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geology-What is the definition of groundwate

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Subject: Geology
Topic: General Geology
Due on: 10/28/2015
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Groundwater 213

12 Groundwater

A. What is the definition of groundwater? Why is the composition of geyser deposits variable
within Yellowstone National Park? How does the total amount of groundwater on Earth
compare with that of surface water?
B. How do the dynamics of groundwater in humid regions differ from those in arid regions?
C. What are two common problems associated with the water table described here? What is
hydraulic gradient, and how does its measurement differ from the way in which geologists
measure slope? What is hydraulic conductivity? What is Darcy’s Law, and how does it
apply in computing the rate at which groundwater flows through a saturated zone?
D. How can one map a water table from well data? How can one use such a map to decipher
the direction and rate of flow of contaminants within the saturated zone?
E. What is a potentiometric surface, and what does it have to do with artesian conditions?
What does a potentiometric surface have to do with flowing and non-flowing artesian wells?
F. What is a perched water table? What is the purpose of a reservoir liner?
G. What is karst topography, and what are its features? How are the direction and rate of flow
of groundwater in Florida measured from a study of ponds within sinkholes?

A. Groundwater defined
In its broadest definition groundwater
is all that water that occurs in otherwise
open spaces within rocks and sediments.
Groundwater that originates from the
precipitation of rain and snow—the
topic of this exercise—is called
meteoric water.

of their deposition in ancient seas; and
juvenile water, water that was born
of magmatic activity. Neither connate
water nor juvenile water is a source of

In addition to meteoric water, there
are two minor sources of subterranean
water: connate water (aka sediment
water), which is water that was
entrapped within sediments at the time


W a te r

Figure 12.1 The vast majority of groundwater is meteoric in origin and is free to
move with vagaries of climate. Rates of
groundwater flow differ with depth, ranging from days to thousands of years to
traverse an area the size of a county.

ta b l

Flow lines


potable water, but connate water can
locally be important as a high-salinity
environmental contaminant associated
with petroleum.


214 Groundwater
Groundwater—the great dissolver, the great precipitator
Groundwater is physically and chemically dynamic. It is
constantly on the move, constantly dissolving and/or precipitating a host of rocks and minerals—depending on the
chemistry of the water and the chemistry of the rocks and
sediments through which it moves (Fig. 12.2).
Dissolves some things

Precipitates many things

Limestone, gypsum, salt
(forms caves and landscapes
in these rocks)

Cave deposits
(stalactites, stalagmites, etc.)

Few minerals from
sandstones, shales, and
igneous and metamorphic
rocks (rarely forms caves in
these rocks)

Cements that hold sedimentary
rocks together (calcareous,
siliceous, ferruginous)
Spring and geyser deposits

Q12.1 In the southern part of Yellowstone National
Park (e.g., the vicinity of Old Faithful), geyserite
consists of varicolored siliceous material; whereas in
the northern part of the park (e.g., in the vicinity of
Mammoth Hot Spring), geyserite consists of snow-white
calcareous material. Examine details in Figure 12.3
and try to explain why this difference in the mineral
compositions of Yellowstone geyserites. Hint: What goes
around, comes around. What water dissolves, water
might precipitate.

Concretions and geodes

Figure 12.2 Groundwater dissolves rocks and minerals,
groundwater precipitates rocks and minerals—depending on
the composition of the water and on the composition of the
rocks and sediments through which it moves.


The variability in the composition of groundwater is illustrated by the variety of geyserites in Yellowstone National
Park. (Geyserite is mineral material that is precipitated from
groundwater as it emerges from the ground and evaporates,
leaving behind elements that were in solution.)



siliceous igneous rocks




Figure 12.3 This is a schematic cross-section showing the
variety of rocks in which the ‘plumbing systems’ of Yellowstone
geysers occur.

Groundwater and geologic wonders—Minerals that grow
within cavities in rocks are most commonly precipitated by
groundwater (Fig. 12.4A and B). And, petrifaction of trees
of Triassic age in the Petrified Forest of northern Arizona
reflects the work of groundwater as well (Fig. 12.4C). In the
world of geologic wonders, cases documenting the effects of
groundwater abound.




Old Faithful


Figure 12.4 A This faceted quartz crystal was precipitated
by groundwater within cavities in sandstone of the Ouachita
Mountains of west-central Arkansas. B This geode was precipitated by groundwater within cavities in volcanic rocks of
Brazil. C. This fossil tree was ‘petrified’ by groundwater in
the Petrified Forest region of Arizona.

Groundwater 215
, MARCH 24, 2002

U.N. water report warns
of impending shortages

The impending global water shortage
not only requires that the world develop
all available potable water resources in
the near future, but we must also do a
better job of minimizing the waste of
water and guard it against pollution.

Crisis will affect 5 billion wo

rldwide by 2025.

VIENNA, Austria (AP)—Wa
people face a critical shortag rning that 2.7 billion
2025, the United Nations ma e of drinkable water by
Friday with a call for a “blue d World Water Day on
and tap the seas for new supplie olution” to conserve
In fewer than 25 years, abo s.
living in areas where it will ut 5 billion people will be
to meet all their needs for be difficult or impossible
looming crisis that overshado sh water, creating “a
the Ea rth ’s pop ula tio n,” ws nearly two-thirds of
a U.N . rep ort sai d.

Groundwater will play a growing role
in efforts to provide water for our
growing global population, given the
fact that it quantitatively competes with
other fresh water resources (Fig. 12.5).

Water on land


(x 1,000,000 km 3)

Figure 12.5 This is a comparison among the four vast reservoirs of accessible water on
Earth. (Each unit is one million cubic kilometers.) Water on land consists of streams, rivers,
lakes, and ponds.

Q12.2 (A) How many cubic kilometers of water reside within groundwater?
(B) How many more times abundant is groundwater than water on land?

Groundwater has several advantages over surface water when it comes to providing
for municipal needs.

Q12.3 Imagine that you are a member of a city council and your town is in
need of a new and larger municipal water supply. Discussion has turned to the
merits of well water compared to those of surface water. In what ways can you
imagine that groundwater might be superior to surface water as concerns the
following points?
(A) Paying the cost of drilling a well, compared to that of constructing a dam.
(B) Contending with the occasional drought in your semiarid region.
(C) Minimizing contamination from surface runoff and from the atmosphere.
(D) Protecting your water supply against the threat of terrorism.

216 Groundwater
B. Anatomy of water tables
Saturated and unsaturated zones—Within the subterranean realm of
groundwater there are two main zones:

A word about arid and semiarid

(1) The saturated zone (Fig. 12.6) is the zone in which open spaces in sediments
and rocks are filled with water. The top of the saturated zone is the water table.
The slow movement of groundwater—toward streams in humid regions and away
from streams in arid regions—is impeded by friction, so water tables are rarely
flat. The shape of a water table in a humid region mimics that of the land surface—
i.e., high under hills and low under valleys, where it intersects perennial streams
and lakes.

Q12.4 Judging from the informa-

(2) The unsaturated zone is the zone in which intergranular spaces and fractures
are filled with air and, at times, films of descending water.

If you answered the above question with,
“in an arid or semiarid region,” you were
correct. Flat-bottomed losing streams
like that in Figure 12.6B (Spanish
arroyos or barrancos) can be dangerous.
There is little rain in arid and semiarid
regions, but when rain does come, it
is typically torrential. Commonly, the
entire annual amount of rainfall arrives
in a single afternoon—often creating
disastrous flash floods. In August 2003
this kind of flash flood swept automobiles and highway dividers from I-35 in
Kansas, killing a number of people.

A Humid conditions
(infiltration is important)

Wa te
table r





B Arid or semiarid conditions
(runoff is important)

Aquifer defined


tion accompanying Figure 12.6, which
kind of stream do you suspect would
rise faster (though briefly) for a given
amount of rain—that in a humid
region or that in an arid or semiarid

Ba r e r ock
tab ter


Figure 12.6 A In a humid region, water moves (‘seeking its own level’ as it were) in its
tendency to develop a horizontal water table, and so the saturated zone feeds a gaining
stream. B In an arid or semiarid region, the water table slopes downward from a losing
stream, the source of water for the saturated zone.

Much of western United States lies
within the Great American Desert
(Fig. 12.7), a region in which the
groundwater situation is like that
in Figure 12.6B. In contrast, eastern
United States is characterized by conditions shown in Figure 12.6A.

Great American Desert

Figure 12.7 Because of arid to semiarid
climate, approximately one-half of the
conterminous 48 states is at risk as
concerns the development and
management of water resources.

Before going further in our discussion
of groundwater, we need to define the
concept of aquifer—a body of sediments
or rocks that yields water sufficient to
meet specific needs. The saturated zone
in Figure 12.6 might or might not be
an aquifer. The definition of ‘aquifer’ is
qualitative; e.g., an aquifer supplying a
particular city might cease to be viewed
as an aquifer were the population to
grow beyond its capacity.

Groundwater 217
C. Dynamics of water tables
Common problems

Hydraulic gradient

In Los Angeles County, a 3.5-mile section of I-105 was constructed below ground
level in an effort to minimize noise and visual pollution. Caltrans (Calif. Dept. of
Transportation) believed the water table to be 30 feet below road level at the time
of construction (Fig. 12.8). However, what Caltrans failed to learn was that the
water table had been drawn down by over-pumping in the 1950s, and another state
agency had recently mandated that the over-pumping cease. (Ref: Calif. State Auditor

Geologists describe the magnitude of
slope as the vertical angle between
slope and the horizontal (Fig. 12.10).

rep’t #99113, 1999.)

30 ft


Wa t e r t a b l e a t t i m e o f c o n s t r u c t i o n

Figure 12.8 Highway engineers recessed a section of I-105 in Los Angeles County in
an effort to mitigate noise and visual pollution. At that time, the water table was 30 ft
below the highway.

Q12.5 So what do you suppose happened when over-pumping of the saturated zone was stopped by that other California state agency?
On more than one occasion a gas station in a low topographic setting has allowed
the level of gasoline in its storage tanks to become too low (Fig. 12.9). Then came
the rains, with runoff making its way into the saturated zone.

Figure 12.10. A Brunton compass, with
its leveling bubble and sight-adjusted
protractor, enables a geologist to measure
the vertical angle between slope and the

But engineers describe the magnitude
of slope as the ratio of vertical drop
to horizontal distance, aka the percent
of grade. Thus, a gradient of 0.05, or
5 percent, designates a vertical drop
of 5 feet per 100 feet of horizontal
distance. This same convention is used
in describing the hydraulic gradient of
groundwater (Fig. 12.11).

Well #1

h1 h2

Well #2
Wa te

r ta b



Hydraulic gradient =
Unsaturated zone

Wa te r ta bl e

Saturated zone

‘X-Ray view’ of gas storage tanks

Figure 12.9 This gas station is very near the water table, which presents a threat
to fuel storage tanks.

Q12.6 Can you imagine what happened when the water table rose?
Hint: Asphalt and concrete are only so strong.

h1 h2

Figure 12.11 h1 is the elevation of
the water table in well #1, h2 is the
elevation of the water table in well
#2, and l (for length) is the horizontal
distance between wells.

Q12.7 If, for the model in Figure
12.11, h1 were 506 ft, h2 were 497
ft, and l were 150 ft, what would be
the hydraulic gradient (in percent)
between well #1 and well #2?

218 Groundwater
Hydraulic conductivity

Darcy’s Law

Just as surface water flows faster down
steeper slopes, groundwater moves
faster down steeper hydraulic gradients.
But hydraulic gradient is not the only
factor affecting the rate of groundwater movement. Equally important is
hydraulic conductivity, which is the
ease with which sediments or rocks
transmit water. Hydraulic conductivity
introduces the concepts of porosity and

The most fundamental questions in
targeting a prospective groundwater
resource are, ‘How much, and how
often?’ This volume per time issue is
analogous to the discharge of a stream.

Porosity is the percentage of a body
of sediments or rocks that consists of
open spaces, called pores. Porosity
determines the amount of water that
sediments or rocks can hold. There are
many kinds of pores—ranging from
pores among sedimented particles, to
pores within volcanic rocks, to cavities within soluble rocks, to fractures in
any kind of rock (Fig. 12.12). And any
of these pores can be filled to differing
degrees by cements.
Permeability is the ability of soil, sediment, or rock to transmit fluid. Material
with low porosity is likely to have low
permeability as well, but high porosity
does not necessarily mean high permeability. In order for pores to contribute to
permeability, they must be (a) interconnected, and (b) not so small that they
restrict flow. For example, clay commonly has high porosity, but clay grains
are so broad in proportion to their
microscopic size (i.e., around 0.005
mm) that the molecular force between
clay particles and water restricts flow.
As concerns the potential of sediments
and rocks to transmit water—a critical
issue in the aquifers—permeability is
Figure 12.12 (White space is open
space available to water. Open
space along fractures is too thin
to illustrate.) Porosity and
permeability can result from
sedimentation, volcanism, solution,
collapse, faulting, and fracturing.
Subsequent cementation can reduce
the volumes of any of these pores.
(Magnification is 5–10x.)

Pebbles and


Limestone with
solution cavities

In 1856, Henri Darcy, a French engineer, attempted to determine whether
a prospective aquifer could yield
water sufficient for the city of Dijon.
Darcy undertook a series of laboratory
experiments in which he measured the
rate of water flow through a variety
of sediments in tubes tilted at various
angles. Not surprising to us now, Darcy
(1) Groundwater flows faster through
more permeable rocks.
(2) Groundwater flows faster where the
water table is more steeply inclined.

Volcanic rock
with vesicles

produced by
collapse or

Darcy identified the four key variables
in groundwater flow (or discharge) as…

Discharge (Q)
Hydraulic conductivity (K)
Hydraulic gradient (h1 – h2 / l)
Area (A) (thickness x breadth of
the aquifer)

…and crafted an algebraic expression
(‘Darcy’s Law’) of their relationship:

Q = (K) (h1 – h2 / l) (A)
Darcy’s Law enables one to calculate
the maximum amount of water that an
aquifer might yield to an array of wells.



Q12.8 Hydraulic conductivity of an
aquifer is known to be 8 ft/day, and
its dimensions are estimated to be
40 ft thick and 18,000 ft wide. Two
test wells drilled one mile apart in
the direction of flow encountered the
water table at elevations 5,030 ft and
5,050 ft. Question: How many gallons
of water flow through the aquifer per
day? (To convert ft3 to gal, see page i
at the front of this manual.)

Groundwater 219
D. Mapping a water table
Map the direction of groundwater
flow within your mapped area
Figure 12.23 on Answer Page 230 is a
contour map on which 26 water wells
have been plotted. Each well site shows
the depth to the water table within that
well as a negative value in feet below
ground level.
For each well location:
(1) Estimate the surface elevation from
the proximity of contour lines.

Rates of groundwater flow—
applying Darcy’s Law to your
map of the water table

(2) Subtract the depth to the water table
from surface elevation in order to determine the elevation of the water table.
Record that elevation (of the water
table) at the well site.

Q12.12 What is the difference in

(3) After repeating the above two steps
for each of the 26 wells, contour the
groundwater elevations with a contour
interval of 20 feet. (A ‘getting started’
example is framed with a gray rectangle
in the lower-left corner of the map.)

Q12.9 Draw an arrow between
Well A and Well B indicating the
direction in which groundwater is
likely to be moving. In which direction is the arrow pointing, northeastward or southwestward?

Q12.10 (A) At what map coordinates is the difference between the
elevation of the ground and the
elevation of the water table the
greatest? (B) Give the coordinates
of a place where you might expect
to find a marsh or spring.

Q12.11 If contaminants were to
find their way into groundwater at
Acme Industries, in which well would
those contaminants be more likely
to appear—the well at the Smith
farmhouse, or the well at the Jones

elevations of the water table at Well A
and Well B?

Q12.13 What is the map distance (in
feet) between Well A and Well B?

Q12.14 What is the hydraulic gradient (h1 – h2 / l) between Well A and
Well B?

Q12.15 If the hydraulic conductivity
(K) of the aquifer is 10 ft/day, and the
cross-sectional area (A) of the aquifer
is 200 ft x 5,000 ft, what is the rate of
flow or discharge, (Q), through the
aquifer in cubic feet per day?

220 Groundwater
E. Artesian conditions and confined aquifers
‘Water seeks its own level,’ which
explains simple artesian conditions. To
illustrate—if we were to add water to
the glass tubing in Figure 12.13A, to a
particular elevation in conduit A, that
water would be driven by hydrostatic
pressure to that same elevation in
conduits B and C. However, were our
tubing (a) filled with sand, and (b) open
at its downstream end (Fig. 12.13B),
water would not rise as high in B and
C. Moreover, the farther from the
recharge area, the less the hydrostatic
potential for lifting water in a conduit.
This reduction in potential away from
the water’s source describes a sloping
potentiometric surface.

Notable artesian examples

In the real world, an artesian aquifer is
more like the situation in Figure 12.13B
(rather than 12.13A) in that (a) an
artesian aquifer is filled with sediments
and/or rocks, and (b) water within an
artesian aquifer is free to move within
that aquifer.

Q12.16. In Figure 12.13B, what two
things account for less hydrostatic
pressure within Conduit C than
within Conduit B? Hint: These two
things are pretty much spelled out by
labels in this figure, and one appears
in the description of ‘the saturated
zone’ (explaining why water tables
are rarely flat) on page 216.

Figure 12.14 Fountains at Trafalgar Square
are a graphic reminder of the famous
London Artesian Basin.

A This system is filled with nothing but water
and is closed downstream

When wells were first drilled in London
circa 1900, fountains at Trafalgar
Square were flowing artesian wells
(Fig. 12.14). But hydrostatic pressure
has since declined, so now water must
be pumped to the surface.





Artesian thermal waters at Hot Springs
National Park, Arkansas, owe their
heat to deep circulation (Fig. 12.15).
Slow descent of rainwater is via a large
collecting system, with rapid ascent via
narrow passageways. (A bent funnel effect.) Thus, heat persists as waters make
their quick escape to the surface.


B This system is filled with sediment
and is open downstream

Hot Springs


Friction impedes flow
through sediments

surfac ometric

even more

1 mi

(some pressure relieved)

Figure 12.13 A This tubing illustrates the simple principle, ‘water
seeks its own level.’ B. This tubing illustrates the variation of this
principle that is applicable to the real world of artesian flow of
groundwater. It’s filled with sediment, and it’s open downstream.

Figure 12.15 Within folded rocks of the
Ouachita Mtns., rainwater enters an artesian
aquifer at 60°F, descends to a depth of one
mile, and emerges at 143°F (the temperature
of hot coffee).

Groundwater 221

Details of confined aquifers—But
first, a look back at unconfined
aquifers. In Sections B and C (pages
216–218), which deal with the anatomy
and dynamics of water tables, aquifers
are unconfined; i.e., the top of the zone
of saturation (i.e., the water table) is
free to rise and fall with the vagaries
of climate. But in a confined aquifer,
such is not the case. A confined aquifer
is one in which water is prevented
from rising and falling by relatively
impermeable intervals of rock called
aquitards (from Latin, retards water).
A visual metaphor: Swiss cheese (with
its interconnected holes) between slices
of dense bread. Aquitards typically
consist of shale—the most common and
the least permeable of all sedimentary
A confined aquifer receives its water
in an area where it intersects the land
surface, called the recharge area (Fig.
12.16). Where a well is drilled into a
confined aquifer, water rises toward

Recharge area

the elevation of the water table in
the recharge area, a condition called
artesian (recall the glass tubing model
on the facing page). Water will not rise
quite as high as the water table in the
recharge area because (a) friction is
associated with water moving through
the aquifer, and (b) the water is free
to move laterally, thereby reducing
hydrostatic pressure. The imaginary
level to which water in a group of artesian wells tends to rise is (as defined
on the facing page) the potentiometric
surface (aka the piezometric surface).
In Figure 12.16 the potentiometric
surface is below ground level in the
vicinity of Wells A and B, so they are
non-flowing artesian wells. The potentiometric surface is above ground
level in the vicinity of Well C, so that
well is a flowing artesian well.

Q12.17 On Figure 12.24 on Answer
Page 230, label each well with the
correct letter as described in the text
beside that figure.

Mapping a potentiometric surface—
Figure 12.25 on Answer Page 231
shows six wells (#1–#6) on a ground
elevation contour map in the area of a
confined aquifer. All six wells penetrated the aquifer. The map includes a
second set of contours (straight dashed
lines) drawn on the aquifer’s potentiometric surface. Flowing artesian wells
should occur where the potentiometric
surface is higher than ground elevation.
Non-flowing artesian wells should
occur where the potentiometric surface
is lower than ground elevation.
At every place where a ground elevation contour line crosses a potentiometric contour line of the same value,
place a small circle. Then connect the
circles with a line. Ground elevations of
wells on one side of that line are lower
than the potentiometric surface, so the
wells should be flowing artesian wells;
whereas ground elevations of wells on
the other side of that line are higher
than the potentiometric surface, so the
wells should be non-flowing artesian

Q12.18 (A) Which of the six wells in


Water table

Horizontal C


ric surface


Figure 12.16 The dotted line is a horizontal projection of the water table in
the recharge area. The solid line is the potentiometric surface. Well A is near
the recharge area, so the water in it rises almost to the elevation of the water
table. Well B is farther away, so friction over a greater distance accounts for
the water’s not rising as high as in Well A. The mouth of Well C is below the
potentiometric surface, so it is a flowing artesian well.

Figure 12.25 on page 231 should be
flowing artesian wells? (B) Darken
the a...
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Tutorial Preview … metamorphicrocks (rarely forms caves inthese rocks)Cements that hold sedimentaryrocks together (calcareous,siliceous, ferruginous)Spring and geyser depositsQ12.1 In the southern part of Yellowstone NationalPark (e.g., the vicinity of Old Faithful), geyseriteconsists of varicolored siliceous material; whereas inthe northern part of the park (e.g., in the vicinity ofMammoth Hot Spring), geyserite consists of snow-whitecalcareous material. Examine details in Figure 12.3and try to explain why this…
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