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Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 1
CHAPTER 1
Preliminary concepts and Overview
Ron Chaoka
Department of Geology, University of Botswana. Gabarone. Botswana.
1.1
Hydrogeology
Hydrogeology is the study of groundwater. It is concerned with, but is not limited
to, the distribution, occurrence, and movement of groundwater; physical and
chemical interactions between groundwater and geologic formations; groundwater
contamination; transport of chemical constituents by groundwater. Hydrogeology
is one of the main branches of hydrology, and is sometimes referred to as
groundwater hydrology.
Hydrogeology is an interdisciplinary field. It requires a good understanding of
chemistry, physics, geology, and mathematics. A basic understanding of
hydrogeology is useful to engineers, soil physicists, planners, and others.
Historically, the study of groundwater was almost entirely motivated by its
importance as a resource. Thus groundwater exploration, evaluation, and
exploitation were given a great deal of attention. Considerable effort was also
devoted to understanding the principles governing groundwater flow. These are
critical to the development of water supplies and the sustainable management of
groundwater resources. While water supply aspects are still important, the
emphasis in groundwater studies, particularly in developed countries, has shifted
over the past several decades from water supply problems to other aspects.
These include geochemical aspects, groundwater contamination and transport
processes, the role of groundwater in environmental and geotechnical engineering
problems as well as in geological processes.
Geotechnical, mining, and civil engineers invariably have to take into
consideration the impact of groundwater on their engineering designs. Massive
inflows of groundwater into a tunnel, excavation, or mine can delay tunnel driving
and mining operations for a considerable length of time. Increased groundwater
pressures beneath a dam can lead to dam failure. Excessive groundwater
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 1
withdrawals can lead to land subsidence and severe problems for engineering
design. While the role of groundwater in geological processes such as crustal
deformation, earthquake generation, ore deposit formation, migration and
accumulation of petroleum is recognized, it is not very well understood.
1.2
Groundwater occurrence
Groundwater may be found almost anywhere in the world and in almost all types
of geologic formations. However, its distribution in terms of quantity and quality
varies from one place to another and from one geologic formation to another. One
of the greatest challenges to a hydrogeologist and /or a geophysicist, therefore, is
to locate geological formations with sufficient water of reasonable good quality for
a particular use.
There are at least three factors that influence groundwater occurrence: hydraulic
properties of geological formations, geological framework, and climate. Hydraulic
properties of geological formations are those properties that govern groundwater
storage and transmission. These include pores, vesicles, lava tubes (and tunnels),
solution cavities, bedding planes, foliation, faults, shear zones, unconformities,
and intrusive contacts. Some of these structures are primary; that is, they were
formed at the same time as the formation. Others were formed after the formation
was formed and are referred to as secondary. The generic term for the relative
volume of a geologic formation in which water can be stored is porosity regardless
of whether that volume consists of pores or other types of openings. Geological
formations with interconnected openings are said to be permeable. The ability of a
geological formation to transmit water and yield it in usable quantities to wells
depends on its permeability.
Geological formations differ considerably in their ability to store and transmit
water. Knowledge of typical values of porosity and permeability of different
geological formations is a prerequisite for successful groundwater exploration.
Virtually all groundwater originates as surface water. In order to reach the
saturated zone, water must not only be available at the surface; it must also be
able to infiltrate to the saturated zone. The availability of water at the surface
depends on climate, while the infiltration rate depends on the thickness and
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Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 1
permeability of the unsaturated zone as well as topography. Geological framework
includes topography, type of geologic formation, physical and chemical
characteristics of surficial or unconsolidated deposits overlying bedrock.
1.2.1 Hydraulic properties of geological formations
Hydraulic properties of geological formations were defined in the preceding
section as those properties that govern groundwater storage and transmission.
The definitions of some of the most important hydraulic properties are provided
below.
Porosity ( n )
Porosity is the ratio of the volume of the voids or interstices in a given volume of
geologic material to the total volume of material
n
Uv
U
(1.1)
Where U v is the volume of the voids and U is the total volume of material. It may
be expressed as a decimal fraction or as a percentage. Porosity ranges from 1
percent to as much as 80 percent in some recently deposited clays, but in most
granular materials it falls between about 5 and 40 percent (Table 1.1). There are
two types of porosity: primary and secondary. The former is developed during the
formation of geological materials, while the latter is formed after.
Specific yield (Sy)
Specific yield is the ratio of the volume of water that drains from a saturated
volume of geologic material under gravity ( U d ) to the total volume of the material:
Sy 
Ud
U
(1.2)
Table 1.1.
Range of values of porosity (Kresic, 1997; Freeze and
Cherry, 1979)
__________________________________________________
n (%)
__________________________________________________
Unconsolidated deposits
Gravel
25-40
Sand
25-50
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Silt
Clay
35-50
40-80
Rocks
Fractured basalt
Vesicular
Tuff
Pumice
Weathered volcanic rocks
Sandstone
Limestone, dolomite
Shale
Fractured crystalline rocks
Dense crystalline rocks
5-50
10-50
15-40
60-90
15-60
5-30
<1-20
<1-10
<1-10
<1-5
In general the term specific yield is used to describe the storage capacity of an
unconfined aquifer (see section 1.2.2). Values of specific yield typically range from
0.1 to 0.3, and are always less than the porosity of the geologic material.
Specific Retention (Sr)
Specific retention is the ratio of the volume of water a saturated volume of
geological material retains against gravity ( U r ) drainage to the total volume of the
material
Sr 
Ur
U
(1.3)
The sum of the specific yield and the specific retention is equal to porosity. Figure
1.1 (Bear, p. 485) shows the relationship between median grain sizes of various
granular materials and the hydraulic properties defined above.
Figure 1.1: Relationship between specific yield, specific retention, porosity
and grain size (Bear, 1972)
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Permeability (k)
This is a measure of the ability of a formation to transmit water. It depends only on
properties of the geological formation. Permeability is measured in units of length
squared or in darcy units. Table 2.2 shows the range of values of permeability.
Table 2.2 Range of values of permeability (Freeze and Cherry, 1979)
Rocks
Unconsolidated
deposits
k
(darcy)
5
10
10
4
10
103
10
2
10
Gravel
10
Unfractured
metamorphic and
igneous rocks
Shale
Unweathered
marine clay
Glacial till
Silt, loess
Silty sand
Clean sand
Karst limestone
Permeable basalt
Fractured igneous and
metarmorphic rocks
Limestone and
dolomite
Sandstone
K
2
(cm )
10
-3
-4
-5
-6
10
-8
1
10
-1
-9
10
10
10-2
10-10
-3
10
-4
10
-5
10
-6
10
10
-7
10
10-8
10-16
10
10
10
10
-11
-12
-13
-14
-15
1.2.2 Hydrological classification of geological formations
Saturated geological formations can be divided into three types depending on
their ability to transmit water: aquifer, aquitard, and aquiclude.
1.
An aquifer is a saturated geological formation that is sufficiently
permeable to transmit water and yield it in usable quantities to a well
or spring.
2.
An aquitard is a saturated geological formation that is not permeable
enough to yield water to a well or spring in usable quantities, but is
permeable enough to allow an interchange of groundwater in
between adjacent aquifers.
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3.
An aquiclude is a nearly impervious geological formation that is
virtually incapable of transmitting groundwater.
These definitions not withstanding, it must be pointed out that in nature
there is no such thing as an impermeable geological formation. All
geological formations have some permeability. However, it is difficult to
provide definitions of aquifer and aquitard in terms of permeability because
the difference between an aquifer and aquitard also depends on local
conditions.
Aquifers may be divided into three main types: confined, unconfined, and
leaky (Figure 1.2). A confined aquifer is bounded above and below by an
aquiclude (Figure 1.2a). Water-level elevations in wells tapping a confined
aquifer are above the top of the aquifer; otherwise the aquifer is
unconfined. The water in a well tapping a confined aquifer may rise above
ground surface. Such a well is referred to as a flowing artesian well. An
imaginary surface joining water-level elevations in a confined aquifer is
known a potentiometric surface.
water level
aquiclude
Confined
aquifer
aquiclude
Figure 1.2a. Conceptual model of a confined aquifer.
An unconfined aquifer, also known as a water table aquifer, is bounded
below by an aquiclude and above by a water table, which is the surface
joining water-level elevations in an unconfined aquifer (Figure 1.2b). There
is a special type of unconfined aquifer known as a perched aquifer (Figure
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1.2c). Perched aquifers are formed where lenses of low-permeability
material occur within otherwise permeable material. The low-permeability
material does not allow infiltrating water from the ground surface to pass
through it.
water level
watertable
Unconfined
aquifer
aquiclude
Figure 1.2b: Conceptual model of an unconfined aquifer.
A leaky aquifer is a type of confined aquifer in which one or both
boundaries are aquitards (Figure 1.2c). As a result, water from overlying or
underlying aquifers is free to move through the aquitard(s). As the
preceding discussion implies, aquitards and aquiclude constitute the
confining units.
perched aquifer
water-table
Unconfined
aquifer
Aquitard
Leaky aquifer
Aquiclude
Confined
aquifer
Aquiclude
Figure 1.2c: Multilayered aquifer system showing a leaky and a perched
aquifer.
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Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 1
1.3
Geological Processes and Formations
The occurrence, distribution, movement, and composition of subsurface waters
are intricately linked to the structure and nature of geological formations. One of
the primary objectives of hydrogeological investigations is to identify geological
formations and structures of importance for the occurrence of groundwater. A
basic understanding of the different types of geological formations, as well as the
events that produce them and their fundamental properties is required to achieve
the objectives of such an investigation. The main objective of this section is to give
a brief description of various geological processes that give rise to the different
geological formations.
Geological formations consist of rocks and their weathering products. A rock
consists of an agregate of one or more minerals. A mineral on the other hand may
be defined as a naturally occurring crystalline substance formed by inorganic or
organic processes. There are three main groups of rocks: igneous, sedimentary
and metamorphic. Once formed, rocks are susceptible to weathering, alteration,
deformation, and transformation. Figure 1.3 is a schematic diagram of the rock
cycle. It illustrates the role of various geological processes in the formation of the
different types of rocks. It also shows how one rock is transformed into another by
processes that act on and within the earth.
1.3.1 Igneous rocks
As Figure 1.3 shows, igneous rocks form mainly from silicate melts (molten rock)
by a process called crystallization. The melts or magmas from which igneous
rocks form are produced by partial melting of pre-existing solid rock. Melting may
be induced by changes in temperature, pressure, or an influx of water which
depresses the melting point of the solid. Partial melting occurs at different levels
within the earth’s crust and upper mantle and gives rise to a wide variety of
magma compositions. However, most igneous rocks are composed mainly of
silicon, oxygen with lesser amounts of aluminium, iron, magnesium, calcium,
sodium, and potassium.
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Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 1
MAGMA
Cooling and
solidification
(Crystallization)
Melting
Heat and pressure
IGNEOUS
METAMORPHIC
ROCK
ROCK
W
ea
the
ri
ng
, tr
an
sp
or
ta
ion
,
Heat and pressure
(Metamorphism)
an
dd
ep
osi
t
Weathering,
transportation,
and deposition
ion
Weathering, transportation,
and deposition
SEDIMENTARY
ROCK
Cementation
and compaction
(Lithification)
SEDIMENT
Figure 1.3: Schematic diagram of the rock cycle
Crystallization of molten rock may occur within the earth to form intrusive igneous
rocks or at the earth's surface following an eruption to form volcanic or extrusive
igneous rocks. In general, extrusive rocks are fine-grained to glassy, while
intrusive rocks are medium- to coarse-grained. Igneous rocks exhibit a wide
variety of fabrics (textures) as well as chemical and mineralogical compositions.
These characteristics have been used as a basis for classification schemes.
Textural classifications are based on the degree of crystallinity (i.e. proportion
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crystalline material to glass), grain size and shape, and the mutual arrangement of
mineral grains within an igneous rock. Mineralogical classification schemes are
based on modal composition; that is, the volumetric proportions of essential
minerals in a rock. In contrast, chemical classification schemes are based on the
bulk or whole rock chemical composition expressed in terms of weight
concentrations on a percentage basis of major elements.
1.3.2 Sedimentary Rocks
Sedimentary rocks are those rocks formed at or near the earth's surface at
relatively low temperatures and pressures. They can be divided into clastic
(detrital) and chemical sedimentary rocks. The main constituents of clastic
sedimentary rocks are rock fragments and minerals. They are produced by
weathering and erosion of pre-existing rocks. The products of weathering and
erosion are transported and deposited by water, wind or ice. Clay minerals and
quartz are the most abundant constituents of detrital sedimentary rocks, followed
by rock fragments.
Chemical sediments are derived from soluble material produced largely by
chemical weathering. Chemical sediments are formed by precipitation of dissolved
constituents. Precipitation occurs a result of inorganic processes such as
evaporation or changes in the saturation status of the solution due to changes in
temperature, pressure, and chemical activity.
Some chemical sediments are
produced by the action of living organisms. These are referred to as biogenic
sediments. One rock type that is neither clastic nor chemiclal is coal. Coal is
formed from accumulations of carbonaceous material derived from vegetation.
The transformation from carbonaceous material to coal involves burial,
compaction, and slight heating.
Sedimentary rocks may be classified on the basis of various criteria such as (a)
composition; (b) grain size; (c) grain shape; (d) orientations of the grains; and (e)
the packing of the grains. For example, detrital sedimentary rocks may be divided
into three groups on the basis of grain size of the detrital fragments and minerals
(Table 1.1). As Table 1.1 shows, however, it may be difficult to distinguish among
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different rock types on basis of one criterion or even two criteria. For example,
both conglomerate and breccia consist of rock fragments and minerals within the
same grain-size range but have different names because their constituent
materials have different shapes. Therefore, additional modifiers and subdivisions
are required to make classification systems more useful.
Table 1.1: Classification of Clastic Sedimentary Rocks
Sediment Name and
Rock Name
Particle Size
Gravel (>2 mm)
Conglomerate
Breccia
Sand (1/16 - 2 mm)
Sandstone
Silt and clay (<1/16 mm)
Siltstone
Shale
Mudstone
Three stages are involved in the transformation of sediments into sedimentary
rocks: compaction, cementation and lithification (Figure 1.3). Cementation refers
to the binding together of constituents of sedimentary rocks. Cements are formed
by precipitation of chemical constituents dissolved in pore fluids.
1.3.3 Metamorphic rocks
Metamorphic rocks form from other rocks by essentially solid-state changes in
mineralogy and/or texture as a result of changes in temperature (T), pressure (P),
and prevailing stress condition. Apart from possible losses or gains of minor
amounts of volatile constituents (e.g. H2O, CO2, O2, and S), most metamorphic
rocks have chemical compositions close to their precursors. However, in the type
of metamorphism called metasomatism, significant changes to the chemical
composition of the original rock body may occur due to the introduction of
chemical constituents from an external source by metasomatic fluids.
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The response of a solid rock body to changing P, T, and X (chemical composition)
conditions depends on (1) the nature of the original rock; (2) the grade of
metamorphism, that is, the relative temperature of metamorphism; and (3)
presence or absence of a fluid phase. Various types of metamorphism are
recognized. These include thermal (contact), dynamic, regional, burial, and
retrograde metamorphism as well as metasomatism. Thermal metamorphism is
associated with magmatic intrusions at shallow crustal levels. It is essentially
isochemical. Dynamic metamorphism occurs in rocks subjected to intense
localized stresses such as those due to meteorite impacts. Rocks produced by
this type of metamorphism are rare. Regional metamorphism, as the name
implies, occurs on a large scale under a wide range of pressure and temperature
conditions. It is associated with mountain building and magmatism and is
characterised by penetrative deformation. In contrast to regional metamorphism,
burial metamorphism is not related orogeny or magmatism. It is due to pressure
and temperature changes associated with thick sequences of volcanic and
sedimentary rocks.
The transformation of high-grade metamorphic mineral
assemblages to low grade mineral assemblages is referred to retrograde
metamorphism.
Metamorphic rocks can be classified on the basis of fabric, field relations,
mineralogical and chemical composition as well as the inferred pressuretemperature conditions of metamorphism. The inferred P-T conditions are based
on characteristic mineral assemblages. The classification most commonly used is
that based on foliation. Foliated texture is characterized by parallel alignment of
minerals and structural features of metamorphic rocks. On the basis of these
characteristics, metamorphic rocks can be divided into strongly foliated, weakly
foliated, and nonfoliated. Strongly foliated rocks possess strong planar fabric and
have a strong parallelism of prismatic minerals. They break easily along foliation
planes. In contrast, weakly foliated rocks have a weak parallelism of prismatic
minerals and/or diffuse planar fabric. Nonfoliated rocks are typically massive.
1.3.4 Rock deformation
Most rocks are deformed. The most common manifestations of rock deformation
are folds and fractures. Folding involves bending or warping of rocks in response
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Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 1
to compressive stresses. It is best observed in rocks containing planar structures
such as rock strata, bedding planes, foliation or cleavage. The most common
types of folds are anticlines and synclines (Figure 1.4a).
Folds are often
associated with extension fractures and normal faults on the convex side and
thrust faults on the concave side of anticlines (Figure 1.4b).
A
n
tic
lin
e
S
y
n
c
lin
e
A
n
tic
lin
e
Y
o
u
n
g
e
s
tro
c
k
O
ld
e
s
tro
c
k
Figure 1.4a. Types of folds
A
B
Ex
t.
t.
Ex
C
N
m
Co
p.
Co
m
p.
A - Tensile fracture
B - Normal fault
C - Thrust fault (rare)
N - Neutral surface
Figure 1.4b. Types of fractures associated with folds.
The term fracture without qualification refers to any break in a material. Many
different types of fractures are found in rocks. These may be divided into two
types: (1) fractures along which displacement has occurred parallel to the fracture
surfaces and (2) fractures along which no movement has occurred. The former
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Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 1
includes faults and shear fractures (shear zones), while the latter includes joints,
gashes, fissures, and veins. Displacements in shear zones and along faults range
from a few meters to hundreds of kilometers.
Folds and faults affect groundwater regimes in many different ways. In general
folding can lead to a complicated distribution of recharge and discharge zones,
aquifers as well as local and regional flow systems. Fractures may facilitate
groundwater storage and transmission or act as barriers to groundwater flow.
They may also affect the distribution and occurrence of aquifers. Due to their
importance in hydrogeology, fractures constitute important targets for groundwater
exploration.
1.4 Soil moisture and groundwater
Subsurface waters can be divided into vadose water and groundwater. Vadose
water occurs in the vadose (unsaturated) zone, which comprises the soil moisture
zone, the intermediate zone, and the capillary zone. Below the vadose zone is the
saturated zone (Figure 1.5). Groundwater only refers to water in the saturated
zone. The boundary between the vadose zone and the saturated zone is defined
as the surface on which the fluid pressure in the pores of the porous medium is
exactly atmospheric. It is referred to as the water table. Above the water table,
fluid pressure is less than atmospheric, whereas below it the pressure is greater
than atmospheric.
The vadose zone typically extends from land surface to the saturated zone and its
thickness varies from place to place depending on climatic conditions. For
example, in wetlands, the thickness of the vadose zone may fluctuate seasonally
or may even be non-existent. By contrast, in arid environments the vadose zone
may not contain water for considerable depths. The interstitial spaces within the
soil moisture zone are partly filled with water and partly filled with air.
Soil moisture refers to all the subsurface water in the unsaturated zone. It occurs
as films around soil particles and is held by surface tension forces. There are two
types of surface tension forces: adhesive and cohesive forces. Adhesive forces
are due to attraction between molecules of dissimilar substances (e.g., water and
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Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 1
soil particles or water and air). They are strong, but very short-range forces.
Cohesive forces are due to attractions between molecules in like substances.
Ground surface
0
+ve
+ve
-ve
0
+ve
Unsaturated
or Vadose
zone
Soil water zone
Saturated
zone
Capillary fringe
Water table
depth
Intermediate
zone
Groundwater
=
(a)
(b)
(c)
Figure 1.5: (a) Zones of subsurface water, (b) profile of moisture content ,
and (c) profile of pressure head  versus depth.
As a result of surface tension (capillarity), water molecules at the water table are
subject to an upward attraction. An expression for the height of capillary rise in
unsaturated soils is usually arrived at by utilising an approach similar to that used
in classical physics for the analysis of the binding of water in a glass capillary. The
phenomenon of capillarity may be illustrated by inserting one end of a fine glass
tube of radius r into water as shown in Figure (1.6). As a result of capillarity, water
will rise in the tube to a height hc.
The rise of the water in contact with the tube is attributed to the attraction between
the sides of the tube and the water molecules (adhesive forces). In contrast, the
rise of the water not directly in contact with the walls of the tube is due to cohesive
forces between water molecules. Water will eventually stop rising when the
downward pull of gravity exactly equals the sum of the adhesive and cohesive
forces (Figure 1.7).
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Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 1
cos
r
hc
Water
Adhesion
Cohesion
Gravity
Adhesion
Figure 1.6: Illustration of capillary rise using a glass tube
b
a
Figure 1.7: Diagram showing the forces acting on a column of water in a
glass tube
An equation for the height of capillary rise hc may be written by considering the
forces acting on the column of water in the glass tube. The force acting downward
Fd is the weight of the column of water and is given by
Fd  hc r 2  w g
(1.4)
Where w is the density of water and g is the acceleration due to gravity. The
force acting upward Fu is the vertical component of the surface tension  which
acts around the circumference of the tube
Fu  2 r cos
(1.5)
Where  is the angle of wetting between the liquid and the glass. At the point of
maximum capillary rise Fd must be equal to Fu. Thus
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hc 
2 cos
 w gr
(1.6)
Fluid pressures in the unsaturated zone are measured with a device known as a
tensiometer. A tensiometer consists of a tube, which is fitted at one end with a
liquid-filled porous ceramic cup, and at another end with a vacuum gauge (Figure
1.5). When a tensiometer is inserted into an unsaturated soil, water will flow out of
the tensiometer through the pores in the ceramic cup. This creates a partial
vacuum in the tube, which can be read from the vacuum gauge (Figure 1.8).
Soil moisture is an important source of water for plant growth. It is also important
for mineral weathering, which provides nutrients for plant growth.
Vaccum gauge
Connecting tube
Ground level
Porous cup
Figure 1.8: Tensiometer with vacuum gauge, porous cup, and connecting
tube.
1.5
Groundwater and geologic processes
Groundwater plays an important role in many geological processes. Examples
include faulting, earthquake generation, ore deposit formation, diagenesis,
metamorphism as well as the migration and accumulation of hydrocarbons. Until
the role of fluids in faulting was understood, it was very difficult to explain the
movement of large slices of continental crust (thrust blocks) over large distances.
The main problem was that the horizontal forces required to move the thrust
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blocks thousands of metres thick tens of kilometres would create stresses that
greatly exceeded the strength of any known rock. The paradox was eventually
resolved when it was realised that pore water pressures in faults at depth greatly
reduced the normal component of the intergranular stress in the fault plane and
hence reduced the critical value of the shear stress required to produce sliding. As
a result, horizontal forces that exceeded the strength of the rock were not needed
to produce sliding.
There is also a strong correlation between earthquake generation and elevated
fluid pressures in the earth's crust. Earthquakes are generally associated with
rupturing of the earth's crust and movements along faults in the earth's crust. The
movement occurs when a rock that has been subjected to stress suddenly
ruptures or whenever the frictional forces on the fault surfaces are overcome. It is
now generally accepted that elevated pore water pressures in faults at depth
contribute to the generation of earthquakes by reducing the frictional resistance at
which the rock should fail.
The origin of some (many) ore deposits is also attributed to mechanisms involving
groundwater flow. These include the so-called Mississippi Valley-type (MVT) leadzinc deposits that are commonly found in porous carbonate rocks; sedimenthosted uranium deposits; and evaporites. In the case of MVT ore deposits there is
a lot of evidence to suggest that they were deposited from deeply circulating
heated saline connate waters at moderately high temperatures (80 to 1500C and
at relatively shallow levels in the earth's crust. Evaporites as the name implies
were formed by precipitation during the evaporation of brines. They can be divided
into marine and nonmarine evaporites. The former precipitated from seawater,
while the source of the latter is believed to be groundwater.
Virtually all rocks undergo some physical and chemical changes after deposition
and/or crystallization from magmas. The main changes in which groundwater
plays a significant role are diagenesis and metamorphism. Diagenesis refers to all
the changes undergone by sediments after initial deposition and during and after
lithification. Most sediments contain pore fluids at the time of deposition. The
increase in temperature and pressure that accompanies burial of a sediment
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affects the solubility of substances and may lead to precipitation or dissolution of
some minerals.
Although metamorphism occurs at temperatures and pressures higher than those
for diagenesis, there is no agreement on exactly where metamorphism begins and
diagenesis ends. Metamorphism involves changes in the texture and mineralogy
of a rock as a result of changes in temperature and pressure and the action of
chemically active fluids. Geologists and geochemists have used oxygen and
hydrogen isotopes to show that meteoric fluids (water) are abundant in many
metamorphic environments.
1.6
Groundwater in the hydrologic cycle
Water on earth resides in the atmosphere, on the surface of the earth, and below
the surface of the earth. However, water does not remain in anyone of these
environments indefinitely. It is constantly moving among them. This unending
circulation of the earth's water is called the hydrologic cycle. The cycle has no
beginning or end. Figure 1.9 shows a box model of the hydrologic cycle.
Atmosphere
ET
Vegetation
E
P
E
P
P-In
Land Surface
Ro
I
Soil
Rg
Rivers and Lakes
Q
Qs
Qg
Qg
Ground Water
Ocean
Figure 1.9: Box model of the hydrologic cycle (ET = evapotranspiration, I n =
interception, Rg = recharge, Qs = interflow, P = precipitation, E =
evaporation, Ro = overland flow, Q = Runoff, and Qg = subsurface flow to
streams and oceans).
1-19
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 1
Various estimates of the quantity of water in different components of the
hydrologic cycle have been made. All of them show that groundwater constitutes
an insignificant proportion of the earth's total water balance. As far as the world's
freshwater resources are concerned, however, groundwater accounts for almost
97% of the utilizable freshwater resources (excluding icecaps and glaciers).
Figure 1.10 shows near-surface hydrologic processes, some of which contribute
to groundwater storage. As Figure 1.10 shows, precipitation that falls on land may
be intercepted by vegetation, pond on the land surface, flow overland to streams
and rivers and eventually end up in the oceans, infiltrate into the ground, flow
through the unsaturated zone, and discharge into streams, percolate through the
unsaturated zone to recharge groundwater, evaporate from the oceans and the
land surface. Some of the infiltrated water is returned to the atmosphere by
evapotranspiration.
Transpiration
Precipitation
Precipitation
Evaporation
Uptake by roots Infiltration
Ov
er
lan
df
low
Int
er
f lo
w
Water table
Stream
Aquifer
Figure 1.10: Near surface hydrologic processes
1-20
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 1
1.7
Utilization of groundwater in the Southern African Development
Community (SADC)
The Southern African
Development Community is a regional grouping of 14
sovereign states with the main objective of fostering co-operation and mutual
benefit from the resources of this region. The SADC member states are shown in
Table 1.2. Several SADC member states have limited water resources.
Furthermore, some of the available groundwater and surface water resources are
shared by two or more states, some of whom belong to the SADC and others are
not. Due to the transboundary nature of some of the water resources in the region,
SADC member states have long recognized the critical importance of water to
regional integration and economic development.
As a result, in 1996, SADC
established a Water Sector Co-ordination Unit.
The SADC Water Sector Co-ordination Unit, among others, collects, analyses,
and archives information on water use in each member state. Table 1.2 shows
statistics on water use established by the SADC Water Sector Co-ordination Unit
from country situation reports for the years 1996 to 1998 (Molapo et al., 2000).
Although these figures are slightly outdated and some are rough estimates, they
nevertheless show that groundwater is an important component of water use in
the SADC region, particularly in Botswana, Namibia, Mauritius, and South Africa
to some extent.
1.8
References
Bear, J. (1972) Dynamics of Fluids in Porous Media. Dover Publications, New
York.
Freeze, R. A., and J. A. Cherry. (1979) Groundwater. Prentice Hall, New York.
Hornberger, G. M., J. P. Raffensperger, and P. L. Wiberg. (1998) Elements of
Physical Hydrology. The Johns Hopkins University Press.
Kresic, N. (1997) Quantitative Solutions in Hydrogeology and Groundwater
Modelling. Lewis Publishers, New York.
Molapo, P., S. K. Pandey, and S. Puyoo (2000) Groundwater Resources
Management in the SADC Region: A Field of Future Regional Co-operation. In
Sililo et al. (eds) Proceedings of a Conference on Groundwater: Past
Achievements and Future Challenges, Balkema, The Netherlands, pp.981-986
1-21
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 1
Tarbuck, E. D., and F. K. Lutgens. (1999) Earth: An Introduction to Physical
Geology. Prentice Hall, New York.
Tindall, J.A., J. R. Kunkel. (1999) Unsaturated Zone Hydrology of Scientists and
Engineers. Prentice Hall, New York.
Table 1.2.
Water use and part of groundwater in the SADC countries (Molapo et al., 2000)
SADC
Domestic water supply
Part of
Total water use
Part of
Member
Surface &
Groundwater
groundwater
Surface &
Groundwater
ground-
states
ground-
only
in domestic
ground
only
water in
water
106 m3/s
water supply
water
106 m3/s
total water
%
106 m3/s
106 m3/s
use
Angola
130
28
22
2,474
35
1
Botswana
36
15
41
119
76
64
D.R. Congo
-
-
-
-
-
-
Lesotho
19
11
58
37
15
41
Malawi
120
35
29
1,161
35
3
Mauritius
170
80
47
620
101
16
que
107
36
34
630
36
6
Namibia
144
53
37
278
140
50
Seychelles
-
-
-
-
-
-
South Africa
2,128
319
15
18,965
2,844
15
Swaziland
24
8
33
1,716
40
2
Tanzania
263
66
25
2,423
108
4
Zambia
271
75
28
2,221
189
9
Zimbabwe
410
40
10
3,930
390
10
SADC
3,823
766
20
34,574
4,009
11.6
Mozambi-
Figures in bold italics are rough estimates
1-22