GEO307: HYDROGEOLOGY AND WATER RESOURCES
LECTURE NOTES PART I
ACADEMIC YEAR: 2022-2023
Dr. T. R. CHAOKA
DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES
BOTSWANA INTERNATIONAL UNIVERSITY OF SCIENCE AND TECHNOLOGY
PALAPYE, BOTSWANA
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THE SCIENCE OF HYDROGEOLOGY
1.1
Basic Definitions
Hydrogeology (Groundwater Hydrology) is the branch of hydrology that deals
with the study of groundwater.
The other two branches of hydrology are
hydrometeology and surface water hydrology.
What is groundwater?

Groundwater is subsurface water that exists at fluid pressures equal to or
greater than atmospheric pressure. The vadose zone is part of the
subsurface that contains water but the fluid pressure in the vadose zone is
less than atmospheric pressure (water in the vadose zone cannot be
pumped out of the ground.

Some hydrogeologists divide the subsurface into the unsaturated zone
and the saturated zone and go on to say that groundwater is water that
occurs in the saturated zone. While this is correct to some extent, we must
realize there is a saturated zone in the subsurface (e.g. the capillary
fringe) in which the fluid pressure is less than atmospheric pressure.
Water in the capillary zone is not groundwater.

The zone in which groundwater occurs is also called the phreatic zone.
The boundary between the vadose zone (capillary fringe) and the phreatic
zone is called the water table when the water in the subsurface is open to
the atmosphere. The fluid pressure at the water table is exactly equal to
atmospheric pressure.

Groundwater is found inside openings within unconsolidated and
consolidated geological formations. There are many types’ openings in the
subsurface. Some are interconnected, while others are not. The openings
in the subsurface include pores, joints, fractures, bedding planes, faults,
interstitial openings between rock fragments and mineral grains.
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Classification of subsurface water according to depth of occurrence.
Soil physicists and hydrologists have divided the subsurface into several zones
as shown below in Figure 1. As shown Figure 1, the zone below the ground
surface is divided into the vadose zone and the saturated zone. In actual fact this
zone should be divided into the zone above the bottom of the capillary fringe (i.e.
the vadose zone) and the zone below the capillary fringe (i.e. thezone in which
groundwater is found/occurs). The fluid pressure in the zone below the capillary
fringe is equal to or greater than atmosphere. This is the zone in which
groundwater occurs. Above this zone is the vadose zone in which the fluid
pressure is less than atmospheric pressure. From the groundwater hydrology
stand point, it is incorrect to divide the subsurface into the unsaturated zone and
the saturated zone because the capillary fringe and the zone below it are both
saturated with water. Water in the vadose zone is sometimes referred to as
vadose water.
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Figure 1. Classification of subsurface water according to depth of occurrence
Figure 2. Classification of subsurface water (from Chilton and Seiler, ?)
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0
Ground surface
+ve
+ve
-ve
0
+ve
Unsaturated
or Vadose
zone
Soil water zone
Intermediate
zone
Saturated
zone
depth
Capillary fringe
Water table
Groundwater
=
(a)
(b)
(c)
Figure 3. a) Zones of subsurface water, (b) profile of moisture content ,and (c)
profile of pressure head  versus depth. Diagrams (b) and (c) are from Freeze,
R.A. and J. A. Cherry (1979, see Figure 2.12) Groundwater. Englewood Cliffs,
N.J., Prentice-Hall. Please download a copy of Freeze and Cherry’s it is available
on the internet.
The symbol Ө is used for the volumetric moisture content which is defined as the
volume of water in a sample of a porous medium divided by the volume of the
sample.
Ө=
Vw
Vt
Ө is less than or equal to porosity. Why?
Characteristics of the Vadose Zone

the vadose zone typically extends from the earth’s surface to the bottom of
the capillary fringe (but this profile may be temporarily reversed, especially
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after a precipitation event). As already stated, it is the zone in which the fluid
pressure is less than atmospheric. Soil physics is primarily concerned with the
study of this zone.

the thickness of the vadose zone varies from place to place depending on
climatic conditions.

the pores, fractures, and other openings in the vadose zone (above the
capillary fringe) are partly filled with water and partly filled with air. This
statement, however, does not apply to the capillary zone which is
part of the vadose zone, but is completely saturated.

Pore water pressure in the vadose zone is less than atmospheric pressure
(i.e., Pw < Patm).
The Soil Water Zone

The soil water zone zone provides water for the growth of vegetation and
is of great interest to agricultural scientists, foresters, and soil physicists

Water in the soil water zone exhibits diurnal (daily) fluctuations as a result
of evapotranspiration. What is evapotranspiration and when does it take
place?
The Intermediate Vadose Zone

This zone occurs below the root zone (does evapotranspiration take place
above or below the root zone? What is the maximum depth at which
evapotranspiration take place called? As in the soil water zone, the void
spaces in the intermediate zone are partly filled with water and partly filled
with air.
The Capillary Fringe (capillary zone)

The capillary fringe extends above the zone in which groundwater occurs.
Even though it is completely saturated, water in the capillary zone exists at
fluid pressures below atmospheric pressure.
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Groundwater

Groundwater is subsurface water in a zone that is completely saturated
with water and in which fluid pressure is equal to or greater than
atmospheric pressure. The water table is the surface at which the fluid
pressure is equal to atmospheric pressure.

Pore water pressure in groundwater is greater than atmospheric pressure
and all the openings are filled with water

In the absence of overlying impermeable strata, the water table forms the
upper surface of the groundwater zone
The following point is worth repeating:
Definition
Groundwater refers only to the water in the saturated zone in which pore water
pressure is greater than or equal to atmospheric pressure. As there are no sharp
boundaries between the different zones, the groundwater (saturated) zone can
only be distinguished from the vadose zone on the basis of fluid pressure relative
to atmospheric pressure. McWhorter and Sunada (1977) state that “the term
groundwater is used to denote subsurface water that exists at pressures greater
than or equal to atmospheric pressure”
Measurement of Fluid Pressures in the Unsaturated Zone
Fluid pressures in the unsaturated zone are measured with a device known as a
tensiometer (but there are other instruments are used). A tensiometer consists of
a (connecting) tube, which is fitted at one end with a liquid-filled porous ceramic
cup, and at another end with a vacuum gauge (see below). 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.
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Vaccum gauge
Connecting tube
Ground level
Porous cup
Figure 4. Tensiometer with vacuum gauge, porous cup, and
connecting tube.
What is groundwater hydrology (hydrogeology)?
Groundwater hydrology (hydrogeology) is the study of groundwater (subsurface
water that exists at fluid pressures equal to or greater than atmospheric
pressure). The scope of hydrogeology is very broad. The study of groundwater
covers, but is not limited to:

the distribution, occurrence, quality and quantity of groundwater

the laws governing the movement of groundwater

the physical and chemical interactions between groundwater and geologic
formations

the origin of groundwater

groundwater exploration, evaluation and recovery

groundwater contamination; transport of chemical constituents by
groundwater

the role of groundwater in environmental and geotechnical problems

the role of groundwater in geological processes
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Classification of Subsurface Water with Respect to Origin
Figure 5. Different genetic types of water (from Singhal and Gupta: hydrogeology
of fractured rocks)
How are isotopes used to distinguish between these different types of water?
Which isotopes are used for this purpose?
Meteoric water includes water currently in the atmosphere as well as relatively
young groundwater that originated as surface water or as atmospheric water
(precipitation)
Connate Water refers to groundwater that has been out of contact with the
atmosphere for a considerable amount of geologic time (millions of years). It may
or may not have been buried with sediments at the time of deposition. In arid
regions, connate water represents the past pluvial climate. Define the term
residence time respect to the occurrence of water in the different reservoirs that
constitute the hydrologic cycle.
Magmatic water is water associated with magma.
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Juvenile water or new water is water that is associated with magma, but has
never circulated in the hydrologic cycle
Metamorphic water is water that has been associated with rocks during
metamorphism.
Classification of Water by Salinity (TDS)
Another important classification of water is by salinity. Salinity or total dissolved
solids (TDS) is the total amount of solids (in mg/l) remaining when a water
sample is evaporated to dryness.
Table 1. Classification of water according to salinity.
______________________________________________
Water
TDS (mg/l)
______________________________________________
Fresh water
<1 000
Slightly saline
1 000 - 3 000
Brackish (moderately saline)
1 000 - 10 000
Very saline
10 000 - 35 000
Sea water
35 000
Brine
>>35 000
______________________________________________
Based on the table above, which of these waters meet the water quality standard
for drinking water in Botswana?
Classification of Water by Hardness
Hardness is a measure of the capacity of water to react with soap (hard water
requires considerably more soap to lather) to produce a lather. Hard water often
produces a noticeable deposit of precipitate in containers, pipes, and “bathtub
ring”. “It is not caused by a single substance but by a variety of dissolved
polyvalent metallic ions, predominantly calcium and magnesium cations,
although other cations (e.g. aluminium, barium, iron, manganese, strontium and
zinc) also contribute. Hardness is most commonly expressed as milligrams of
calcium carbonate equivalent per litre. Water containing calcium carbonate at
concentrations below 60 mg/l is generally considered as soft; 60–120 mg/l,
moderately hard; 120–180 mg/l, hard; and more than 180 mg/l, very hard
(McGowan, 2000).
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The hardness of water refers to the concentration of ions in the water that will
react with soap to form precipitates and with certain anions in the water to form
scale (insoluble residue). Total hardness, HT, is usually expressed as mg/l of
equivalent CaCO3
CaCO3
CaCO3
 Mg
Ca
Mg
100.09
100.09
H T  Ca
 Mg
40.08
24.05
H T  Ca
where HT, Ca, and Mg are in mg/l and the ratios in equivalent weights.
Table 2. Classification of water according to hardness
______________________________________________
water quality
TDS (mg/l as CaCO3)
______________________________________________
soft
0 - 60
moderately soft
61 - 120
hard
121 - 180
very hard
> 180
______________________________________________
Water occurs in the atmosphere above the surface of the earth, on the surface of
the earth, and below the surface of the earth. It is continuously circulating
between the atmosphere, the earth’s surface, and below the surface of the earth.
This is what constitutes the hydrologic cycle. The various components of the
hydrologic cycle are further divided into components.
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GROUNDWATER IN THE HYDROLOGIC CYCLE
Atmosphere
ET
Vegetation
P
E
E
P
P-In
Land Surface
Ro
I
Soil
Rg
Rivers and Lakes
Q
Qs
Qg
Qg
Ground Water
Ocean
Figure 6. 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).
The hydrologic cycle refers to the interconnected cyclic movement of water
between the atmosphere, earth’s surface, and the region below the earth’s
surface. Water above the surface of the earth is found in rivers, ponds, wetlands,
and oceans. There is also water below the surface of the earth in a zone in which
the fluid pressure is less than atmospheric pressure. The vadose zone is a
subsurface zone in which the fluid pressure is less than atmospheric pressure.
Groundwater constitutes an insignificant percentage of the earth's total water
balance. However, as far as the world's freshwater resources are concerned,
groundwater accounts for almost 100% of the utilizable (potable) freshwater
resources (excluding icecaps and glaciers).
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The amount of water in the different components of the hydrologic cycle is shown
in Table 2 (from Chilton and Seiler, ?).
Table 2. Percentage of water in the various components of the hydrologic cycle
The residence time may be defined as the average length of time a water
molecule spends in various components of the hydrologic cycle.
Table 3 shows trends in water use (including groundwater use) in the SADC countries
(Molapo et al., 2000).
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Table 3. Trends in water use (including groundwater use) in the SADC countries
(Molapo et al., 2000)
SADC
Domestic water supply
Member
Surface &
states
ground
water
Part
of Total water use
Groundwat
groundwat
er only
er
106 m3/a
domestic
106 m3/a
Surface &
in
water
ground
water
Part
Groundwat
groundwat
er only
er in total
106 m3/a
water use
106 m3/a
supply
%
Angola
130
28
22
2,474
35
1
Botswana
36
15
41
119
76
64
D.R.
-
-
-
-
-
-
Congo
19
11
58
37
15
41
Lesotho
120
35
29
1,161
35
3
Malawi
170
80
47
620
101
16
Mozambi-
107
36
34
630
36
6
que
144
53
37
278
140
50
Namibia
-
-
-
-
-
-
Seychelles
2,128
319
15
18,965
2,844
15
South
24
8
33
1,716
40
2
Africa
263
66
25
2,423
108
4
Swaziland
271
75
28
2,221
189
9
Tanzania
410
40
10
3,930
390
10
3,823
766
20
34,574
4,009
11.6
Mauritius
Zambia
Zimbabwe
SADC
Figures in bold italics are rough estimates
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of
GROUNDWATER OCCURRENCE
Groundwater may be found nearly everywhere (in the world) in most geological
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 hydrogeologists and/or a geophyscists is to locate
geological formations with sufficient water of reasonable good quality for a
particular use.
Hydrogeological Classification of Saturated Geological Formations (in
which the fluid pressure is equal to or greater than the atmospheric
pressure).
From a hydrogeological point of view, saturated geological formations in which
water exists at fluid pressures equal to or greater than atmospheric pressure can
be divided into aquifers, aquitards, and aquicludes, depending on whether they
are capable of yielding sufficient or usable quantities of water to wells.
An aquifer is a saturated geological formation that contains sufficient
permeable material to yield significant quantities of water to wells under
ordinary hydraulic gradients. In simple terms groundwater in an aquifer can be
easily withdrawn using pumps, buckets, etc.
An aquitard is a saturated geologic formation that is insufficiently
permeable to be considered an aquifer, but contains appreciable amounts of
water. The response of an aquitard to pumping in an overlying or underlying
aquifer is very important.
An aquiclude is a saturated geologic formation that is incapable of
transmitting groundwater
NOTE:
The distinction between an aquifer and an aquitard depends on local conditions,
and also include availability of water
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Types of Aquifers
Aquifers may be divided into three main types: confined, unconfined, and
leaky. A confined aquifer is bounded above and below by an aquitard/aquiclude.
water level
aquiclude
Confined
aquifer
aquiclude
Figure 7a. Conceptual model of a confined aquifer (from Kruseman and de
Ridder, 1994). A conceptual model is a simplified pictorial representation
of an aquifer.
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. A similar surface in an
unconfined aquifer is called a water table.
An unconfined aquifer, also known as a water table aquifer, is bounded below
by an aquitard/aquiclude and above by a water table, which is the surface joining
water-level elevations in wells tapping an unconfined aquifer.
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water level
watertable
Unconfined
aquifer
aquiclude
Figure 7b. Conceptual model of an unconfined aquifer.
In an unconfined aquifer the fluid pressure at the depth at which groundwater is
encountered (the so-called water strike) is exactly equal to atmospheric pressure.
Furthermore, the water strike and the water table are at the same depth.
However, during pumping the water table falls below the water strike.
In
contrast, in a confined aquifer the fluid pressure at the depth at which water is
encountered is higher than atmospheric pressure. How is fluid pressure in a
cylinder filled with water calculated? This is the reason why water levels in
confined aquifers are higher than water strikes.
There is a special type of unconfined aquifer known as a perched aquifer.
Perched aquifers are formed where lenses of low-permeability material occur
within otherwise permeable material, but above the water table. The lowpermeability material does not allow infiltrating water from the ground surface to
pass through it.
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perched aquifer
water-table
Unconfined
aquifer
Aquitard
Leaky aquifer
Aquiclude
Confined
aquifer
Aquiclude
Figure 8. Occurrence of different types of aquifers in a multi-layered aquifer
system
A leaky aquifer or semi-confined aquifer is a type of confined aquifer in which
one or both boundaries are aquitards. During pumping, water from overlying or
underlying aquifers is free to move through the aquitard(s) into the overlying or
underlying aquifer.
It is important to appreciate what confinement really means with respect to the
aquifer whose upper water surface (water table) is at atmospheric pressure,
whereas a confined aquifer is an aquifer whose upper water surface is at a fluid
pressure that is higher than atmospheric pressure.
Dual porosity aquifers and fractured aquifers: these are aquifers that consist
of porous matrix blocks (blocks of fractured rock) and/or fractures.
The preceding classification may be further refined by considering the types of
openings in which groundwater occurs. These openings are of various types,
such as interconnected pores between rock fragments and mineral grains,
fractures, joints, bedding planes, solution cavities, etc.
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The classification of aquifers on the basis of the degree of confinement depends
largely on the ability of the driller to make accurate assessments of the water
strikes relative to the static water level.
Understanding diagnostic plots for well test interpretation by Renard et al.
(2009) offers a different and more objective classification of aquifers. This
is a very nice paper in which the classification of aquifers is based on
diagnostic plots of the derivative of the drawdown with respect to the
derivative of the natural log of time since pumping started versus time
since pumping started.
Aquitards do not impede the movement of groundwater from one aquifer to
another in a multi-layered aquifer (Figure 8). They are, together with
aquiclude are incapable of transmitting large quantities of water to wells..
Aquifers may also be distinguished on the basis of diagnostic plots of the
derivative of the drawdown with respect to the derivative of the natural log
of time since pumping started versus time since pumping started (Renard
et al. 2009).
Properties of Porous Media
Porosity ()
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

Vv
V
Where Vv is the volume of the voids and V is the total volume of material. It may
be expressed as a decimal fraction or as a percentage. Porosity ranges from 1
20
percent to as much as 80 percent in some recently deposited clays, but in most
granular materials it falls between about 5 to 40 percent. 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.
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Secondary Porosity: Fractured bedrock (the top of the bedrock is highly
weathered)
(a) Fractured rock (matrix blocks have zero porosity
(b) Fractured rock (major and microfractures present)
(c) Double porosity (Fractures constitute secondary porosity, while matrix
porosity constitutes primary porosity)
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