Cec 207 Theory - Hydrogeology

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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

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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

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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.

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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

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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.

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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.

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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.

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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.

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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.

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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)

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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.

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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

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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.

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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.

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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

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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

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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.

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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)

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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

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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

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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.

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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

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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

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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.

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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

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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.