Ocaan
Glaciers and other ice
Ground water
Lakes-Fresh
-saline
Soll moimre
Atmosphere
Rbars
Porosity and Permeability
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 considerable amount of water. A deposit of loose sand may have a
Po-%
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 depressions. 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 transition 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 corner 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 finegrained day minerals hold water and mnke it available for
plant growth. Most plants *drownnif 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 purrnabsPod4such as sandstone. If the perched wzar table inrhr l.d
*I
Cmund wnrrr
4s
Vadose zone
The Movement of
I
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 following the slope of the water table (figure 17.3). See
box 17.1 for Darcy's Law.
Figure 17.3
The circulation of ground water in the satuMovement 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 limbeds 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
-wd
Dry well
I
AwollmuMbe~lnanaqukto~wPtw.Ma
~ 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 twithmrcr,Pnda
~,I
"
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
mnerion
h y mind s haw for wrer molecule6 (see chapter 12), prrvmts ~ p t n
&
thqdchcflowof@~il~.)~e&rhar~
~fRcarndhwseua,maybe~adpcrm&~
m ~ a f a i r l y d c p c r P d r b t c ~ mndk(figun17.5).
Flgurc 17.6 s h m the &&ilnce
an unconhd
aqa& whkh has a water & h u h it in only pvrly filled
%
Wlh
A well is a deep hole, +y
cylindrical, that i
drilled into the ground to penetrate an aquikr within
rated ZOQC (@art 17.4). U s d y wr*ter d r ~ ff
t ona inm chc
I
Flgum 17.6
'1
.;t
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 surface, 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)
:$bkiia"U W @ ! S & m f
ill
w8
h S @fld
~ W i ~ 0 t t h l ~ I O r H k A O
am hiah: WIW
: . - titit~gddhn
. ~.
nidily.(8)Ov m : m t s r t a b l s a;ld&&erd
#;somaw
a
. ha
- @
, d wells dry
. up.
.
Nkdw6~
o
ljiw
,
..
. ,
chCuru1pt6d ~
~
~
(
~
b
e
i
i
.As fihre 17.7 s h m , a will diipini Mnni w d v has to
'
o
r
~
~
p down shorter distvlce to hit &r than ;well d;$ 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 -I*M
table during rhc nnt rainy &n noro the drv'+
The addition of
h a of e a r g .
k a r to the saturated z&e is called redwee.
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 h e l i n g aE,the
~
m
.
~
~
iha m+.r t a ~ . inm s rrnm A+ bnv.-slnn
,
~rimplarddwit$a,
rheEndofarapki+ateicrnnottic
Oudvtorignifieanrhr h c s the Waw
,.Inunconfinwl aquifers, war risar in &dmv wells
the l m l of the water table. In eontinedaquifar, rho
under pressure and r
k in wells-to a level
the top of the aq&r (figure 17.6). Such a wen is
..
m#um 17.e
oakta-,S,,
414
wvrrl
v-m
L. 4
~
IIa i-8-y
~
1
."
.
.
.....
+
unumum
&--
4
cat
d i n d muihr bscawB l k M 1 W # d a n d ~ - t 0 t h 0 ~ W b y * o l
water in &welie mw a h the land surface when the
mrr
tapped In the 1800s.
G
d
W
F
m 17.10
Artesian well rpouts water abow W eurlace In South Dakota,
early I-.
H ~ w use
y of UUs aquWh6 reduced water pressure
80 much that 6
w
B
p
o
u
s
t 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 17.11). Sbinc ap&g d k h q c where thc
wzru nble intemm the land s&,
bttt they a
h ~ z uwhere
r
warv flows out from cpverns or along fr~cnuer,fpulci, or rock
(A) A k r ~ q
8 i n g Issuing fmm a m r n in limestons, Jarper
contam that come to the surfice (figure 17:12).
National Pvk, AUlerta, Canadaa( 6 )A line of oprlngs seep^ from
Climate determines the relationship bnmwn s u c ~ mflow
the gmund at the wn(rot k M * n lees permeae shale and ihr
and the water table. In rainy regions most streams are
owlying per8aWtow.Sauthern Utah.
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 a
d hrh rshu %.$day &
water table, Because of the addd gmvnri b u r , the h
e
w
of these stre~maincreases domrstnam. Whw the warn table
Ground water in its n a n d etatc~knbto be nla.tivdy
intersem the land surfaec ov+r a bmad area, ponds, l a b , and
concaminma in marcame.. &aurc it is a widely used m
swamps am fbund.
ofdrinking wpm, pollution of gmund
watw can be a very
In drier climates rivers t a d to be I+
rtnrrm; that is,
oua problck.
they arc loding water to the maturated mnc (figure 17-13),
The channels of losing streams lie above h e water table. The '
&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
water percolating into the ground b e n d a losing stream
ground water when rain or irrigation rnw leaches the p o w
may cause the water cable below the stream to rise. This
downward into the soil F M h art also a concern. Nimce,
ground-water mound remains beneath the stream even wfim
oneofrhcmo~wirWyUMCL~,ishprmzlin~cn
and in a deberr rhe nnrcst source of
htance under a dry stream bed.
8
Pollution of Ground Water
.. . .. .
1-
.q;sgl-J<,:; .. .
..
Chapter 17
KWiW
F4
Land stniace
Lostno stream
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 partially 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 groundwater 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 concentrations 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
D
C
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 DfrornUSDA-Soil Conservation Service
ponds leak, ground water can become polluted. Deep wells
may be safe for liquid waste disposal if they are deep enough,
but contamination of drinking water supplies and even surface
water has resulted in some localities from improper design of
the disposal wells.
Acid mine dminage from coal and metal mines can contaminate both surface and ground water. It is usually caused by
sulfuric acid formed by the oxidation of sulfur in pyrite and
other sulfide minerals when they are exposed to air by mining
activity. Fish and plants are often killed by the acid waters
draining from long-abandoned mines.
Radioactive waste is both an existing and a very serious
potential source of ground-water pollution. The shallow
burial of low-level solid and liquid radioactive wastes from
the nuclear power industry has caused contamination of
ground water, particularly as liquid waste containers leak
Chapter 17
into the saturated zone and as the seasonal rise a
water table at some sites periodically covers the waste
ground water. The search for a permanent disposal sit
solid, high-level radioactive waste (now stored tem
on the surface) is a major national concern for the
States. The permanent site will be deep undergrou
must be isolated from ground-water circulation
sands of years. Salt beds, shale, glassy tuffs, and
rock deep beneath the surface have all been studie
larly in arid regions where the water table is hundreds of
below the land surface. The likely site for disposal of hi
' level waste, primarily spent fuel from nuclear reactors,
Yucca Mountain, Nevada, 180 krn (1 10 miles) n
Las Vegas. The site would be deep underground in vo
tuff well above the current (or predicted future) water
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
who did not want the site, essentially chose Nevada in late
1988 by eliminating the funding for the study of all alternarive sites, but the final decision regarding the safety of Yucca
Mountain will not be made until after much additional
study. Even if the site is deemed safe, it could not open
before the year 2010. It could be delayed much later than
this: in 1992, a 5.G-magnitude aftershock of the Landers,
California, earthquake occurred 19 kilometers (12 miles)
from the proposed disposal site. The quake caused $1 million damage to a U.S. Energy Department office building
near the sire, and may indicate that the region is too seismicdly active for the site to be built here at all.
Not all ground-water pollutants form plumes within the
saturated zone as shown in figure 17.15. Garoline, which leaks
from gas station storage tanks at tens of thousands of U.S.
locations, is less dense than water, and floats upon the water
table (figure 17.16). Some liquids such as TCE are heavier
than water and sink to the bottom of the saturated zone, perhaps traveling in unpredicted directions upon the surfacc of an
impermeable layer (figure 17.16). Determining the extent and
flow direction of ground-water pollution is a lengthy process
requiring the drilling of tens, or even hundreds, of costly wells
for each pollution site.
Not a11 sources of ground-water pollution are rnanmade. Naturally occurring minerah within rock and roil may
Ground Warrr
iBd
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 elements 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.
Chapter 17
A
B
Flgun 17.1 7
Rock type and distance control possible sewage contamination of neighboring wells. (A) As little as 30 meters (100 feet) of movement
effectivelyfilter 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
and tens o f millions o f dollars to complete.
Chapter 17
Bdancing Withdrawal
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
und-water pollutlon problems caused or aggravated by
ing wells. ( A )Water table steepens near a dump, l m a r i n g
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 (beforepumping). Fresh water floats on salt water.
C beains ~ u m ~ i nthinning
a , - the freshwaterlens and
Ing salt water into the well.
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
Sinkholes%
and Karst Topography
Cam (or CIVUIU) are naturally formed underground chambers. Most caves develop when slightly acidic ground water
dissolves limestone along joints and bedding planes, opening
up cavern Wtems as calcite is carried away in solution (figure
17.20). Natural ground water is commonly slightly acidic
because of dissolved carbon dioxide (CO,) from the atmosphere or from soil gases (see chapter 12).
Geologists disagree whether limestone caves form above,
below, or at the water table. Most caves roba ably are formed by
p u n d water circulating below the water table, as shown in
figure 17.20. If the water table drops or the land is elevated
abwe the water table, the cave may begin to fill in again by calcite precipitation. Read the equation below from left to tight
for calcite solution, and from right to left for the calcite precipitation reaction (see also table 12.1).
Ground water with a high concentration of calcium (CaT*)
and bicarbonate (HCO;) ions may drip slowly from the ceiling of an air-filled can. As a water drop hangs on the ceiling of
Hz0
water
+
CO2
carbon
dioxide
CaCO,
calcite in
limestone
t
<
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
ceiling. When the water drop falls to the cave floor, the i
may cause more C O loss, and another small amount of
may precipitate on t i e cave floor. A falling water
fore, can precipitate small amounts of calcite on both
ceiling and the cave floor and each subsequent drop ad
calcite to the first deposits.
Deposits of calcite (and, rarely, other minerals) built
caves by dripping water are d c d dr)stone.
like pendants of dripstone hanging from
17.208). They are generally slender
aligned along cracks in the ceiling, which act as c
ground water. Sdogrnita are cone-shaped masses o
stone formed on cave floors, generally
rites. Splashing water precipitates calci
the cave floor, so stalagmites are usually t
Iactites above them. As a stalactite grows downward
stalagmite grows upward, they may eventually join to
column (figure 17.20B). Figure 17.21
intriguing features formed in caves.
2
b
Ca*
calcium
ion
t
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 sediment, including most of the coarse-grained material found on
cave floors, may be carried into the cave by streams, particularly when surface water drains into a cave system from openings 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
fracturesand 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 formsstalactltes, stalagmites, and columns
above the water table.
438
Chapter 17
!
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I
depressions found on land surfaces underlain by limestone (figure 17.22).They form either by the collup~rof 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 (figure 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, howeve6 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 probably 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~sMedia
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 r o d , a hard rounded
Concretlon8 that have weathered out of shale. Concretions
contain more cement than the surrounding rock and thereforeare
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
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.)
4
Hot Water Underground
-
Geodes. Concemrlc layers of amorphous slllca are ~ ~ n witn
e d wellformed 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~hotoQHal BeralNieual8 Unllmlted
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 cooling igneous rock. In the United States most hot springs are
found in the western states when they are associated with relatively recent volcanism. The hot springs and pools of Yellowstone 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 supplied 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 condensing 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. Travrrtine is a deposit of calcirr that often forms around hot springs
(figure 17.28), while dissolved ~ilicaprecipitates 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
Chapter 17
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 ~ ana,
l
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
i
I
i Figure 17.30
Geothermal power plant at The Geysers, California. Underground
steam, piped from wells to the power plant, is being discharged
from the cooling towers in the background.
: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 subsidence 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
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
steam.
Nonelectric uses of geothermal energy include space heating (in Boise, Idaho; Klamath Falls, Oregon; and Reykjavik,
the capital of Iceland), as well as paper manufacturing, ore processing, and food preparation.
Photo by M. Smith, US. Geological Survey
:
About 15 percent of the water that falls on
land percolates underground to become
%roundwater. 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
k aquifer 427
nrtesian well 429
uvc (cavern) 438
pncmion 440
mne of depression 429
confined (artesian) aquifer 428
kawdown 429
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 surface, hsingsmamr contribute to the ground
water in dry regions.
Ground water can be polluted by city
dumps, agriculture, industry, or sewage diiposal. Some pollutants can be filtered out by
passage of the water through moderately permeable geologic materials.
A pumped well causes a cone of drprrrsion that in turn can cause or aggravate
ground-water pollution. Near a coast, it can
cause saltwater inmion.
Artificial recharge can help create a balance 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, concretions, geodes, and alkali soils.
Grysm and hor springs occur in regions
of hot ground water. Geothermal energy can
be tapped ro gencrate electricity.
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
I Testing Your Kno
- -
Use the questions below to prepare for exams based on this chapter.
1. What conditions are necessary for an artesian well?
2. What distinguishes a geyser from a hot spring?Why does a
-_-__
(b) the capacity of a rock to transmit a fluid (4 the
sediment to rnvd weer (d) none of the above
17. The subsurface wne in which 111 rockopenings arc fdledd
4
limestone? For stalactites to develop in a cave?
5. What causes a perched water table?
18.
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.
19.
20.
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?
13. What happens to the water table near a pumped well?
14. How does a confined aquifer differfrom an unconfined aquifer?
4
openings (b) the capacity
of a rock to transmit a fluid (c)
.
ability of a sediment to retard warn (d) none of the a k : g
~
4. What chemical conditions arc necessary for caves to develop in
4
16. Petmeability is (a) the pcrccntagc of a rodis volume that kkk
gcy,syos, s,uyr;>
3. What is karst topography?How does it form?
abiq
21.
22.
water is called the (a) saturated wne (b) water table (c)
wne
-3
.$
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
Which rock type would m& the best aquifer? (a) sh
(b) mudstone (c) sandstone (d) d of the above
Which of the following determines how quickly gro
flows? (a) elevation (b) water pressure (c) permeabil
the above
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~
The drop in the water table around a pumped well is the
(a) drawdown (b) hydraulic head (c) porosity (d) fluid potentid
15. Pomsity is (a) the percentage of a rock's volume that is openings
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
m .
Baldwin, H. L., and C. I.
McGumness. 1963. Aprimcron
ground water. Washington, D.C.: U.S.
Geological Survey.
Bouwer, H. 1978. Groundwater hydmlogy
New York: McGraw-Hill.
444
Chapter 17
state or community uses nuclear
power, where is your local waste
stored?
3. Should all polluted ground water be
cleaned up? How much money has
been set aside by the federal
government for cleaning polluted
ground water?Who should pay for
ground-water cleanup if the company
that polluted the water no longer exists?
Davis, S. N., and R J. M. De Wiest. 1966.
Hydmgcology New York: John Wiley &
Sons.
Driscoll, F. G. 1986. Gmundwatrrand wells.
2d cd. St. Paul, Minnesotl: Johnson
Division.
Should some aquifers be
let? conruninated if t h m is
use of the water, or if future
be banned?
4. Why arc most of North America's
springs and gepcn in the western
and provinces?
Fetter, C. W. 1993. AppliedLydmgcolog)r
3d ed. New York: Maunillan Publishing
Company, Inc.
-.
1993. Contaminanthydrgcology.
New York: Maunillan Publishing Company,
Inc.
I1
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.
Groundwatercontamination 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/
web site that has a lot of links
Good
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