Download - Cec 207 Theory - Hydrogeology
1
UNESCO-NIGERIA TECHNICAL & VOCATIONAL EDUCATION
REVITALISATION PROJECT-PHASE II
YEAR 2- SE MESTER I
THEORY/PRACTICALS
Version 1: December 2008
NATIONAL DIPLOMA IN
CIVIL ENGINEERING TECHNOLOGY
HYDROGEOLOGY COURSE CODE: CEC 207
2
CIVIL ENGINEERING TECHNOLOGY HYDROGEOLOGY CEC 207 COURSE INDEX WEEK 1 1.0 INTRODUCTION 3
1.1 SOUCES OF GROUNDWATER 4
1.2 USES OF GROUNDWATER 6
1.3 EFFECTS OF GROUNDWATER ON ENGINEER-
ING CONSTRUCTION 6 1.4 GROUNDWATER FLOW 7 WEEEK 2 2.0 FACTORS AFFECTING MOVEMENT OF WAT- ER IN SOILS 8 2.1 DARCY’S LAW 9 WEEK 3 3.0 GROUNDWATER BEARING FORMATIONS 13 3.1 AQUIFER GEOLOGIC FORMATIONS 14 3.2 CATEGORIZATION OF AQUIFERS 15 WEEK 4 4.0 FLOW PATTERN OF AQUIFERS 19 4.1 GROUNDWATER EXPLORATION 22 4.2 GROUNDWATER EXPLORATION TECHNI- QUES 22 WEEK 5 5.0 SURFACE GEOPHYSICAL METHODS 26 WEEK 6 6.0 SEISMIC METHOD 33 6.1 SURFACE INVESTIGATION OF G/WATER 35 WEEK 7 7.0 RESISTIVITY LOGGING 39 WEEK 8 8.0 WATER TABLE AND G/WATER EXPLOITA- TION 42 8.1 FACTORS AFFECTING AQUIFER YIELD 43 WEEK 9 9.0 GROUNDWATER EXPLOITATION TECHNI- QUES 48 9.1 DUG WELLS 48 9.2 BORED WELLS 49 9.3 DRIVEN WELLS 50 WEEK 10 10.0 DEEP WELLS 52 10.1 INFILTRATION GALLERIES 53 10.2 ARTESIAN WELLS 54 WEEK 11 11.0 GROUNDWATER QUALITY 55 11.1 SOURCES OF IMPURITIES IN G/WATER 56 WEEK 12 12.0 CAUSES OF SPECIFIC TYPES OF IMPURI- TIES 60 12.1 TOLERANCE LIMITS OF IMPURITIES 62 WEEK 13 13.0 EXAMPLE 13.1 69 WEEK 14 14.0 EXAMPLE 14.2 74 WEEK 15 15.0 COMMON G/WATER CONTAMINANTS 80
3
HYDROGEOLOGY (CEC 207)
WEEK 1
1.0 INTRODUCTION
Hydrology is the study of the Circuit of water occurrence. This is
illustrated by the hydrologic cycle. Water occurring below the
ground surface (sub surface water) is an integral part of the
endless circulation of water.
Hydrogeology is based on the subsurface aspect of the hydrologic
cycle. Under ground water is made up of water in unsaturated zone
(Vadose zone) and water in fully saturated zone. (Ground water
phreatic water). The term ground water is used for subsurface
water existing below the point where pressure is equal to
atmospheric and geologic formations that are fully saturated. This
line differentiates between the two types of subsurface water.
Hydrogeology is defined as the study of ground water or as the
science of the occurrence, distribution and movement of water
below the surface of the earth. It also involves studying the quality
(the chemistry) and relation of ground water to the geologic
environment.
4
The subdivision of underground water is illustrated below
Fig. 1.1: General Distinction of Subsurface Water
In the Vadose (aeration) zone the voids are filled with water and air
and terminates at the ground surface. Water entrapped in this zone
is of importance for agricultural purposes. Water in the zone of
saturation is considered in engineering works, geologic studies and
water supply developments.
1.1 SOURCES OF GROUND WATER
Ground water is principally derived from the origin of the hydrologic
cycle. Thus atmospheric precipitation is the main source of fresh
ground water. Specific areas of the earth crust with water bearing
capacity acts as conduits for transmission of and as reservoir for
storage of water. Virtually all ground water originates as surface
water.
Bed Rock
Zone of Saturation
Water table
Vadoze or water in unsaturated water zone
Ground water
5
The source of ground water can be either natural or artificial. The
natural sources are: -
i. Precipitation
ii. Stream flow
iii. Lakes
iv. Sea waters or marine water: This is water that has moved
into aquifers from oceans.
v. Juvenile water (primary water). This is subsurface water
which are not originally part of this hydrologic cycle and are
formed within the earth and of a volcanic or magmatic origin.
vi. Magmatic water are a derivative of magma and include
volcanic water (shallow magma) and plutonic water (deep
magma).
vii. Metamorphic water, water that were in rocks during the
period of metamorphism.
Artificial Sources
i. Water from excess irrigation
ii. Seepage from canal
iii. Direct supply of water to shore up ground water supply.
iv. Reservoirs
There are referred to as sources of recharge of ground water.
The motion of ground water through the saturated zone is in a
direction determined by the surrounding hydraulic situation.
6
1.2 USES OF GROUND WATER
Ground water is an important water resource employed to meet
water requirement in varied areas. The specific uses include: -
i. Irrigation: - This is the largest application of ground water. It
involves the use of irrigation wells that are dug into zones of
saturation.
ii. Industrial Uses: - Due to its unique properties ground water
is used in oil refineries, paper manufacturing, metal working
plants, chemical manufacturing, air conditioning, refrigeration
units and distilleries.
iii. Municipal water supply.
iv. Rural Water supply: This is meant by the use of hand dug,
bored or driven wells.
1.3 EFFECTS OF GROUND WATER ON ENGINEERING
CONSTRUCTION
For most Engineering construction work a basic understanding of
subsurface water occurrence and flow pattern is necessary.
Ground water, depending on the water table, affects the structural
performance/integrity of most sub structures, foundations and
basements. As an example, the foundation of most buildings are
designed such that the depth of footing is taken to a suitable
bearing stratum.
7
The design of buried utilities including pipelines, communication
lines and cables are done to cut them off from interference of
underground water. The amount of settlement/consolidation
expected of a given structure (foundation/footing) is influenced by
the ground water level, there is increase settlement under
saturated conditions with equal loading on the structure.
1.4 GROUND WATER FLOW
Ground water is usually in a state of constant motion. The
movement of ground water is usually subject to surrounding
hydraulic conditions and hydraulic theories. This is basically
facilitated by the fact that most ground water bearing formation
(aquifers etc) are porous media.
The flow of water through soils which as typified by ground water flow is
usually laminar.
8
WEEK 2
2.0 FACTORS AFFECTING MOVEMENT OF WATER IN SOILS
The major factors affecting movement of water through soils are
the permeability, porosity and hydraulic gradient.
Permeability is the measures of the ease of flow through a given
medium or the ability of the soil medium to conduct water. The
hydraulic gradient, ¿, is the difference in energy levels (heads) of
water flowing through a soil mass. Thus ground water moves from
levels of higher energy to levels of lower energy.
Other factors which affect flow of water through soils include:
Porosity: - The percentage of voids present in a material given
by 1.2−−−−−−−−−−−−−−−−−−−−=Vo
Vvn
Where Vv – volume of voids
Vo – total volume of porous medium
And Vo – Vs + Vv -------------------------------2.2
Where Vs – volume solids.
9
For consolidated materials, n depends on degree of
documentation, state of solution and fracturing of the rock.
For unconsolidated materials, n depends on packing of grains,
shape, arrangement and size distributions.
2.1 DARCY’S LAW
It has been experimentally observed that the flow of water through
a sand medium is accompanied by an energy/head loss. This loss
has being proven to be proportional to the velocity of flow or
discharge/flow rate, Q.
This is more appropriately defined as Darcy’s Law which holds that
the flow rate, through a porous media, or velocity of flow is
proportional to the head loss and inversely proportional to the
length of flow path. Simply the flow velocity is proportional to the
hydraulic gradient and is given as:
Q/A = K. ∆h-----------------2.3 ∆1
Where Q – Discharge
A – X – sectional area of porous medium
K – constant of proporationality
∆h – head loss
∆1 – length of flow path.
∆h ∆1 – hydraulic gradient.
10
Q/A – velocity of flow.
This is simulated by the experimental set-up shown below for flow
through packed sand contained in a cylinder of cross-sectional area, A.
having piezometers spaced ∆1 m apart.
h1
Fig. 2.1: Head loss in flow through a sand column.
We have that
∆h = h1 – h2 are total energy heads at points 1 and 2 respectively.
From Bernoulli we have that
H1 = P1/Υ + Z1
And h2 = P2/Υ + Z2
Since we are connecting velocity head
Z1 Z2
h2 (2)
(1)
∆h = h1 - h2
∆L
P1/dz P2/dz
11
V2/2g
Total energy above the datum plane is given by
P1 + Z1 = P2 + Z2 + ∆h
Y Y
∆h = (P1 + Z1) – (P2 + Z2)
Y Y
h1 – h2
From Darcy we have that
Q/A α ∆h and Q/A α1/∆L
Q/A = K. ∆h/∆L
Where K is the constant of proportionality or coefficient of permeability.
Thus discharge through porous media is given by
Q = KA ∆h/∆L-------------------------------------2.4
∆h/∆L is defined as the hydraulic gradient and denoted by i or s
Table 2.1: Representative Values of K for Different Materials
Rock type App. Co-efficient of (cmls) permeability
Discharge Capacity
Clean gravel 5 – 10
12
Coarse sand Fine sand
0.4 – 0.02 Good
Silt sand + gravel Silty sand
10-5 – 10-4 Fairly Good
Sandy clay Colloidal clay
10-6 – 10-9 Poor
13
WEEK 3
3.0 Ground Water Bearing Formation – Aquifers
It has been shown that water beneath the water table or in the
phreatic zone is termed ground water. The material in these zone
must be formations having structures that permit appreciable water
to move through them under ordinary field condition.
The permeable geologic formations in which ground water occur
are defined as aquifers. They are alternatively referred to as
ground water reservoirs or water bearing formation. Only small
fractions of most phreatic zone will yield significant amount of water
to wells.
Aquiclude: Impermeable formation which may contain water but is
incapable of transmitting significant water quantities e.g. clay.
Aquifuge: An impermeable medium, like solid granite, which
neither contains nor transmits water.
Aquitard: Natural material that stores water but does not transmit
enough to supply individual wells e.g. silty clay.
Ground water retention and transmitting capabilities of most of the
formation diversified is facilitated by the presence of spaces not
occupied by solid mineral matter. These spaces retention and
transmitting capabilities of most of the formation diversified is
facilitated by the presence of spaces not occupied by solid mineral
14
matter. Theses spaces termed voids, interstices, pores or pore
space acts as storage and conducts of ground water in aquifers.
3.1 AQUIFER GEOLOGIC FORMATIONS
A significant proportion of all formed aquifer are made up of
unconsolidated rocks, mainly gravel and sand. These occur as:
Water Courses: These are the alluvium that underlie stream
channels and form adjacent flood plains.
Buried or Abandoned Valleys: Valleys no longer occupied by
streams.
Plains: There are usually underlain by unconsolidated sediments.
The aquifers under such plains are composed of graved and sand
beds.
Intermontane Valleys: Aquifers made up of tremendous volumes
of unconsolidated rock materials derived by erosion of bordering
mountains.
Alluvial: Made of sand and earth that is left by rivers or floods.
Generally, aquifers can develop from limestone in which a
considerable proportion of the original rock has been dissolved and
removed.
Permeable aquifers can also be formed from volcanic rocks for
which permeable zones might be due to flow breccias, porous
zones between lava beds, lava tubes, shrinkage cracks and joints
15
sand store aquifers develop occur from sands and gravel on which
the constituent particles are partially cemented or in which water
yield are from their joints. Clay aquifers only provide enough yield
of water to meet small domestic demands.
3.2 CATEGORIZATION OF AQUIFERS
Aquifers are classed as either confined or unconfined depending
on the presence of a restraining medium or phreatic surface above
the water bearing medium.
Confined aquifers are restrained or overlain by comparatively
impermeable strata. They are also referred to as artesian or
pressure aquifers and they are usually confined at pressures
greater than atmospheric.
Free, phreatic or non artesian aquifer are unconfined aquifers in
which water table serves as the upper boundary of the saturation
zone. The water level coincides with the points at which the
pressure equals atmospheric and at varies according to areas of
recharge and discharge, Pum page from wells and permeability.
There are as idealized in the figure shown.
Artesian well – hole made in the ground through which water rises
to the ground surface by natural pressure.
Phreatic describes soil or rock below the water level, in which all
the pores and inter-granular spaces are full of water.
Recharge area
16
Fig. 2.3: Confined and unconfined aquifers
The following points should be noted
i. The recharge area serves as the region of supply of water
enters a confined aquifer in areas where the restraining bed
rises to the surface. Where this bed ends under ground an
unconfined aquifer results.
Impermeable Strata
Confined aquifer
Unconfined aquifer
Water table
Water table
Ground surface
Piezometric surface
Flowing well
17
ii. The piezometric surface of a confined aquifer as an
imaginary surface coinciding with the hydrostatic pressure
level of the water in the aquifer.
iii. A flowing well is the resultant effect of a piezometric surface
which lies above the ground surface.
iv. A perched aquifer results when a ground water body is
separated from the main ground water by relatively
impermeable stratum of small area extent. This is typified by
clay lenses in sedimentary deposits which often have shallow
perched water bodies overlying them.
Perched aquifer
Impermeable strata
Perched aquifer
Ground surface
Water table
18
Fig 2.4: Perched aquifers within an unconfined aqui fers.
Additionally the piezometric surface is to the surface to which water
would rise in the confined aquifer if it could.
Unconfined aquifer
19
WEEK 4
4.0 FLOW PATTERN IN AQUIFERS
Entrance of water into aquifers, which serve as underground
reservoirs, is through natural or artificial recharge. The flow of
water through aquifers is under the influence of gravity and thus the
flow is aligned along the induced hydraulic gradient. This owes to
the differential created by water existing at different elevations
which corresponds to pressure levels.
Specifically for underground water flow in aquifers with defined
extremes the flow pattern can be simulated by the use of flow
rates. This is the resultant of the plot of the flow lines, depicting the
direction of flow and the lines joining all points of equal pressure
(equal-potential lines).
There are mutually perpendicular (orthogonal) set of line and is as
shown below
20
Fig. 2.5: Part of a typical flow net developed from ψψψψ and
ΦΦΦΦ lines
Apart from illustrating the direction and pattern of flow in an aquifer
flow rate are near accurate means of evaluating amount of
discharge from a given waters bearing stratum as proved.
For the flow net shown the hydraulic gradient is given by
i = dh/ds
Where dh – change in pressure across each square formed by the
flow net ds = length of square
The flow through each square, between two flow lines is
dq = K dh/ds dm : - Azdm x 1, Q = KIA = Kdh/ds x dm x 1
dm
dm
h h – dh
dq
dq
ds ds
Equal potential
Line ( ΦΦΦΦ)
Flow lines ( ψψψψ)
21
This for square thickness (Lar to page) as derived from Darcy’s (Q
= KiA)
For each of the squares of the flow net it is assumed that
ds dm
thus
dq = K dh
But dh = h/x For a total head drop of h across n squares)
Thus for a flow net of m channel or m + 1 flow lines, total flow Q is
given by Q = mdq = Km h/n-------------------2.6
= K x Nf/Nd x Hw------------------------------------2.7
Where K – Permeability constant
Nf – of flow channels
Nd – Number of pressure drops
Hw – Total pressure head.
For the special cases of flow through aquifers, flow nets are
constructed using a contour map of static water table levels since
flow is induced by the different level of water table, flow lines are
direction of movement. Flow lines parallel impermeable
boundaries, for confined aquifers, because no flow crosses such
extremes and no flow crosses the water table of an unconfined
aquifer.
22
4.1 GROUND WATER EXPLORATION
Ground water exploration methods are used to determine the
location, movement and quality of water in geologic formations. It
also aids in determining thickness. Composition, permeability and
yield of ground water for large scale usage.
Proper exploration techniques is used to estimate the qualitative
and quantitative parameters of water bearing zones within the earth
crust and other impermeable and non-retaining geologic structures.
4.2 GROUND WATER EXPLORATION TECHNIQUES
The main methods for investigating ground water are surface and
subsurface techniques. Surface methods involve studying ground
water occurrence by working from the surface while subsurface
investigation entails a detailed underground survey of ground water
and conditions governing its occurrence. The two methods
supplement each other and for a thorough investigation, surface
survey can serve as a sort fo reconnaissance.
4.3 SURFACE INVESTIGATION OF GROUND WATER
The various type of surface investigation are: -
i. Geologic methods
ii. Hydrologic methods
iii. Surface geophysical methods.
Geologic Methods
23
This gives preliminary information of the occurrence of subsurface
water in a short time. It is suited for areas of complex geology with
difficulty in locating water bearing zones as opposed to areas of
uniform water bearing formations (thickness and depth).
Investigation is carried out by the use of aerial photographs,
regional geologic maps and rapid ground reconnaissance.
The specific geologic methods are: -
1. Petrography: - Involves appraising rock types with regards
to porosity and permeability. These are the parameters
controlling the amount of water that can be stored and
transmitted through different materials. It is complemented
by hydrologic maps showing surface extent of various rock
types (lithologic units) and their water bearing characteristics.
2. Statigraphy: - The study of the position and thickness of
water bearing regions as well as presence and extent of
confining strata (beds).
3. Structural Geology: - This gives an indication of displaced
water bearing zones due to earth movement and fractured
areas in dense brittle rock.
4. Geomorphology: - Used in locating areas of glacial
sediments and studying occurrence of subsurface water in
areas of recent deposits.
24
The specific information obtained from geologic work include extent
and regularity of water bearing formation, magnitude of water yield
from aquifers, occurrence of aquifers beneath unsuitable upper
strata, continuity and interconnection of aquifers and aquifer
boundaries.
Basically, ground water occurrence is directly dependent on
geologic structure.
Hydrologic Methods
This is used mainly to recharge, ease of recharge as well as
location and quantity of ground water discharge at the surface.
The probability of ground water discovery increases with available
recharge. This is also a function of the ease of recharge because
recharge is a measure of the infiltration capacity of the surface.
Thus impermeable surface such as shale, clay and quartzite leads
to rapid and high run off in place of infiltration and hence
inadequate recharge.
Hydrologic and geologic investigation should be done concurrently
for enhanced result because a geologically adverse region may not
be suitable for ground water development even with favorable
hydrologic conditions.
25
WEEK 5
5.0 Surface Geophysical Methods
This is the scientific measurement of physical properties of the
earth’s crust for the purpose of investigation ground water. This is
based on the relationship between the values of measured physical
properties and the presence of ground water in the formation for
which physical measurement is done. It involves detecting
difference of physical properties.
The properties most commonly measured are density, magnetism,
electrical potential and resistivity, elasticity, seismic refraction and
electrical conductivity. The slight but distinct variation in the
measured quantities of these parameters are interpreted in terms
of geologic structure, rock type and porosity, water content and
water quality.
The different geophysical methods are; Electrical resistivity
method, electrical potential methods, seismic refraction method,
gravity and magnetic methods.
Electrical Resistivity Method
This is used to establish measured resistivity values of rock types
at different depths to give information on suitable water bearing
rock types. It is a widely employed means of geophysical
26
exploration method due to ease of operation and portability of
equipment.
The electrical resistivity of a formation limits the amount of current
passing through the formation and is given as
Q = RA/L or 2π a R = 2π a V/I
Where a – electrode spacing (in units of length)
A – cross – sectional area
L – length of material
V – potential
I – current
Resistivity for rock formations vary over a wide range based on
material, density, porosity, pore size and shape, water
content/quality and temperature.
The resistivity (apparent) Ra, increases with increasing porosity of
the material, decreasing water content and decreasing salt content
of water in the formation.
Resistivity Range for Different Materials
Resistivity (Ra) ( ΩΩΩΩ – m) Material type/characteristic
102 – 108 Igneous/Metamorphic rocks
>108 Solid igneous rock/quartzite
100 – 104 Sedimentary/unconsolidated rocks
< 1 Clay with salty water
27
15 – 600 Sand/gravel aquifers with salt water
15 – 20 Aquifers with high salt content
300 – 600 Aquifers with salt free water
<10 Aquifers with brackish/saline water
50 Fresh water.
For porous media depends largely on water content and quality
and thus control the for aquifers. Specifically for aquifers can
be expressed in terms of ground water resistivity Pw and porosity,
α in the relationship.
/w = 3 – α/2 α------------------------------------------3.1
Where – aquifer resistivity
w – ground water resistivity
α – porosity
Resistivity can be shown to vary with depth for different rock types,
hence measured resistivity values can give relative extent of
different rock formations. This is based on their distinct or range of
resistivity values.
Also since resistivity values of aquifers are distinct due to the
porosity and moisture content, resistivity measurement are reliable
methods of locating water bearing formations.
28
Resistivity values are obtained from potential differences and
current measurement. This is facilitated by placing electrodes in
the ground surface and connecting the relevant meters to measure
potential difference and current as shown in the arrangement
below.
Fig. 5.1: Electrical circuit for determining networ k of current
and potential lines
The network shown in formed from orthogonal current and potential
lines. There is an observed variation of apparent resistivity with
variation of electrode spacing. This is due to penetration of the
electric field with increased electrode spacing. Thus apparent
resistivity vary with depth but is restricted to relatively shallow
depth.
There are two practical arrangement for measurement of resistivity;
(a) Shlumberger arrangement (b) Wenner arrangement
I
V Current electrodes
Current electrodes
P C C
Voltmeter
Ammeter
Potential electrodes
Current lines
Equi potential lines
I (i)
29
Fig. 5.2: Electrodes arrangement for Ra measurement s
For case (i)
Qa = 2π a V/I------------------------------------------3.2
Where Qa – apparent resistivity
a – electrode spacing
V – potential
I – current
And for case (ii)
a a a
V
C C
Wenner
P P
b
V
I
C C
Shlumberger
(ii)
P P
L
30
Qa = π(L/2)2 – (b/2)2 V/I -----------------------------3.3 b
A plot of measured apparent resistivity at varying electrodes
spacing is made and can be interpreted to depict spacing
approximately as depth of rock types. This is illustrated below
showing resistivities of various layers with precise interpretation of
the geologic type as indicated.
Apparent resistivity, a ( 500Ω – m)
Soil and gravel + till
Soil/Sandy
Glacial till little sand and gravel
Precambrian Rock
Dep
th to
gro
und
surf
ace
(m)
150
30
15
3.0
1.5
100 500 100
150
30
15
3.0
1.5
Ele
ctro
de S
paci
ng (
m)
Fig. 5.3: Apparent resistivity of subsurface materi al
determined by the expanding electrode method
31
WEEK 6
6.0 Seismic Method : - This method is based on the different
velocity of travel of wave across different medium. The specific
method suited for ground water investigation is the seismic
refraction method which provides information on geologic formation
at relatively shallow depth.
Velocity of sound in underground material increases with
increasing density and water content. Thus seismic survey result
are used to interpret type, porosity and water content of the
material. The wave velocities are indication of geologic formation.
Alternation in seismic wave velocity are due to change in elastic
properties and the contrasts indicate demarcation of material
formation and boundaries.
The waves generate in seismic studies are induced by small
shocks applied at the earth’s surface either by the impact of a
heavy instrument (sledge hammer) or a small detonator and
measuring the time required for the resulting sound or shock wave
to travel known distance.
Consider the formation below with a saturated and unsaturated
zones delineated as shown.
32
Fig. 6.1 Seismic wave front advance
Shock wave is applied at S, the distance of separation between
saturated and unsaturated layers is d.
The velocity of the wave front in the unsaturated and saturated
zones are V1 and V2 respectively. This is depicted by the refraction
simulated below.
The distance d is determined thus:
Wave travel from S to B via path SB or
Indirectly via SDEB
Direct travel time
Saturated
Unsaturated
Ground Surface
S
D E
B
Boundary
Path of reflection
Path of reflection
r
i
V1
V2
33
t1 = SB/V1 = X/V1 x – distance from shot point to point of
observation
indirect travel time t2 is
t2 = SD/V1 + DE/V2 + EB/V1
But SD = EB = d/Cosi [d/SD = cosi SD = d/cosi]
t2 = 2d/V1cosi + DE/V2
And t1 = t2
x/V1 = 2d/V1cosi + DE/V2
but DE = x – 2d tani
x/V1 = 2d/V1cosi + x/V2 – 2d tani/V2
1212 /2/ VVVVxd +−=
6.1 SUBSURFACE INVESTIGATION OF GROUND WAT ER
Subsurface investigation of ground water is conducted from the
surface with equipment extending underground. It gives more
detailed and precise information of subsurface water occurrence.
They are mainly classified as Test drilling (wells) and logging.
Test drilling provides information on substrata in a vertical line from
the surface. Logging gives data on properties of the formation,
water quality, size of well cavity and rate of ground water
movement.
Test Drilling: - Test wells, made up of small diameter holes bored
to ascertain geologic and ground water conditions before proper
34
well drilling. Subsequently, successful test holes are red riled or
reamed to a larger diameter to form pumping wells. Observation
wells are often test holes and are used for measuring water levels
or for conducting pumping tests.
Logging: - Logging is the method of stratifying the different kinds
of geologic formation and their variation with depth from surface. It
result in a graphical inventory of the different kinds of rocks within a
certain station and their relative positions and extents.
The following are the different forms of logging.
Drilled well logs: - This is also referred to as geologic logs and is
constructed from drilling samples of rocks strata encountered in
boring the well. The log is precisely a record of phases of well
drilling. Aquifers can be delineated from well logs and water quality
is indicated by the water sample collected.
Drilling - time log is a derivative of well logging which is due to
changes in rock characteristics that are indicated by changes in
drilling rate or by vibration in rotary drilling.
35
A typical driller’s well log is shown below
Fig 6.1A Drillers well log
Cemented clay
Gravel
Coarse gravel
Sand and gravel
Blue clay
Sand and gravel
Yellow clay
Gravel
Blue clay
Fine sand
Blue clay
Top soil and salt
Material Depth mo
4.5
22.5
28.4
32.6
34.2
38
42.7
45.1
46.4
51.2
59.5
60.4
36
Drilling rate
Fig. 6.2 Drilling time Log and Strata Penetrated
Gray clay
Coarse Clean Sand
Pebbly Clay
Depth, m
67
67
67
67
67
67
67
0 3 6 9 12 15
37
WEEK 7
7.0 Resistivity Logging
It has been shown that the resistivity of different rock types vary
depending on mineralogy and other properties. Resistivity logging
is employed to establish a trace of the variation of resistivity with
depth by measuring underground resistivity at several intervals.
The resultant is a resistivity (or electric) log. The log is affected by
fluid within the well, well diameter, character of surrounding strata
and by ground water.
The most commonly employed mode of measuring underground
resistivity is the multi electrode method. It consists of four
electrodes, two for emitting current and two for measuring potential.
The curves that result can be either normal or lateral according to
electrode arrangement.
Resistivity curves indicate the lithology of rock strata pentrated by
the well and enable fresh and salt waters to be distinguished in the
surrounding material. However an accurate interpretation of
resistivity log is difficult and requires specialized know-how. Typical
electrode arrangement for measuring resistivity and the resultant
38
resistivity (or electric) curve are as indicated below.
Fig. 3.9 (a): Electrode arrangement for (b) Late ral resistivity will logs
Measuring normal resistivity logs m easuring arrangements
A and B are current electrodes
M and N are Potential electrodes
AM
AM<<AB
B
A
M
M
A
B
AO
AM<<AB
Ammeter
Potentiometer Ammeter
Potentiometer
Dep
th Normal
1000
Spontaneous Potential muh voits
500
Normal
20 +
Resistivity ΩΩΩΩ hm – m2/m(ΩΩΩΩ – m)
39
Fig.7.1 Spontaneous Potential and Resistivity logs of a well.
Other modes of logging include potential logging. Temperature
logging caliper logging, radio activity logging, acoustic logging fluid
resistivity and fluid velocity logs.
Resistivity to determined aquifer porosity from the relationship.
αm = Qw/Qw
Where α – Porosity
Qw – Ground water resistivity
Q – Formation resistivity
m – Void distribution co-efficient
Which ranges from 0.97 – 2.71
Variation of Seismic Velocity for Materials
Velocity Range Material type
250mls Loose unsaturated material
≥ 5000mls Dense crystalline rock
300 – 1000mls Deep unconsolidated unsaturated material
1500 – 25000mls Deep saturated unconsolidated material
3000 – 5500mls Bed Rock
40
WEEK 8
8.0 WATER TABLE AND GROUND WATER EXPLOITATION
Water existing beneath the ground surface has been classified as
vadose water and ground water. The dividing line between these
zones is the water table which is the limiting surface of the
saturation zone. The groundwater zone, usually extend
downwards to an underlying impermeable strata like clay beds or
bedrock. Thus, the upper surface of the zone of saturation is the
water table and is technically defined as the surface of atmospheric
pressure. It is usually revealed as the level to which water stands
in a well penetrating the aquifer. The water table is technically
defined as the surface in unconfined material along which the
hydrostatic pressure is equal to the atmospheric pressure. This is
practically manifested by the equalized level f observed for both
arms of manometers placed at the water level in a hypothetical
well.
41
Fig. 8.1: Water table in a uniform water bearing me dium
as indicated by the inverted U – tubes.
The above definition of water table assumes horizontal
pattern of ground water flow.
8.1 FACTORS AFFECTING AQUIFER YIELD
The yield of a typical aquifer is the amount of water which can be
drained from the bearing formation under the effect of gravity. The
water retaining and water – discharging capabilities of subsurface
strata are of overriding importance in evaluating the yield of water
bearing formations.
Specifically the major factors which influence aquifer yield are the
rock properties, porosity and permeability. Aquifers should process
structures that make for appreciable water to move through them
thus the rock properties are of paramount importance.
For manometer at Y Pressure = Atmospheric/ pressure at X
Manometer X - Atmospheric
Y
42
Porosity is the amount of voids present in a given rock or soil body.
It is the portion of the material not occupied by solid mineral matter.
Voids are usually occupied by ground water. These void also
called interstices, pores or spaces act as water conducts.
Porosity is a measure of contained interstices and expressed as
the percentage of void space to the total volume of the mass thus.
α = 100w/V
α – porosity
w – volume of water required to saturate all pore
spaces.
v – total volume of soil/rock.
Porosity is characterized by size, shape, irregularity and distribution
and can be original interstices, created by geologic processes
governing the origin of the rock. Secondary interstices are
developed after the rock was formed like joints, fractures, openings
by plants and animals.
Permeability is the ability of is given soil body/rock type to
discharge/conduct water through its pores. Permeability is given as
K (cm/s) and though dependent on porosity it is not a direct
function of the pore spaces present.
All the voids in a saturated zone are filled with water, but not all the
water held in the interstices can be discharged/conducted (under
43
the influence of K). This water retained is held in place by gravity
and is the retentive ability of the rock/soil type.
The specific retention Sr. is given by
Sr = 100wr/v
wr - volume occupied by retained water
v – gross volume of the rock/soil.
The water that can be drained is specific yield and is defined as the
rates expressed as a percentage of the volume of water which can
be drained by gravity to volume of the formation given by
Sy = 100wr/v
wy - volume of water drained
v – gross volume of the rock/soil.
Since w = wy + wr
Then α = Sr + Sy
α = void ratio
Specific yield is a fraction of porosity of an aquifer.
For uniform sand Sy 10 – 30%
For alluvial aquifer Sy 10 – 20%
Specific yield of an aquifer can be determined by saturating
samples in the laboratory and allowing them to dawn or saturating
in the field a considerable body of material situated above.
Explain safe yield and overdraft.
44
The water table and above the capillary zone and allowing it to
drain downward naturally.
Range of yield for different formation are as given below.
Maximum 10% grain size
PE
RC
EN
T
5
10 15 20
25
30 35
40 45
Cla
y &
si
lt
San
dy
Cla
y F
ine
San
d S
and
Med
ium
S
and
Coa
rse
San
d
Gra
vely
Fin
e gr
avel
M
ediu
m
grav
el
Med
ium
gr
avel
M
ediu
m
grav
el
Coa
rse
grav
el
Coa
rse
grav
el
Boi
ler
Specific retention
Specific yield
Porosity
1/16 1/8 ¼ ½ 1 2 4 8 16 32 64 128 256
45
Fig. 4.2: Porosity, specific yield and specific ret ention
variation with grain size.
WEEK 9
9.0 GROUND WATER EXPLOITATION TECHNIQUES
The common method for exploiting ground water is by the use of
wells. A water well is the universal term used for holes or shafts,
usually vertical, excavated in the earth for bringing ground water to
the surface.
Wells are broadly classified as either shallow or deep wells.
Other categories of wells include boreholes, sunk wells,
infiltration galleries and artesian wells. Dug. Bored, driven or
felted wells are all types of shallow wells and deep wells are
drilled by mechanical methods. Deep wells are usually for
optimum yield and are tested before installing a pump.
Which ever method is employed for tapping ground water
depends on the water supply requirement, quantity of water
required, depth of ground water, geologic conditions and
economic consideration. Shallow wells are usually less than
15m in depth.
9.1 DUG WELLS
The most common method of furnishing water supply is the Dug
well which is a form of shallow wells. Basically dug wells are
excavated by simple hand implements but large dug wells are
46
constructed with portable excavating equipment (clean shell and
peel buckets). Walls of the wells are braced against caving by
lining/casing is provided using termed curb. The curb should be
perforated to permit water entrance. The dug should extend a
considerable depth beneath.
The water table (4.5 – 6m below water table).
9.2 BORED WELLS
For low lost supply of relatively small quantities of water bored
wells are used where water table exists at a shallow depth in an
unconsolidated aquifer. Boring implement can be either hand -
operated or power – driven earth augers the boring operation of
hand augers are facilitated by the use of cutting blade at the bottom
which bore into the ground.
Power – operated driven augers consists of cylindrical steel
buckets with a cutting edge projecting from a slot in the bottom.
Reamers are used to enlarge holes to diameters exceeding the
auger size. Augers are well suited for formation of loose sand and
gravel and supplementing drilling methods where sticky clay is
encountered. Hand – bored wells vary between 6 – 8” in diameter
and 15m in depth while wells bored using power assisted methods
are usually up to 36” in diameter and 30m deep.
9.3 DRIVEN WELLS
47
A driven well is developed by connecting a series of pipe lengths
and driving them into the ground below the water table. Driving is
done using a maul, sledge hammer, drop hammer or air hammer.
Entrance of water into the well is through drive (sand) point at the
lower end of the well. Driven wells are between 1¼ – 4 in diameter
and usually do not exceed 15m in depth.
Extraction of water from driven wells is by suction type pumps and
for continuous water supply the water table should be near the
ground surface. Driven wells can be used for domestic supplies,
temporary supplies, exploration (observation wells) and for
dewatering excavation for foundations and other subsurface
construction work. Unconsolidated formations with no large gravel
or rocks that can damage well point are areas well suited for driven
well.
9.4 JETTED WELLS
Wells that are constructed through the action of high velocity steam
of water directed downward are jeffed wells. The speed of the
water stream washes the earth away. Water and formation
material cuttings are conducted up and out of the well. The
diameter of such wells are usually (1½ – 3 inches) and depth might
exceed 15m. These wells are easily constructed in unconsolidated
formations and the yield is small. They are suited for investigative
exploration purposes and well point – system.
48
WEEK 10
10.0 DEEP WELLS
These are large high-capacity (high yield) wells developed to
extensive depths and constructed by drilling. There main
construction (Drilling) method are employed.
(i) Cable tool (also known as percussion or standard)
(ii) Hydraulic rotary method
(iii) Reverse rotary method.
(i) Cable too method – This is adapted for drilling wells through
consolidated rock materials. The method is less effective in
unconsolidated sand, gravel and quick sand as the loose
materials slumps and cave around drilling bits. The entive
component comprises of a standard well drilling rig,
percussion tools includes a rope socket which attaches
drilling rope to string of tools, set of jars which are connecting
links to loosen entire tool when stuck, drill stem which adds
weight and length to the drill for raped and vertical cutting.
The drilling bit does the actual drilling and is of various sizes
shapes and weights. The bailer removes excavated
materials/cuttings from the well.
49
(ii) Hydraulic rotary include – In this method drilling is
facilitated by a hollow rotating bit through which a mixture of
clay and water (drilling mud) is forced. The rising mud serves
the purpose of lifting loosened material upward in the hole.
All category of drill bits have hollow shanks and one or more
centrally located orifices for jetting mud in to the bottom of the
holes. The bit is attached to a rod/shaft, turned by a rotating
table that allows the drill rod to slide downward as the hole
deepens. This is a fast method for drilling in unconsolidated
strata and deep wells of up to 18 dia – can be developed.
(iii) Reverse Rotary method This is a variation of the hydraulic
rotary method but the cuttings are removed by suction pipe.
The procedure is thus a suction dredging method. The main
components include a large – capacity centrifugal pump, a 6”
dia drill pipe and a bit similar to a dredge cutter head.
10.1 INFILTRATION GALLERIES (Horizontal wells)
An infiltration Gallery is developed within a permeable aquifer and
as a horizontal permeable conduit for intercepting and collecting
ground water which flows by gravity. High yield galleries should be
located within conducting aquifers with a high water table source.
Thus many infiltration galleries are laid parallel to river beds where
induced infiltration ensures adequate and consistent water supply
is assured.
50
10.2 ARTESIAN WELLS
These are wells that are developed to penetrate a confined aquifer
in which water is confined under a pressure greater than
atmospheric. The water level in artesian wells rises above the
bottom of the confining strata. Some artesian wells create enough
pressure to generate an upward flow greater than 45m high and
discharge of 1000 gallons per minute (gpm).
Fig 10.1 Artesian Well
Unconfined aquifer
Confining strata aquifer
Impermeable strata
Water table well
Artesian well
51
WEEK 11
11.0 GROUND WATER QUALITY
The quality of ground water is a measure of the amount of
impurities as well as chemical and biological characteristics of the
water. Specifically the impurities in ground water are soluble salts
which originate primarily from solution of rock materials.
The term salinity is used to describe the Concentration of salt in
ground water. Criteria of ground water quality are established
based on physical, chemical and bacterial constituents. Limits of
water quality are established for proper safeguard and
improvement of ground water storage.
The quality of ground water depends on its use. Thus, requirement
for drinking, (domestic) industrial and irrigation are markedly
different.
Development of Ground Water Salinity: - The concentration of
salts in ground water is due to the reaction of precipitation, which
contains trace amount of dissolved mineral matters of the soils and
rocks of the earth. This is further illustrated sinematically.
Precipitation
Rain Water
+ Small amount of mineral
Soil and Rock of the Earth/aquifers
+ Saline ground water
52
11.1 SOURCES OF IMPURITIES IN GROUND WATER
The various source of impurities can be grouped into three:
a. Diffuse Sources which impair ground water quality over
large areas like percolation from intensely farmed filed.
b. Point source like septic tunk and garbage disposal sites.
c. Line sources of contamination like seepage from polluted
streams.
The specific sources of ground water contaminants are varied and
affect quality to certain extents. The major form are dissolved salts
which are carried in solution and these are more in ground water
due to exposure to soluble material in geologic strata.
The main source are:
i. Infiltrating surface water which are saline enough to alter
ground water quality. This occurs mainly in areas recharging
large volume of water underground like alluvial streams.
Channels and artificial recharge area.
Reaction results in dissolution of the mineral matter and the amount dissolved depend on the chemical composition and physical structure of the rocks as well as pH and redox potential of water.
Resulting from the solvent action of water on rocks and mineral matters in earth and aquifers.
53
ii. Dissolved mineral product which are due to absorbed gases
of magmatic origin.
iii. Imparities that are product of weathering and erosion by
rainfall and flowing water.
iv. Excess irrigation water which might contribute substantial
quantity of salt.
v. Ground water of arid regions contain high amount of
impurities due to lack of leaching by inadequate rain which
affects dilution of salt solutions.
vi. Ground water formations (aquifers) dissolve and form
solution depending on their solubility lead to impurities in
ground water. This is exemplified by ground water flowing
through sedimentary or igneous rock aquifer.
vii. Salt water intrusion, which occurs in coastal aquifers is an
invasion of saline water into fresh ground water. This is the
movement of seawater inland when ground water level
declines.
viii. Direct entry of sewage/sludge into the ground from septic
tanks, cesspools and sewage systems leads to impairment of
quality of surrounding ground water. This can also result
from unintended infiltration of sewage into underground water
from leakage of sewers and seepage and industrial waste
54
disposed in land fills upon decomposition contaminate
underlying ground water.
ix. Petroleum leakage and spills: This is the impairment of
ground water quality by petroleum products which are
accessible to soils and aquifers from leaking pipelines and
burned steel gasoline storage tanks in gasoline stations.
x. Excessive repeated exploration of ground. This might be due
to construction of shafts and tunnels that other ground water
courses, coal mining in which oxidation of pyrite resulting in
sulphuric acid and the tailings and processing waste from
mining and milling metal ores which affect proximate local
ground water quality.
xi. Deep well storage of liquid waste which is adopted for waste
fluids that are difficult to dispose pose serious hazards to
ground water quality. This is due to the migration of the
waste fluid over long distance into fresh water aquifers.
xii. Underground disposal of radio active waste leads to the
formation of radio nuclides which are completely undesirable
in ground water.
55
WEEK 12
12.0 CAUSES OF SPECIFIC TYPES OF IMPURITIES
The impurities, like dissolved solids and sediments in ground water
are elements molecules and compounds which on dissolution in
water alters its chemical composition. These include Na, Ca, Mg,
K, Cl, SO4, HCO3, CO3, Fe, N, NH3, and other elements as well as
deleterious substance in trace amounts.
These result in increased hardness of ground water alkalinity and
change of pH which alters the acidic content of water.
The trace elements in ground water, which occur from the various
sources of ground water impurities, constitute a minor proportion of
dissolved solids. They are present at concentration below 0.1mg/L
because of the level of solubility of minerals.
The include Arsenic, Barium, Chromium, Copper, Lead, Mercury
and Zinc. These have the effect of increasing the toxicity of ground
water especially when used for human consumption. Most of the
mentioned trace elements have been proven to seriously impair the
function of vital human physiology.
Radioactive nuclides lead to serious degradation of ground water
especially where they exceed the concentration guide limits.
56
Dissolved gases entrapped in rain water, upon percolation result in
allied chemical compound which changes the taste of ground
water.
Pathogenic organisms, from sewage/sludge and solid waste lead to
serious contamination of ground water. This is because microbial
thrive in most underground materials as aquifers and other deep
formation are known to be conducive for most microbes lives.
The colour of ground water is altered by certain types of acidic
impurities and protein compounds made up of stable organic
matter (humus). The colour and taste of ground water is altered by
infiltration of gasoline and other petroleum products.
Importantly the alteration of ground water quality due to petroleum
product contamination is taste. This is apparent at concentration
less than 0.005mg/L.
Solids suspended in ground water result in turbidity of ground water
impurities that might cause increased turbidity include clay, silt and
other fines.
12.1 TOLERANCE LIMITS OF IMPURITIES
The salt concentration of ground water is given in terms of the Total
Dissolved Solids (TDS) contents. It is measured in Parts Per
Million (PPM) or Milligrams per liter (PPM) or Milligrams per litre
(mg/L).
57
One PPM indicates one part by weight of the ion to a million parts
by weight of water. The standard limits for classifying ground water
in terms of concentration of solids are
Fresh < 100mg/l
Moderately saline 3000 – 10,000mg/l
Very saline 10,000 – 35,000mg/i
Briny > 35,000mg/l
The other kind of elements/compounds present in ground water
has lesser tolerable amounts allowed in ground water.
Iron: Recommended to a maximum of 0.03mg/l in drinking water.
Manganese: Maximum concentration is set at 0.05m/l
Aluminum: Rarely exceeds 0.5mg/l
Carbonate and Bicarbonate: Concentrations are not more than
10 and 200mg/l respectively.
Chloride: Recommended as a maximum of 250mg/l in drinking
water.
Silica: In the range of 20mg/l
Sulphate: Concentration in drinking water < 250mg/l
Fluorine: Exceed 10mg/l in ground water but it limit in drinking
water is pegged at 1.4 – 2.4mg/l.
Most natural ground waters contain less than 0.1mg/l of
phosphorous which is considered safe. Most of the trace elements
should not exceed 0.1mg/l in ground water.
58
Consider the case shown below with ground water elevation known
from wells, an estimated ground water contour (lines joining pts of
equal pressure head/water level) and flow directions are as
indicated.
Fig. 12.1: Ground water Contours
An enlarged case for a wider network is as depicted below
Fig. 12.2: Contour map of ground water Surface show ing flow
lines.
95
105
100
97.5
102.5
100 Ground water contours
Direction of Ground water flow
Water table Elevation
(AHGSD)
260
350.5 350 349.5
348.5 348 347.5 346.5 346
0.5m contours of ground water surface
59
For special cases of flow through aquifers consider the following
cases.
1. Flow through a phreatic aquifer. With a free water surface
resisting on an impermeable base as shown.
Impervious horizontal base
Fig. 12.3: Flow in a Phreatic aquifer
From Darcy’s we have that
de
dKV
Φ−=
Assume small dΦ, such that dx
d
d
d Φ≡
Φ1
And that dx
dh
dh
d=
Φ
Then:
q = - KH dx
dh [Flow/unit width]-----------------------2.8
dx
hdk
dx
dq )(=
2 2
2
-1/2
n
h
Phreatic surface
dh = dΦΦΦΦ
Ground surface ΦΦΦΦ
χχχχ
Potential line
dl
dx
60
( )dx
dvnv
dx
Vd =
dx
dhh
dx
hd 2
)(=
½ ( )
dx
dhh
dx
hd−=
Since flow is uniform
9.20 −−−−−−−−−−−−−−−−−=dx
dq
Thus d2 ( )10.20 −−−−−−−−−−−−−=
dx
h
dx
dhkhq −=
( )dx
hKq =
( )dx
hdK
dx
dq
2. Flow in a confined aquifer with permeability K as shown in fig
below with ground water flowing from left to right line of
potential head (Energy grade line) is declining as indicated by
piezometers.
n-1
2
2
2
2
-1/2
2
2
2
2 2
-1/2
61
Fig. 12.4: flow in an artesian aquifer
From Darcy’s
dx
dKVx
Φ−=
Thus flow per unit width
dx
dKHq
Φ−=
11.2−−−−−−−−−−−−−−−−Φ
−=dx
dKH
dx
dq
Since flow is assumed to be uniform
12.20 −−−−−−−−−−−−−−−−−−−=dx
dq
Flow
H
φφφφ
Potential head
Piezometers
φφφφ
χχχχ
2
2
2
62
Thus 13.20 −−−−−−−−−−−−−−−−=Φ
dx
d
These are fundamental equation for flow through a confined
aquifer. For case 1 assume that the aquifer is being recharged at a
net infiltration rate N units (rain failing on the ground, then.
dq = N. dx
( )N
dx
hdK
dx
dq==
( )K
N
dx
hd
2−=
Fig. 2.10: Flow in a Phreatic aquifer with rainfall
2
2 2 2 - 1/2
2
2 2
q + dq dχχχχ
q
Net infiltration N.
63
WEEK 13
13.0 Example 13.1 Given two canals at different levels separated by a
strip of aquifer 1000m wide, of permeability K = 12m/day as shown in fig
below. The aquifer is 20m in depth is an impermeable base and the
higher canal is 2m higher than this with the lower canal exactly coincides
with the top of the aquifer. Find the inflow or discharge from each canal
per meter length of aquifer assume annual rainfall is 1.20m per annum
and assume 60% infiltration.
From reference origin x = 0, h = 20m
And x = 1000m, h = 22m
N = 1.2m x 0.6/year = 0.72/365m/day
( )K
N
dx
hd 2−=
( )K
CNx
dx
hd 12 +−=
h K = 12m/day 20m
2m
N
h 1000m
χχχχ Impermeable base
2 2
2
2
2
64
21 CxCK
Nxh ++−=
At x = 0, h = 20
202 = C2
C2 = 400
At x = 100, h = 22
222 = 400100012365
,1072.01 +C
x
x
C1 = 0.248
We have that q = - dx
dhKh
=h √ ( )400248.0 ++− xNx K
−Let uK
Nx=++ 400248.0
h2 U
1/2
( )K
Nx
udx
du
udx
dh 248.02
2
1
2
1 +−==
at x = 0
( ) mdaymu
Kuq 1/49.1248.02
1−=−=
( ) ( ) 49.1248.06248.02
12−=−=−=
u
uq
at x = 1000
2
6
2
2
1/2 1/2
3
1/2
1/2
1/2
1/2
65
( )12365
248.072.02000
2 x
xKq
+−=
= ( )
4380
248.014406
+−
6(-0.08076) = - 0.48 m3/day
Thus there is a discharge into both canals from the aquifer of 1.49m3/day
to the lower canal and 0.48m3/day to the upper canal per meter length of
aquifer.
Example 2.2
Two observed wells were used to evaluate underground water flow
pattern through an unconfined aquifer. If the aquifer is made up of
permeable material of K – 3 x 10-3cm/sec calculate the seepage loss to
the well at the upstream per meter width of aquifer end of flow using flow
net for which nf = 4 and nd = 10. The sketch of the stated condition is as
shown.
12m
1.8m
10.2m
∆h=1.02
66
Given Hw = 12 – 1.8 = 10.2m
M = 4, n = 10
Flow from flow net discharge equation is
q = Km Hw = (3 x 10-4 x 10-2 x 24 x 60 x 60m/day) x 4 x 10.2
n 10
= 1.08m3/day.
Example 2.3
A well was sunk into a confined aquifer to augment water supply for a
small rural set-up. The aquifer is 1.8m thick, 2.5m in x-section and
extends 250m from the area of recharge to the well. The well is
developed such that water rises to an elevation of EL + 248m and the
water table at the recharge area is at elevation EL ≠256m. If the aquifer
is made up of material with a K of 1.7cm is calculate:
i. The total discharge, Q expected into the well assuming water for
the well is to be sourced from precipitation trapped in the recharge
area.
ii. If water supply requirement for the area is 50,000gal/day, is the
aquifer yield sufficient?
Solution
Aquifer: = 1.8m X-section = 2.5m
Length between recharge area and well = 250m
Head of water, ∆h = E1 + 256 – El + 248 = 8m
Hydraulic gradient = ∆h = 8/250 = 0.032m/m
67
L
X – sectional area of aquifer = 1.8 x 2.5 = 4.5m2
Discharge from aquifer into the well is computed from Darcy’s as
Q = KiA = K x ∆h x A L
= (1.7 x 10-2 x 24 x 60 x 60m/day) x 0.032 x 45
= 212m3/day (1m3 = 264 – 172gal)
56,001 gal/day.
68
WEEK 14
14.0 Example 14.1 A hillside underlain by an aquifer drains into a stream
at its lower end as shown.
The aquifer is composed of permeable material 20m thick and with a
permeability of 0.5m/day. Calculate the seepage into the stream per unit
length of stream assuming the stream is located 1000m away from the
idealized recharge area of the aquifer and the top water level is 50m
lower than this point.
From Darcy’s Axh
hxKQ
∆∆
=
∆h2 50m, ∆h = 1000m
A = 20m x 1 (per unit length of stream)
daymxxxQ /1201000
505.02 =
Example 2.5
A confined aquifer has a transmissivity of 40m2/day. The slope of the
piezometric surface is 0.25m/km. How much water per day flows through
the aquifer per kilometer width of the aquifer?
Q = KiA
I = 0.25/1000m2/m
But transmissivity = 40m2/day
3
69
= K x depth of aquifer
Q = 40m2/day x 0.00025 x wdith of aquifer
= 40m2/day x 0.00025m/m x 1000m
= 10m3/day
Example 2.6
The horizontal contour of ground water elevation for an unconfined
aquifer is as shown with a coefficient of permeability K = 5.79 x
10-5cm/s.
Assuming the aquifer is bordered by a steam at the upstream end and a
hypothetical well is located as indicated on the figure into the well in
ms/day. Take the piezometric elevation between the stream and well as
10m.
Solution
K = 5.8 x 10-5cm/s
Hw = 10m, m = 4, n = 12
55
50
45
40
35
30
25
20
15
10
5
Well
Stream line
Equipotential line
Stream
70
Q = K Hw m n
= (5.8 x 10-5 x 24 x 60 x 60 x 10-2m/day) x 10 x 4 12
= 0.17m3/day/unit depth of aquifer
Example 2.7
In an area of 1,011,725m2, the water table drops by 5m. If the porosity is
0.30 and the specific retention is 0.10, compute the specific yield and
change in storage in m3.
Solution
Specific yield = porosity – specific retention pg 30(α = Sr + Sy)
= 0.30 – 0.10 = 0.2
Change in storage = 0.2 x 5 x 1, 0011, 725
= 1,011,725m3
Example 2.8
A water table drop of 5m occurs during a certain year. Lab analysis
shows that the specific yield of the alluvial material is 0.20.
What is the amount of storage during the year if the area of the region is
4046.9m2? what is the area retention of the material if lab sample shows
a porosity of 0.3
Solution
Sy = 0.20
Area = 4046.9m2
Water table drop = 5m
71
Amount of storage = 0.2 x 5 x 4046.9m2
= 4046.9m3
α = 0.3, Sy = 0.2 Sr = ?
α = Sy + Sr
0.3 = 0.2 + Sr Sr = 0.1
Example 2.9
A 500m wide aquifer (unconfined) has a permeability of K = 6m/day and
is bordered on both sides by two canals at different levels. The aquifer is
15m deep to an impermeable base and the higher canal is 1.2m higher
than this which the lower canal coincides exactly with the top of the
aquifer. Find the inflow or discharge from each canal per meter
length/width of aquifer assuming annual rainfall is 1.8m per annum and
80% infiltration rate.
From reference origin x = 0, h = 15m
And x = 500m, h = 16.2m
N = 0.8m x 1.8/year = 1.44/365m/day
( )K
N
dx
hd 2−=
( )K
CNx
dx
hd 12 +−=
21 CxCK
Nxh ++−=
2 2
2
2
2 2
72
At x = 20, h2 = 15m
152 = C2
C2 = 225m
At x = 500m, h = 16.2m
16.22 = 22515006365
,50044.1++ C
x
x
C1 = 0.404
We have that q = - dx
dhKh
=h √ ( )225404.0 ++− xNx K
−=uLet 225404.0 ++ xK
Nx
h2 u
/2
( )K
Nx
udx
du
udx
dh 404.02
2
1
2
1 +−==
at x = 0
( ) )404.01022
1+−−= N
uKuq
= - 1.212m3/day/m
at x = 500
( )6365
404.050044.12
2 x
xxKq
+−=
2
2
1/2 1/2
2
1/2
1/2
73
0.76m3/day/m
Thus there is a discharge into both canals of 1.2 12m3/day to the lower
canal and 0.76m3/day into the higher canal.
74
WEEK 15
15.0 COMMON GROUNDWATER CONTAMINANTS
Most groundwater contaminants are derived from agricultural,
urban and industrial land uses.
As recharge percolates through the soil to the water table it
transport a variety of contaminants derived from land uses within
the recharge area. Point sources of contamination such as landfills
and industrial seepage pits, release large quantities of
contaminants which often forms an underground plains.
Non point sources of pollution include;
• Septic systems
• Fertilizers
• Pesticides and street drainage
The following are common groundwater contaminants:
1. Nitrates- Dissolved nitrogen in the form of NO3 is the most
common contaminant in groundwater. High level contaminant
can cause methaeoglobinaemia (baby syndrome) in infants,
may form carcinogens, and accelerate the
eutrophication of surface waters.
Sources of nitrates- includes sewage, fertilizers, air pollution,
landfills and street drainage.
2. Pathogens-are bacteria and viruses which causes
75
3. waterborne diseases such as typhoid, cholera, dysentery,
polio and hepatitis.
Sources includes sewage, landfills, livestock and wild life.
3. Trace metals -include cadmium, chromium, copper, mercury
and lead. These metals can have toxic and carcinogenic
effects.
Sources include industrial discharge, pesticides and street
drainage.
4. Organic compounds -includes volatile and semi volatile
organic compound (e.g. petroleum, derivatives, and
pesticides, Sources include agricultural activities, steel
drainages, sewage, landfills, industrial discharges, spills, air
pollution, leaking underground storage tanks, car exhausts.
15.1 Possible prevention of groundwater pollution
• Water treatment by chloride to get rid of bacteria germ.
• Boiling the water
• filtration/settlement of water reduces turbidity of water
Proper grouting of wells will prevent surface drainage/septic tank
pollution.
76
REFERENCES
1. Essentials of Geology, Frederick K. and Edward J., (2000),
Prentice Hall.
2. Hydrogeology, Wister G. O., (1959). John Wiley.
3. Hydrogeology, Davis S. W., (1956). John Wiley.