may be carried into the cave by streams, particu- larly when surface water drains into a cave system from open- ings on the land surface.

Solution of limestone undergroun tures that are visible on the surface. Ext can undermine a region so that roofs collapse and form depressions in the land surface above. Sinkholes are closed

Flgure 17.20 Solution of llmestone to form caves. ( A ) Water moves along fractures and bedding planes in limestone, dissolving the limestone to form caves below the water table. (6) Falling water table allows cave system, now greatly enlarged, to fill with air. Calcite preclpltatlon forms stalactltes, stalagmites, and columns above the water table.

! 438 Chapter 17 hrtp://wwwmhhr.com/canh~i/gm/ogy/p~rnmcr I

depressions found on land surfaces underlain by limestone (fig- ure 17.22). They form either by the collup~r of a cave roof or by solution as descending water enlarges a crack in limestone. Limestone regions in Florida, Missouri, Indiana, and Kentucky are heavily dotted with sinkholes. Sinkholes can also form in regions underlain by gypsum or rock salt, which are also soluble in water.

An area with many sinkholes and with cave systems beneath the land surface is said to have karat topography (fig- ure 17.23). Karst areas are characterized by a lack of surface streams, although one major river may flow at a level lower than the karst area.

Streams sometimes disappear down sinkholes to flow through caves beneath the surface. In this specialized instance, a true undrrground rtrcum exists. Such streams arc quite rare, how- eve6 as most ground water flows very slowly through pores and cracks in sediment or rodt. You may hear people with wells describe the "underground stream" that their well penetrates, but this is almost never the w e . Wells tap ground water in the rock pores and crevices, not underground streams. If a well did tap a true underground river in a karst region, the water would proba- bly be too polluted to drink, especially if it hiid washed down from the surface into a cavern without being filtered through soil and rock.

[ Figure 17.21

Stalactites, stalagmites, and flowstone in Great Onyx Cave, Kentucky.

Photo courtesy Stanley Fagerlin

, Kentucky, (8) A collapse sinkhole that formed suddenly in Winter

Gmund Water

Rgum 17.-

Petrified log in the Painted Desert, Arizona. Smaii amounts of iron and other elements color the silica in the log. Photo @ Eric & David HoskinglCorb~s Media

Other ERects Ground water is important in the preservation offisih such a9 p d e d woad, that develops when porous buried wood is either filled in or replaced by inotganic silica carried in by ground water (figure 17.24). The result is a hard, permanent rock, commonly preserving thc growth rings and other details of the wood. Calcite or silica carried by ground water can dso replace the original material in marine shells and animal bones.

Sedimentary mdc ccmmt, usually silica or calcite, is carried into place by pound water. When a considerable amount of cementing material precipitates locally in a rod , a hard rounded

Concretlon8 that have weathered out of shale. Concretions contain more cement than the surrounding rock and therefore are very resistant to weathering.

mas called a concretion develops, typically around an or+ nucleus such as a leaf, tooth, or other fossil (figure 17.25).

Geodu are partly hollow, globe-shaped bodies found 4 some limcstoncs and locally in other rodts. The outer shell L; amorphous silica, and well-formed ctystals of quartz, dcite, ot other m i n e d project in& coward a central cavity (figurr 17.26). The origin of geodes is complex but clearly related m )ground water. Crystals in geodes may have filled original caviria or have replaced fossils or other crystals.

In arid and semiarid climates, alkali soil may develop because of the precipitation of great quantities of sodium salts by evaporating .ground water. Such soil is unfit for plant growth. Alkal'i soil generally forms at the ground surface in low-lying areas. (See chapter 12.)

Geodes. Concemrlc layers of amorphous slllca are ~ ~ n e d witn well- formed quartz crystals growlng inward toward a central oavily. (Scale Is In oentimeters.)

, Clgum 17.27 -Eruptive history of a typical geyser In A through D. Photo shows the eruption of Old Faithful geyser in Yellowstone Natlonal Park, Wyoming. See text for explanatlon.

j~hoto QHal BeralNieual8 Unllmlted

Hot Water Underground - Hot aphga are springs in which the water is warmer drM human body temperature. Water can gain heat in two ways while it is underground. First, and more commonly, ground water may circulate near a magma chamber or a body of cool- ing igneous rock. In the United States most hot springs are found in the western states when they are associated with rela- tively recent volcanism. The hot springs and pools of Yellow- stone National Park in Wyoming arc of this type.

Ground water can also gain heat if it circulates unusually deeply in the earth, perhaps along joints or fiults. As discussed in chapter 11, the normal geothermal gradient (the increase in temperature with depth) is 25"Clkilometer (about 75"Flmile). Water circulating to a depth of 2 or 3 kilometers is warmed substantially above normal surface water temperature. The famous springs at Warm Springs, Georgia, have been warmed by deep circulation. Warm water, regardless of its origin, is lighter than cold water and readily rises to the surface.

A geyaw is a type of hot spring that periodically erupts hot water and steam. The water is generally near boiling (100°C). Eruptions may be caused by a constriction in the underground "plumbing" of a geyser, which prevencs the water from rising and cooling. The events thought to lead to a geyser eruption are illustrated in figure 17.27. Water gradually

Water Water and ble layers

Water

condu~t I Ver, hot water

Figun 17.PB Precipitation of calclte In the form of travertine terraces around a hot sprlng (Mammoth Hot Sprlngs, Yellowstone National Park). Algae llvlng In the hot water provide the color. Photo by Dlane Carlson

seeps into a panidly emptied geyser chamber and heat sup- plied from below slowly warms the water. Bubbles of water vapor and other gases then begin to form as the temperature of the water rises. The bubbles may clog the constricted part of the chamber until the upward pressure of rhe bubbles pushes out some of the water above in a gentle surge, thus lowering the pressure on the water in the lower part of the chamber. This drop in pressure causes the chamber water, now very hot, to flash into vapor. The expanding vapor blasts upward out of the chamber, driving hot water with it and con- densing into visible steam. The chamber, now nearly empty, begins to fill again and the cyde is repeated. The entire cycle may be quite regular, as it is in Yellowstone's Old Faithful geyser, which averages about 65 minutes between eruptions (though it varies from about 30 to 95 minutes). Many geysers, however, erupt irregularly, some with weeks or months between eruptions.

As hot ground water comes to the surface and cools, it may precipitate some of its dissolved ions as minerals. Travrr- tine is a deposit of calcirr that often forms around hot springs (figure 17.28), while dissolved ~ilica precipitates as sinter (called geys&tc when deposited by a geyser, as shown in figure 17.29). The composition of the subsurfice rocks generally determines which type of deposit forms, although sinter can indicate higher subsurface temperatures than travertine because silica is harder to dissolve than calcite. Both deposits can be stained by the pigments of algae living in the hot water. The algae can be used to estimate water temperature because their color changes from green to brown to orange to yellow as '

the temperature rises. A mudpoc is a special type of hot spring that contains

thick, boiling mud. Mudpots are usually marked by a small amount of water and strongly sulfurous gases, which combine

Clgum 17.PO Qeyserlte dbpoalts amund the vent of Castle Qeyser, Yellow National Park.

to form strongly acidic solutions. The mud probably ftom intense chemical weathering of the surrounding ro these strong acids (see figure 12.16).

Geothermal Energy Electriciry can be generated by harnessing naturally occur steam and hot water in areas that are exceptionally hot un ground. In such a g c o t h ~ l ana, wells can cap stea superheated water that can be turned into steam) that is piped to a powerhouse where it turns a turbine that sp generator, creating electricity

Geothermal energy production requires no burnin fuel, so the carbon dioxide emissions of power plants burn coal, oil, or natural gas are eliminated (as are nuclear waste and dangers of nuclear power p Although geothermal energy is relatively dean, it has s o d

Chapter 17

:nvironmentd problems. Workets nee toxic hydrogen sulfide gas in the steam, a commonly contains dissolved ions and metals, such as lead and mercury, that can kill fish and plants if discharged on the surface. Geothermal fluids are often highly corrosive to equipment, and their extraction can cause land subsidence. Pumping wastewater underground can help reduce subsi- dence problems.

Geothermal fields can be depleted. The largest field in the world is at The Geysers in California (figure 17.30), 120 kilometers (80 miles) north of San Francisco. The Geysers

i field increased its capacity in recent years to 2,000 megawatts of electricity (enough for 2 million people), but production has declined, and the field may soon run out of

I steam. i Figure 17.30 Nonelectric uses of geothermal energy include space heat-

Geothermal power plant at The Geysers, California. Underground ing (in Boise, Idaho; Klamath Falls, Oregon; and Reykjavik, steam, piped from wells to the power plant, is being discharged the capital of Iceland), as well as paper manufacturing, ore pro- from the cooling towers in the background. cessing, and food preparation. Photo by M. Smith, US. Geological Survey

About 15 percent of the water that falls on land percolates underground to become %round water. Ground water fills pores and joints in rock, creating a large reservoir of usable watcr in most regions.

Pomw rocks can hold watcr. Penneablr rocks permit water to movc through them.

The water table is the top surface of the : saturated zone and is overlain by the uadw

zone. Local variations in rock permeability

may develop apmhed watcr tab& above the main water table.

Ground-water velocity depends on rock permeability and the slope of the water table.

An aquz* is porous and permeable and

can supply water to wells. A confined aquifrr holds water under pressure, which can create artesian wellr.

Gainingstreams, springs, and lakes form where the water table intersects the land sur- face, hsingsmamr contribute to the ground water in dry regions.

Ground water can be polluted by city dumps, agriculture, industry, or sewage dii- posal. Some pollutants can be filtered out by passage of the water through moderately per- meable geologic materials.

A pumped well causes a cone of drprrr- sion that in turn can cause or aggravate ground-water pollution. Near a coast, it can cause saltwater inmion.

Artificial recharge can help create a bal- ance between withdrawal and recharge of ground-water supplies, and help prevent subsidence.

Solution of limestone by ground water forms caws, sinkholes, and kant rep& Calcite precipitating out of ground water forms stahctitcs and stahgmitcs in caves.

Precipitation of material out of solution by ground water helps form petrified wood, other fossils, sedimentary rock cement, con- cretions, geodes, and alkali soils.

Grysm and hor springs occur in regions of hot ground water. Geothermal energy can be tapped ro gencrate electricity.

k aquifer 427 nrtesian well 429 uvc (cavern) 438 pncmion 440 mne of depression 429 confined (artesian) aquifer 428 kawdown 429

ground water 424 hot spring 44 1 karst topography 439 losing stream 430 perched water table 425 permeability 424 petrified wood 440 porosity 424 recharge 429 saturated zone 425

sinkhole 438 spring 430 stalactite 438 stalagmite 438 unconfined aquifer 428 vadosc zone 425 water table 425 well 428

Ground Water

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Use the questions below to prepare for exams based on this chapter. (b) the capacity of a rock to transmit a fluid (4 the a b i q 1. What conditions are necessary for an artesian well? sediment to rnvd weer (d) none of the above 4 2. What distinguishes a geyser from a hot spring? Why does a 16. Petmeability is (a) the pcrccntagc of a rodis volume that kkk

- _-__ > openings (b) the capacity of a rock to transmit a fluid (c) 4 gcy,syos, s,uyr; . ~

ability of a sediment to retard warn (d) none of the a k : g 3. What is karst topography? How does it form?

17. The subsurface wne in which 111 rockopenings arc fdledd 4. What chemical conditions arc necessary for caves to develop in water is called the (a) saturated wne (b) water table (c) 4

limestone? For stalactites to develop in a cave? wne -3 .$

I Testing Your Kno

5. What causes a perched water table?

6. Describe several ways in which ground water can become polluted.

7. Discuss the difirence between porosity and permeability

8. What is the water table? Is it h e d in position?

9. Sketch four different origins for springs.

10. What controls the velocity of ground-water flow?

11. Name several geologic materials that make good aquifers. D e h e aqw$?r.

12. How does petrified wood form?

18. An aquifer is (a) a body of sammed rock or sedimen which water can move easily (b) a body of rock that flow of ground water (c) a body of rock that is impe

19. Which rock type would m& the best aquifer? (a) sh (b) mudstone (c) sandstone (d) d of the above

20. Which of the following determines how quickly gro flows? (a) elevation (b) water pressure (c) permeabil the above

21. Ground water flows (a) always downhill (b) from areas of high hydraulic head to low hydraulic head (c) from high elm% to low elmxion (d) from high permeability to low petmeab'ili~

13. What happens to the water table near a pumped well? 22. The drop in the water table around a pumped well is the

14. How does a confined aquifer differ from an unconfined aquifer? (a) drawdown (b) hydraulic head (c) porosity (d) fluid potentid I 15. Pomsity is (a) the percentage of a rock's volume that is openings 1

1. Describe any difference between the amounts of water that would percolate downward to the saturated zone beneath a flat meadow in northern New York and beneath a rocky hillside in southern Nwada. Discuss the factors that control the amount of percolation in each case.

2. Where should high-level nuclear waste from power plants be stored? If your

state or community uses nuclear Should some aquifers be power, where is your local waste let? conruninated if t h m is stored? use of the water, or if future

3. Should all polluted ground water be be banned? cleaned up? How much money has 4. Why arc most of North America's been set aside by the federal springs and gepcn in the western government for cleaning polluted and provinces? ground water? Who should pay for ground-water cleanup if the company that polluted the water no longer exists?

Baldwin, H. L., and C. I. m . Davis, S. N., and R J. M. De Wiest. 1966. Fetter, C. W. 1993. AppliedLydmgcolog)r McGumness. 1963. Aprimcron Hydmgcology New York: John Wiley & 3d ed. New York: Maunillan Publishing

ground water. Washington, D.C.: U.S. Sons. Company, Inc. Geological Survey. Driscoll, F. G. 1986. Gmundwatrrand wells. -. 1993. Contaminanthydrgcology. Bouwer, H. 1978. Groundwater hydmlogy 2d cd. St. Paul, Minnesotl: Johnson New York: Maunillan Publishing Company, New York: McGraw-Hill. Division. Inc.

444 Chapter 17

Washington, D.C.: U.S. logical Survey Water-Supply Paper

Id, L. B. 1974. Watm:Aprimn: San uaw: W. H. Frceman & Co.

rc, G. W., and G. Nicholas. 1964. logy: Thc I+ ofcavn. Boston: D. C.

Palmer, A. N. 1991. Origin and morphology of limcrtonc cavcr. Geological Society of America Bulletin, v. 103, pp. 1-21. Palmer, C. M. 1992. Pn'nciphof eonminant hydrogeology Chelsea, Michigan: Lewis Publishers, Inc. Pyc, V. I., R. Patrick, and J. Quarlcs. 1983. Groundwater contamination in the United States Philadelphia: Univ. of Pennsylvania. Rimer, D. F., R C. Kochel, and J. R Miller. 1995. Roce~~geomorphology. 3d cd. Dubuque, Iowa: Wm. C. Brown Publishers. Swenson, H. A,, and H. L. Baldwin. 1915. Aprimer on water q1*11i@ Washington, D.C.: U.S. Geological Survey. Todd, D. K. 1980. &und water hydrology. 2d cd. New York: John Wiley & Sons. Wallcr, R M. 1988. &undwamand rhc r u d homeowner. Washington, D.C.: U.S. Geological Survey Genera Interat Publication. Walthun,T. 1975. Caves. New York: Cmwn Publishers.

. . .

e http:lltoxics.usgs.gwItoxicsl Various sites and informatio dean up of toxics in surface and gro hnp:llwater.usgs.govI~ 'widlh

bioremed.html Information about using biorcmediation clean up toxiu in the soil, surfice, and ground water.

http:llwater.wr.usgs.gov/gwadaslindar.html Ground Water Adas for the United Snrcs. Good general information about aquifers.

http:llwater.usgs.gov/ Good web site that has a lot of links to water topics in the United States from the USGS.

http:llwww.caves.org! Home pagc of the National Speieo&cal Sociery wntains links to web pages of local interest and acwa to the NSS boobtore.

Ground Water