Hydrology – From Measurement To Hydrological Information (6)

CHAPTER 6

GROUNDWATER

6.1 GENERAL [HOMS L]
Groundwater underlies most of the Earth’s land
surface. In many areas it is an important source of
water supply and supports the fl ow of rivers. In
order to understand the full extent of the hydrological system, it is necessary to understand the groundwater system (Fetter, 1994; Freeze and Cherry, 1979). The purpose of this chapter is to
provide an overview of those basic concepts and
practices that are necessary to perform an appraisal
of groundwater resources. Generally, a groundwater resource appraisal has several key components:


(a) Determination of the types and distribution of
aquifers in the area of investigation;
(b) Evaluation of the spatial and temporal variations of groundwater levels (potentiometric
surface) for each aquifer, resulting from natural and man-made processes. The construction
of wells and measurement of water levels facilitates this aspect;
(c) Assessment of the magnitude and distribution of hydraulic properties, such as porosity and permeability, for each aquifer. This
is a requirement for any type of quantitative
assessment;
(d) An understanding of the processes facilitating or affecting recharge to and discharge
from each aquifer. This includes the effective
amount of precipitation reaching the water
table, the effects of evapotranspiration on
the water table, the nature of groundwater–
surface water interaction, and the location of
and amount of discharge from springs and
pumped wells;
(e) An integration of the groundwater data
in order to corroborate information from
multiple sources, understand the relative
importance of the various processes to the
groundwater system, and appraise the capacity or capability of the groundwater system to
meet general or specifi c (usually water supply)
goals. This can be facilitated with the development of predictive tools using various analytical options that range from water budgets to
computer-based digital groundwater fl ow
modelling.
6.2 OCCURRENCE OF GROUNDWATER
6.2.1 Water-bearing geological units
Water-bearing geological material consists of either
unconsolidated deposits or consolidated rock.
Within this material, water exists in the openings
or void space. The proportion of void space to a
total volume of solid material is known as the
porosity. The interconnection of the void space
determines how water will fl ow. When the void
space is totally fi lled with water the material is said
to be saturated. Conversely, void space not entirely
fi lled with water is said to be unsaturated.
6.2.1.1 Unconsolidated deposits
Most unconsolidated deposits consist of material
derived from the breakdown of consolidated rocks.
This material ranges in size from fractions of a millimetre (clay size) to several metres (boulders).
Unconsolidated deposits important to groundwater
hydrology include, in order of increasing grain size,
clay, silt sand and gravel.

Figure I.6.1. Examples of water-bearing sediments of rocks with primary (intergranular is shown) and
secondary (fractured and dissolution are shown) pore space (Heath, 1983)


6.2.1.2 Consolidated rock
Consolidated rocks consist of mineral grains that
have been welded by heat and pressure or by chemical reactions into a solid mass. Such rocks are
referred to as bedrock. They include sedimentary
rocks that were originally unconsolidated, igneous
rocks formed from a molten state and metamorphic
rocks that have been modifi ed by water, heat or
pressure. Groundwater in consolidated rocks can
exist and fl ow in voids between mineral or sediment grains. Additionally, signifi cant voids and
conduits for groundwater in consolidated rocks are
fractures or microscopic- to megascopic-scale voids
resulting from dissolution. Voids that were formed
at the same time as the rock, such as intergranular
voids, are referred to as primary openings
(Figure I.6.1). Voids formed after the rock was
formed, such as fractures or solution channels, are
referred to as secondary openings (Figure I.6.1).
Consolidated sedimentary rocks important in
groundwater hydrology include limestone, dolomite, shale, siltstone and conglomerate. Igneous rocks include granite and basalt, while metamorphic rocks include phyllites, schists and gneisses.


6.2.1.3 Aquifers and confining beds
An aquifer is a saturated rock formation or deposit
that will yield water in a suffi cient quantity to be
considered as a source of supply. A confi ning bed is
a rock unit or deposit that restricts the movement
of water, thus does not yield water in usable
quantities to wells or springs. A confi ning bed can
sometimes be considered as an aquitard or an
aquiclude. An aquitard is defined as a saturated bed
which yields inappreciable quantities of water
compared to the aquifer but through which
appreciable leakage of water is possible. An aquiclude
is a saturated bed which yields inappreciable
quantities of water and through which there is
inappreciable movement of water (Walton, 1970).

6.2.1.4 Confi ned and unconfi ned aquifers
In an unconfined aquifer, groundwater only
partially fills the aquifer and the upper surface of
the water is free to rise and fall. The water table
aquifer or surficial aquifer is considered to be the
stratigraphically uppermost unconfined aquifer.
Confi ned aquifers are completely fi lled with water
and are overlain and underlain by confining beds.
The impedance of flow through a confining bed
can allow the water level to rise in a well above the
top of the aquifer and possibly above the ground.
This situation can result in wells that flow naturally.
Confined aquifers are also known as artesian
aquifers.


6.2.2 Development of a hydrogeologic
framework [HOMS C67]
Information about aquifers and wells needs to be
organized and integrated to determine the lateral
and vertical extent of aquifers and confining beds.
On that basis, determination of such characteristics
as the direction of groundwater fl ow, and effects of
hydrological boundaries, can be undertaken. The
compilation of the lateral and vertical extent of
aquifers and confining beds is commonly referred
to as the hydrogeologic framework. To be useful,
this concept of a framework needs to be based, as
much as possible, on actual and quantitative data
about the existence, orientation and extent of each
aquifer and confining unit where applicable. Where

actual data are not available, one must rely on the
conceptual knowledge of the subsurface
conditions.
The development of a hydrogeological framework
requires an accurate view, in a real sense, of the
subsurface conditions. This can be accomplished in
several direct and indirect ways. Direct methods
include recovery of aquifer and confining bed material during the process of drilling, in the form of
cuttings and core samples. Indirect methods include
sensing earth properties using borehole or surface
geophysical properties. A robust approach to collect
these data involves combining all available methods,
ultimately piecing together information to produce
a detailed picture of the aquifer and confining unit
extents, thicknesses, orientations and properties.
6.2.2.1 Well-drillers’ and geologists’ logs
Information about the nature of subsurface materials can be found in the records of construction of
wells, mine shafts, tunnels and trenches, and from
descriptions of geological outcrops and caves. Of
particular usefulness to groundwater studies is the
record of conditions encountered during the drilling of a well. This can be done either by the driller
or a geologist on site monitoring conditions, by
bringing the drill cuttings to the surface and examining any core samples taken. A driller’s log or
geologist’s log (depending on who prepared the
information) is a continuous narrative or recording
of the type of material encountered during the drilling of a well. Additionally, these logs may contain
remarks such as the relative ease or difficulty of
drilling, relative pace of advancement and amount
of water encountered.
6.2.2.2 Borehole geophysical methods
Borehole geophysical logging is a common approach
to discerning subsurface conditions. A sonde is
lowered by cable into a well or uncased borehole.
As it is lowered or raised, a sensor on the sonde
makes a measurement of a particular property or
suite of properties. These data are then transmitted
up the cable as an analog or digital signal that is
then processed and recorded by equipment at the
land surface. The data are typically shown in strip
chart form, referred to as a log. These measurements
provide more objectivity than that of a geologist’s
log of core or drill cutting samples, allowing for
more consistency between multiple sources of data.
Table I.6.1 provides a brief overview of the borehole
geophysical methods commonly used in
groundwater investigations: caliper, resistivity
including spontaneous potential (SP), radiation
logs including natural gamma, borehole temperature
and borehole fl ow (Keys and MacCary, 1971).
6.2.2.3 Surface geophysical methods
Surface geophysical methods are used to collect
data on subsurface conditions from the land surface
along transects. Depending on the instrument,
various types of probes are either placed in contact
or close proximity with the ground surface to
produce the measurements. There are four basic
surface geophysical methods: seismic, electrical
resistivity, gravimetric and magnetic (Zohdy and
others, 1974). These are summarized in Table I.6.2.
Accurate interpretation is greatly aided by core
samples or bore hole geophysical data.

Table I.6.1. Summary of borehole geophysical methods commonly used in groundwater investigations


6.2.2.4 Hydrostratigraphic correlation
The integration of hydrogeological information
collected from a network of individual wells, surface
geophysical transects and geologic outcrops to
formulate a large-scale, comprehensive understanding of the lateral extent and vertical nature of
aquifers and confi ning beds in an area, referred to
as the hydrogeological framework, relies on the
process of correlating those data from different
locations. Correlation, in this sense, can be defined
as the demonstration of equivalency of units
observed at different locations. The essence of the
problem for the practitioner is to determine whether
an aquifer identified at one location is connected or
equivalent to one at other locations. When
approaching this type of task, geologists focus on
equivalent geologic age units or rock types. The
hydrogeologist, however, must be concerned with
equivalence from a hydraulic standpoint that may
transcend rock type or geologic age. The reliability
and accuracy of the resultant hydrogeological
framework is directly related to the density of well
and transect information. In areas of complex geology and topography, a relatively higher data density
is required than in simpler areas.
The approach is to identify a preferably unique
lithologic or hydraulic feature that is directly related
to an aquifer or confining bed at one location. The
feature could be, for example, the presence of a
certain layer with a particular composition or colour
within or oriented near the aquifer or confining
bed under study. This is referred to as a marker unit.
A unique signature of particular strata on a borehole geophysical log may be of use. Once identified
in the data related to a particular well or location,
data from nearby wells are examined for the existence of the same marker. Because of variations in
geology and topography, the depth at which the

marker is found may be different. If the marker is
then identifi ed, it may be postulated that the aquifer or confining bed at a similar relative location as
identified in the original well is correlated, and thus
may indicate that the aquifer or confining unit is
continuous between the data points. If a particular
marker is not identifiable at other nearby locations,
the available data must be re-examined and additional attempts at correlation made. The inability
to make a correlation and define continuity may
indicate the presence of a fault, fold or some type of
stratigraphic termination of the unit. Knowledge of
the geology of the area and how it is likely to affect
the continuity and areal variation in the character
of aquifer and confining beds is essential. It may be
necessary to consult with geologists familiar with
the area in order to proceed with this task. It cannot
be overstressed that geological complications and
the possible non-uniqueness of a marker unit could
lead to erroneous conclusions.


6.3 OBSERVATION WELLS
6.3.1 Installation of observation wells
Since ancient times, wells have been dug into waterbearing formations. Existing wells may be used to
observe the static water table, provided that the
well depth extends well below the expected range
of the seasonal water level fluctuations and that the
geological sequence is known. An examination
should be made of existing wells to ascertain which,
if any, would be suitable as observation wells.
Existing pumped wells can also be incorporated
into the network if the annular space between the
outer casing of the well and the pump column allow
free passage of a measuring tape or cable for measuring the water level. Whenever existing drilled or
dug wells are used as observation wells, the water
level in those wells should be measured after the
pump has been turned off for a sufficient time to
allow recovery of the water level in the well.
Abstractions in the vicinity of an observation well
should also be stopped for a time long enough for
the depth of the cone of depression at the observation well to recover. If new wells are required, the
cost makes it necessary to plan the network
carefully.

Table I.6.2. Summary of surface geophysical methods commonly used in groundwater investigations


In those parts of aquifers with only a few pumped
or recharge wells that have non-overlapping cones
of influence, it is generally preferable to drill special
observation wells far enough from the functioning
wells in order to avoid their influences. The principal advantage of dug wells is that they can be constructed with hand tools by local skilled labourers. Depths of 3 to 15 m are common, but
such wells exist as deep as 50 m or more. Dug wells
may be constructed with stone, brick or concrete
blocks. To provide passage of the water from the
aquifer into the well, some of the joints are left
open and inside corners of the blocks or bricks are
broken off.


When the excavation reaches the water table, it is
necessary to use a pump to prevent water in the
well from interfering with further digging. If the
quantity of water entering the well is greater than
the pump capacity, it is possible to deepen the well
by drilling. The technique of excavating wells to
the water table and then deepening the well by
drilling is common practice in many parts of the
world. The finished well should be protected from
rain, flood or seepage of surface waters, which might
pollute the water in the well and hence the aquifer.
The masonry should extend at least 0.5 m above
ground level. The top of the well should be provided
with a watertight cover and a locked door for safety
purposes. A reference mark for measuring depth to
water (levelled to a common datum) should be
clearly marked near the top of the well.
Where groundwater can be reached at depths of
5 to 15 m, hand boring may be practical for
constructing observation wells. In clays and some
sandy looms, hand augers can be used to bore a
hole 50 to 200 mm in diameter that will not collapse
if left unsupported. To overcome the difficulty of
boring below the water table in loose sand, a casing
pipe is lowered to the bottom of the hole, and
boring is continued with a smaller diameter auger
inside the casing. The material may also be removed
by a bailer to make the hole deeper.


In areas where the geological formations are known
in advance and which consist of unconsolidated
sand, silt or clay, small-diameter observation wells
up to 10 m in depth can be constructed by the
drive-point method. These wells are constructed by
driving into the ground a drive point fitted to the
lower end of sections of steel a pipe. One section is
a strainer (filter) consisting of a perforated pipe
wrapped with wire mesh protected with a perforated brass sheet. Driven wells, 35 to 50 mm in
diameter, are suitable for observation purposes.


To penetrate deep aquifers, drilled wells are
constructed by the rotary or percussion-tool methods. Because drilling small-diameter wells is cheaper,
observation wells with inner diameters ranging
from 50 to 150 mm are common. Hydraulic rotary
drilling, with bits ranging in diameter from 115 to
165 mm, is often used. The rotary method is faster
than the percussion method in sedimentary formations except in formations containing cobbles,
chert or boulders. Because the rock cuttings are
removed from the hole in a continuous flow of the
drilling fluid, samples of the formations can be
obtained at regular intervals. This is done by drilling down to the sampling depth, circulating the
drilling fluid until all cuttings are flushed from the
system, and drilling through the sample interval
and removing the cuttings for the sample.
Experienced hydrogeologists and drillers can
frequently identify changes in formation characteristics and the need for additional samples by keeping
watch on the speed and efficiency of the drill.
The percussion-tool method is preferred for drilling
creviced-rock formations or other highly permeable
material. The normal diameter of the well drilled by
percussion methods ranges from 100 to 200 mm to
allow for the observation well casing to be 50 to
150 mm in diameter. The percussion-tool method
allows the collection of samples of the excavated
material from which a description of the geological
formations encountered can be obtained.
In many cases, the aquifer under study is a
confined aquifer separated by a much less permeable layer from other aquifers. Upper aquifers
penetrated during drilling must be isolated from
the aquifer under study by a procedure known as
sealing (or grouting). The grout may be clay or a
fl uid mixture of cement and water of a consistency that can be forced through grout pipes and
placed as required. Grouting and sealing the
casing in observation wells are carried out for the
following reasons:


(a) To prevent seepage of polluted surface water to
the aquifer along the outside of the casing;
(b) To seal out water in a water-bearing formation
above the aquifer under study;
(c) To make the casing tight in a drilled hole that is
larger than the casing.
The upper 3 m of the well should be sealed with
impervious material. To isolate an upper aquifer,
the seal of impervious material should not be less
than 3 m long extending above the impervious
layer between the aquifers.
In consolidated rock formations, observation wells
may be drilled and completed without casings.
Figure I.6.2 shows a completed well in a rock
formation. The drilled hole should be cleaned of
fi ne particles and as much of the drilling mud as
possible. This cleaning should be done by pumping or bailing water from the well until the water clears.

Casing is installed in wells in unconsolidated deposits. The main features of such an installation are
shown in Figure I.6.3. It should be noted that:
(a) The normal diameter of the casing in observation wells is 50 mm;
(b) At the bottom of the hole, a blank length of
casing (plugged at the lower end) is installed.
This blank casing should be at least 3 m long
and serves to collect sediment from the perforated part of the casing.

Figure I.6.2. Observation well in a rock formation

This is referred to as the debris sump;
(c) A perforated or slotted length of casing, known
as the strainer or screen, is secured to the debris
sump and ensures free interchange of water
between the aquifer and the observation well.
The screen should be about 2 m long;
(d) The blank casing above the screen should be
long enough to protrude above ground level by
about 1 m. The top of this blank casing forms
a convenient reference point for the datum of
the observation programme;
(e) Centring spiders ensure proper positioning of
the screen column in the drilled hole;
50-mm plug
Reference mark
50-mm coupling
Clay seal
Concrete seal
Clay fill (grouting)
Water table
Drilled hole
Rock formation 2.00 m 0.50 0.50 1.00
Figure I.6.2. Observation well in a rock formation
(f) In aquifers with fi ne or silty sand, the mesh
jacket and slotted casing should be protected
from clogging by fi ne material. Graded coarse
material should be packed around the screen
to fi ll the annular space between the screen
and the wall of the drilled hole. In the case of
a 150-mm hole and 50-mm casing pipe, the
normal thickness of the gravel packing should
be approximately 45 mm but should not be
less than 30 mm thick. The material may be
river gravel, ranging from 1 to 4 mm in diameter. The gravel should be placed through a guide pipe of small diameter, introduced into
the space between the casing and the wall of
the hole. The amount of gravel that is used
should be sufficient to fill both the annular
space and the bottom of the hole, that is, the
whole length of the debris sump as well as the
length of the screen and at least 500 mm of the
casing above the perforation;
(g) At ground level, a pit should be excavated
around the casing. The recommended dimensions of the pit are 800 x 800 mm at ground
level going down as a cone with a lower base
approximately 400 x 400 mm at a depth
of 1 m. Clay grout should be placed around the
casing to a depth of 2 m to make the casing
tight in the drilled hole and to prevent seepage
of polluted surface water into the aquifer. The
pit should be fi lled partly by a clay seal and the
upper part with concrete. The concrete should
be poured to fi ll the pit and form a cone around
the casing to drain precipitation and drainage
water away from the well;
(h) The upper end of the protruding casing above
the concrete cone should be closed for security purposes. Figure I.6.3 shows details of the
installation of the well. The outer 50-mm plug
is screwed to the casing by using a special tool,
and the iron plug inside the casing can be lifted
by the observer using a strong magnet.

Figure I.6.3. Observation well in a sand formation


The part of the casing extending above ground level
should be painted a bright colour to make it easy to
detect from a distance. Depth-to-water table is
measured from the edge of the casing (after removal
of plugs). This reference mark should be levelled to
a common datum for the area under investigation.
Observation wells should be maintained by the
agency responsible for the monitoring or
investigation. The area around the well should be
kept clear of vegetation and debris. A brass disc may
be anchored in the concrete seal at ground level
bearing the label “observation well” and the name
of the agency or organization. This brass disc may
also serve as a benchmark for survey purposes.

Should the protruding part of the well casing be
replaced because of damage, then the levelling of
the new reference mark is simplified by the
proximity of the benchmark. Pre-existing wells that
serve as observation wells should be maintained
and labelled in the same manner as wells drilled
specifically as observation wells.


In the area under study, several aquifers at different
levels may be separated by impervious layers of
different thicknesses. In such cases, it is advisable to
observe the following routine (Figure I.6.4):
(a) A large diameter well should be drilled, by the
percussion-tool method, until the lowest aquifer is penetrated;
(b) A small-diameter observation pipe with a proper
screen is installed in the lowest aquifer;
(c) The outer casing is lifted to reach the bottom
of the impervious layer above this aquifer. A
gravel pack is then placed around the screen
of the observation pipe and the top end of the
lower aquifer is then sealed by cement or other
suitable grout;
(d) Another small-diameter observation pipe with
a screen is then lowered to the next higher
aquifer that is again gravel packed and sealed
off by grouting from the aquifer lying above it;
(e) Steps (c) and (d) are repeated for each additional aquifer that is penetrated.
In this case, the sealing of each of the aquifers
should be done very carefully to prevent damage to
the water-bearing formation either by the interchange of water with different chemical properties
or by loss of artesian pressure. If the geology of the
area is well known and the depth to each of the
aquifers can be predicted, it may be advisable to
drill and construct a separate well in each aquifer.

Figure I.6.4. Schematic vertical cross-section of an observation well in a multiple aquifer system

Such boreholes are spaced only a few metres apart.
This procedure may prove to be more economical.
Where privately owned pumping wells are incorporated into the observation network, arrangements
could be made for such wells to be maintained by
the owners.


6.3.2 Testing of observation wells
The response of an observation well to water-level
changes in the aquifer should be tested immediately after the construction of the well. A simple
test for small-diameter observation wells is
performed by observing the recharge of a known
volume of water injected into the well, and measuring the subsequent decline of water level. For
productive wells, the initial slug of water should be
dissipated within three hours to within 5 mm of
the original water level. If the decline of the water
level is too slow, the well must be developed to
remove clogging of the screen or slots and to remove
as much as possible of the fine materials in the
formation or the pack around the well. Development
is achieved by alternately inducing movement of
the groundwater to and from the well.
After cleaning the well, the depth from the reference mark to the bottom of the well should be
measured. This measurement, compared with the
total length of casing, shows the quantity of sediment in the debris sump. This test should be
repeated occasionally in observation wells to check
the performance of the screen. If the measurement
of the bottom of the well shows that debris fills the
whole column of the sump and the screen, then the
water level in the well might not represent the true
potentiometric head in the aquifer. If the reliability
of an observation well is questionable, there are a
number of technical procedures that can be used to
make the well function adequately again.


6.3.3 Sealing and filling of abandoned
wells
Observation wells and pumping wells may be abandoned for the following reasons:
(a) Failure to produce either the desired quantity
or quality of water;
(b) Drilling of a new well to replace an existing
one;
(c) Observation wells that are no longer needed for
investigative purposes.
In all these cases, the wells should be closed or
destroyed in such a way that they will not act as
channels for the interchange of water between
aquifers when such interchange will result in significant deterioration of the quality of water in the
aquifers penetrated.
Filling and sealing of an abandoned well should be
performed as follows:
(a) Sand or other suitable inorganic material
should be placed in the well at the levels of the
formations where impervious sealing material
is not required;
(b) Impervious inorganic material must be placed
at the levels of confining formations to prevent
water interchange between different aquifers
or loss of artesian pressure. This confining
material must be placed at a distance of at
least 3 m in either direction (below and above
the line of contact between the aquifer and
the aquiclude);
(c) When the boundaries of the various formations are unknown, alternate layers of impervious and previous material should be placed
in the well;
(d) Fine-grained material should not be used as fi ll
material for creviced or fractured rock formations. Cement or concrete grout should be used
to seal the well in these strata. If these formations extend to considerable depth, alternate
layers of coarse fi ll and concrete grout should
be used to fill the well;
(e) In all cases, the upper 5 m of the well should
be sealed with inorganic impervious material.


6.4 GROUNDWATER-LEVEL
MEASUREMENTS AND OBSERVATION
WELL NETWORKS
[HOMS C65, E65, G10]
6.4.1 Instruments and methods of
observation
Direct measurement of groundwater levels in observation wells can be accomplished either manually
or with automatic recording instruments. The
following descriptions relate to principles of measurement of groundwater levels. The references
include descriptions of certain instruments.
6.4.1.1 Manually operated instruments
The most common manual method is by suspending a weighted line (for example, a graduated
fl exible steel or plastic-coated tape or cable) from a
defi ned point at the surface, usually at the well
head, to a point below the groundwater level.
On removal of the tape, the position of the groundwater level is defi ned by subtracting the
length of that part of the tape which has been
submerged from the total length of the tape
suspended in the well. This wetted part can be identified more clearly by covering the lower part of the
tape with chalk before each measurement. Colour
changing pastes have been used to indicate submergence below water, although any such substance
containing toxic chemicals should be avoided.
Several trial observations may have to be made
unless the approximate depth-to-water surface is
known before measurement. As depth-to-water
level increases and the length of tape to be used
increases, the weight and cumbersome nature of
the instrument may be difficult to overcome.
Depths-to-water surface of up to 50 m can be measured with ease and up to 100 m or more with greater
difficulty. At these greater depths, steel tapes of
narrower widths or lightweight plastic-coated tapes
can be used. Depths-to-water level can be measured
to within a few milimetres but the accuracy of
measurement by most methods is usually dependent on the depth.
Inertial instruments have been developed so that a
weight attached to the end of a cable falls at constant
velocity under gravity from a portable instrument
located at the surface. On striking water, a braking
mechanism automatically prevents further fall. The
length of free cable, equivalent to the depth-to water level, is noted on a revolution counter. The
system is capable of measurement within 1 cm,
although with an experienced operator this may be
reduced to 0.5 cm.
The double-electrode system employs two small
adjacent electrodes incorporated into a single unit
of 10 to 20 cm in length at the end of the cable.
The system also includes a battery and an electrical current meter. Current flows through the
system when the electrodes are immersed in water.
The cable must have negligible stretch and plasticcoated cables are preferred to rubber sheathed
cables. The cable is calibrated with adhesive tape
or markers at fi xed intervals of 1 or 2 m. The exact
depth-to-water level is measured by steel rule to
the nearest marker on the cable. Measurement of
water level down to about 150 m can be undertaken with ease and up to 300 m and more, with
some difficulty. The limits to depths of measurement are essentially associated with the length of
the electrical cable, the design of the electrical
circuitry, the weight of the equipment (particularly the suspended cable), and the effort in
winding-out and winding-in the cable. The degree
of accuracy of measurement depends on the operator’s skill and on the accuracy with which markers
are fixed to the cable. The fixed markers should be
calibrated and the electrical circuitry should be
checked at regular intervals, preferably before and
after each series of observations. This system is
very useful when repeated measurements of water
levels are made at frequent intervals during pumping tests.
In deep wells that require cable lengths in the order
of 500 m, the accuracy of the measurement is
approximately ±15 cm. However, measurements of
change in water level, where the cable is left
suspended in the wells with the sensor near the
water table, are reported to the nearest millimetre.
The electrochemical effect of two dissimilar metals
immersed in water can be applied to manual measuring devices. This results in no battery being
required for an electrical current supply. Measurable
current flow can be produced by the immersion in
most groundwaters either of two electrodes (for
example, magnesium and brass) incorporated into
a single unit, or of a single electrode (magnesium)
with a steel earth pin at the surface. Because of the
small currents generated, a microammeter is
required as an indicator. The single-electrode system
can be incorporated into a graduated steel tape or
into a plastic-coated tape with a single conductor
cable assembly. The accuracy of measurement
depends upon the graduations on the tape, but
readings to within 0.5 cm can be readily achieved.
A float linked to a counterweight by a cable that
runs over a pulley can be installed permanently at
an observation well. Changes in water level are
indicated by changes in the level of the counterweight or of a fixed marker on the cable. A direct
reading scale can be attached to the pulley. The
method is generally limited to small ranges in
fluctuation.
When artesian groundwater fl ows at the surface, an
airtight seal has to be fi xed to the well head before
pressure measurements can be undertaken. The
pressure surface (or the equivalent water level) can
be measured by installing a pressure gauge (visual
observations or coupled to a recording system) or,
where practicable, by observing the water level
inside a narrow-diameter extension tube made of
glass or plastic, fitted through the seal directly above
the well head. Where freezing may occur, oil or an
immiscible antifreeze solution should be added to
the water surface.


All manual measuring devices require careful
handling and maintenance at frequent intervals so
that their efficiency is not seriously impaired. The

accurate measurement of groundwater level by
manual methods depends on the skill of a trained
operator.
6.4.1.2 Automatic recording instruments
Many different types of continuous, automatically
operated water-level recorders are in use. Although
a recorder can be designed for an individual installation, emphasis should be placed on versatility.


Instruments should be portable, easily installed,
and capable both of recording under a wide variety
of climatic conditions and of operating unattended
for varying periods of time. They should also have
the facility to measure ranges in groundwater fluctuation at different recording speeds by means of
interchangeable gears for time and water-level
scales. Thus, one basic instrument, with minimum
ancillary equipment, can be used over a period of
time at a number of observation wells and over a
range of groundwater fluctuations.
Experience has shown that the most suitable
analogue recorder currently in operation is float
actuated. The hydrograph is traced either onto a
chart fixed to a horizontal or vertical drum or onto
a continuous strip chart. To obtain the best results
with maximum sensitivity, the diameter of the float
must be as large as practicable with minimum
weight of supporting cable and counterweight. As a
generalization, the float diameter should not be less
than about 12 cm, although modifications to
certain types of recorders permit using smaller diameter floats. The recording drum or pen can be
driven by a spring or by an electrical clock. The
record can be obtained by pen or by weighted stylus
on specially prepared paper. By means of interchangeable gears, the ratio of drum movement to
water-level fluctuation can be varied and reductions
in the recording of changes in groundwater levels
commonly range from 1:1 up to 1:20. The tracing
speed varies according to the make of instrument,
but the gear ratios are usually so adapted that the
full width of a chart corresponds to periods of 1, 2,
3, 4, 5, 16 or 32 days. Some strip-chart recorders can
operate in excess of six months.
Where fl oat-actuated recorders have lengths of calibrated tape installed, a direct reading of the depth
(or relative depth) to water level should be noted at
the beginning and at the end of each hydrograph
when charts are changed. This level should be
checked against manual observations at regular
intervals. The accuracy of reading intermediate
levels on the chart depends primarily upon the
ratio of drum movement to groundwater-level fluctuations, and therefore is related to the gear ratios.
The continuous measurement of groundwater level
in small-diameter wells presents problems because
a float-actuated system has severe limitations as the
diameter of the float decreases. Miniature floats or
electrical probes of small diameter have been developed to follow changes in water level. The
motivating force is commonly provided by a servomechanism (spring or electrically driven) located in
the equipment at the surface. The small float is
suspended in the well on a cable stored on a motor driven reel that is attached to the recorder pulley. In
the balanced (equilibrium) position, the servomotor is switched off. When the water table in the
well moves down, the float remains in the same
position and its added weight unbalances the cable
(or wire), causing the reel to move and, by this small
movement, causing an electrical contact to start the
small motor. The reel operated by this motor releases
the cable until a new equilibrium is reached, and
the motor is switched off. When the water level in
the well rises, the cable is retrieved on the reel until
the new equilibrium is reached. This movement of
the cable on or off the reel actuates the pen of the
recorder, and water-level fluctuations are recorded.
The servo-motor, which rotates the cable reel, may
be activated by an electrical probe at the water table
in the well. This attachment consists of a weighted
probe suspended in the well by an electric cable
stored on the motor-driven reel of the water-level
recorder. Water-level fl uctuations in the well cause
a change in pressure that is transmitted by a
membrane to the pressure switch in the probe. The
switch actuates the reel motor, and the probe is
raised or lowered, as required, until it reaches a
neutral position at the new water level. Float and
fl oat-line friction against the well casing can affect
the recording accuracy of water-level recorders,
especially in deep wells.


The largest error is caused by fl oat line drag against
the well casing. A small-diameter float may be
provided with sliding rollers (fixed at both ends of
the float) to reduce friction against the casing.
Round discs (spiders) with small rollers attached to
the cable at 10-m intervals keep the cable away from
the well casing and significantly reduce friction.
Figure I.6.5 shows some details of this device. The
sensitivity of water-level recorders with attachments
for small-diameter fl oats may be 6 mm of water-level
movement, but the switching mechanism of the
fl oat may not be this sensitive. The accuracy of the
mechanism is decreased by weak batteries. To avoid
this effect, the batteries should be replaced after a
maximum of 60 to 90 days of normal use.
An alternative approach is an electrode suspended
in an observation well at a fixed distance above the water level. At specified time intervals, the probe
electrically senses the water level and the movement occurs by a servo-mechanism at the surface.
The depth-to-water level is then recorded. This
system can be adapted to various recording
systems.


Although these instruments have particular value
in small-diameter wells, they can be installed in
wells of any diameter greater than the working
diameter of the probe.
Analogue-to-digital stage recorders used for stream
discharge measurements can be readily adapted to
the measurement of groundwater levels.
Automatic recording instruments require comprehensive and prompt maintenance otherwise records
will be lost. Simple repairs can be undertaken on
the site, but for more serious faults, the instrument
should be replaced and repairs should be undertaken in the laboratory or workshop. Adequate
protection from extremes of climatic conditions,
accidental damage and vandalism should be
provided for these instruments. Clockwork is particularly susceptible to high humidity, thus adequate
ventilation is essential, and the use of a desiccant
may be desirable under certain conditions.
In some research projects, instruments have been
designed to measure fluctuations in groundwater
level by more sophisticated techniques than those
described above, such as capacitance probes, pressure transducers, strain gauges, and sonic and
high-frequency wave reflection techniques. At
present, these instruments are expensive when
compared with fl oat-actuated recorders, have limitations in application, particularly in the range of groundwater fluctuations, and commonly require
advanced maintenance facilities. Float-actuated
systems are considered more reliable and more
versatile than any other method, although future
developments in instrument techniques in the
sensor, transducer and recording fields may provide
other instruments of comparable or better performance at competitive costs.


6.4.1.3 Observation well network
An understanding of the groundwater conditions
relies on the hydrogeological information available;
the greater the volume of this information the
better the understanding as regards the aquifers,
water levels, hydraulic gradients, flow velocity and
direction and water quality, among others. Data on
potentiometric (piezometric) heads and water quality are obtained from measurements at observation
wells and analysis of groundwater samples. The
density of the observation well network is usually
planned on the data requirement but in reality is
based on the resources available for well construction. Drilling of observation wells is one of the main
costs in groundwater studies. The use of existing
wells provides an effective low-cost option
Therefore, in the development of an observation
network, existing wells in the study area should be
carefully selected and supplemented with new wells
drilled and specially constructed for the purposes of
the study.


6.4.1.4 Water-level fl uctuations
Fluctuations in groundwater levels reflect changes
in groundwater storage within aquifers. Two main
groups of fluctuation can be identified: long-term,
such as those caused by seasonal changes in natural
replenishment and persistent pumping, and shortterm, for example, those caused by the effects of
brief periods of intermittent pumping and tidal and
barometric changes. Because groundwater levels
generally respond rather slowly to external changes,
continuous records from water-level recorders are
often not necessary. Systematic observations at
fi xed time intervals are frequently adequate for the
purposes of most national networks. Where fluctuations are rapid, a continuous record is desirable, at
least until the nature of such fluctuations has been
resolved.
6.4.1.5 Water-level maps
A useful approach to organize and coordinate water level measurements from a network of observation
wells is to produce an accurate map of well locations and then to contour the water-level data
available at each well. Two types of maps can be
produced, based on either the depth-to-water level
measured in a well from the land surface or the
elevation of the water level in the well relative to an
established datum, such as sea level. Generally,
these maps are produced on a single aquifer basis
using data collected on a synoptic basis for a discrete
period of time, to the extent possible. Seasonal fluctuations in water levels, changes in water levels
over a period of years as a result of pumping, and
similar effects can cause disparate variations if a
mixture of data is used.
6.4.1.5.1 Depth-to-water maps
The simplest map to produce is based on the measurement of the depth-to-water level in a well relative
to land surface. This is referred to as a depth-to-water map. Maps of this type provide an indication
as to the necessary depth to drill to encounter water,
which can be useful in planning future resource
development projects. A map based on the difference in depth to water between two measurement
periods could be used to show, for example, the
areal variation of seasonal fluctuations. A signifi –
cant limitation of a depth-to-water map is that it
cannot be used to establish the possible direction of
groundwater flow because of the independent variation of topographic elevation.

Figure I.6.5. Small-diameter fl oat with sliding rollers in an observation well


6.4.1.5.2 Potentiometric (Piezometric) surface
maps/water table maps, potentiometric
cross-sections
A water-level map based on the elevation of the
water level in a well relative to a common datum,
such as sea level, is referred to as a potentiometric
surface map (Figure I.6.6). When produced for the
water table or the surficial aquifer, this map may be
referred to as a water table map. This type of map is
more difficult to produce than a depth-to-water
map because it requires accurate elevation data for
the measuring point at each observation well. Each
depth-to-water measurement collected must be
subtracted from the elevation of the measuring
point relative to the datum to produce the necessary
data. A significant benefit of this type of map is that
it can be used to infer the direction of groundwater
fl ow in many cases.
The accuracy of the map is dependent on the accuracy of the measuring point elevations. The most
accurate maps will be based on elevations that have
been established using formal, high-order land surveying practices.

Figure I.6.6. Example of a potentiometric surface map (Lacombe and Carleton, 2002)

This can entail substantial effort and expense. Several alternatives exist. These are the use of elevations determined from topographic maps, if they exist for the study area, or the use of an altimeter or GPS unit to provide elevation information. Any report showing a potentiometric surface map must have an indication of
the source and accuracy of the elevation data.


Maps portray information in two spatial dimensions.
As groundwater flows in three dimensions, another
view is required to understand the potentiometric
data in all directions. With potentiometric surface
data from multiple aquifers or depths at each or
many data sites of an observation well network it is
possible to produce potentiometric cross-sections
(Figure I.6.7). Potentiometric cross-sections are an
accurately scaled drawing of well locations along a
selected transect indicating depth on the vertical
axis and lateral distance on the horizontal axis. A
particular well’s water level is plotted with respect

to the depth axis. It is customary to also indicate a
well’s open interval on the diagram. These crosssections can show the relative differences in water
levels between aquifers and can be very useful in
determining the vertical direction of groundwater
flow.


6.4.1.6 Well discharge measurements
Pumping wells can have a significant effect on
groundwater flow and levels. The measurement of a
pumping well’s discharge is important to facilitate
comparisons of drawdown effects and for quantitative analysis. The common methods of measurement
include the timed fi ll of a calibrated volume, flow
meters and orifice discharge measurements
(American Society for Testing and Materials
International: ASTM D5737-95, 2000). The discharge
of a pumping well will vary with changes in groundwater level. This may require repeated measurements
to keep track of the rate. When a pump is turned
on, the water level in a well drops accordingly,
thereby causing the discharge to vary. Stability in
pumping rate is generally reached in a matter of
minutes or hours. Water-level changes that could
affect pumping rate can also occur as a result of
recharge from precipitation or changes in pumping
of nearby wells. Changes in the configuration of
the discharge plumbing, such as pipe length or
diameter to a point of free discharge, can also
have an effect and should be avoided. These flow
measuring procedures can also be applied to measuring the discharge of a naturally flowing well.


6.4.1.6.1 Calibrated volume
The simplest method of determining the rate of
discharge from a pumping well is by measuring the
time the pumped discharge takes to fi ll a calibrated
volume. Dividing that volume by the time yields
the unit pumping rate. The accuracy of the measurement is dependent on the accuracy of the time
measurement and the logistics of fi lling the calibrated volume. For relatively low pumping, this
measurement is easily handled using a bucket or
drum with calibration marks. However, at relatively
high discharge rates, a measurement of this type
may require some logistical planning in order to
direct the discharge into an appropriate vessel or
container for measurement. The force of the
discharge stream or the presence of entrained air
can complicate the situation.


6.4.1.6.2 Flow meters
A variety of mechanical, electrical and electronic
meters have been developed to measure fluid flow
inside a pipe. Many of these can be easily used to
measure the rate of discharge from a pumped well.
Some meters provide an instantaneous discharge
reading while others compile a totalized reading of
flow. Either type can be used. Some versions have the ability to interface with electronic data logging
equipment. The appropriate instructions from the
manufacturer should be followed to ensure an accurate measurement. Flow meter readings can be
sensitive to the presence of turbulence in the flow.
Operational instructions may require that a
prescribed length of straight pipe precede the meter
to minimize turbulence effects. Additionally, a fullpipe condition is required for most meters. When a
relatively large-diameter pipe serves as a conduit for
a relatively small discharge rate, the pipe may not
be entirely filled with water. To maintain a full-pipe
condition, a valve positioned downstream of the
meter can be partially closed. Entrained air or sediment in the flow can possibly affect the accuracy of
the reading and in the case of sediment, can possibly damage the sensing equipment.


6.4.1.6.3 Orifice discharge
Another common method for measuring the
discharge from a pumped well is the use of a free
discharge pipe orifice. An orifice is an opening in a
plate of specified diameter and beveled-edge
configuration that is fixed, usually by a flange,
over the end of a horizontal discharge pipe
(Figure I.6.8). The diameter of the orifice should be
smaller than the diameter of the pipe. The water
flowing through the discharge pipe is allowed to
freely exit through the orifice. As the orifice somewhat restricts the flow, a back pressure results that
is proportional to the flow. This pressure is measured, usually by direct measurement of a
manometer tube, located about three pipe diameters upstream of the orifice and at the centre line
of the pipe. The measured pressure value, the
discharge pipe diameter and the orifice diameter
are used to enter an “orifice table” to determine
the flow. These tables and the specific requirements for the design of the discharge pipe and
orifice can be found in ISO 5167-2 (2003b).
6.4.1.6.4 Specifi c capacity
A useful index to facilitate a comparison of water level drawdown and discharge rates among wells is
specific capacity. This parameter is defined as the
well’s steady-state discharge rate divided by the
drawdown in the pumped well from its nonpumping state to the steady-state pumping level
(m3 s–1m–1).
6.4.1.7 Drawdown from a pumped well;
cone of depression
The movement of water from an aquifer into a
pumped well is impeded by frictional resistance
with the aquifer matrix. This resistance results in a
lowering or decline in the water level in a well being
pumped and in the adjacent parts of the aquifer.
This decline is referred to as drawdown. Drawdown
is defi ned as the change in water level from a static
pre-pumping level to a pumping level. Water-level
decline resulting from pumpage diminishes nonlinearly with distance away from the pumped well.
The resulting shape is referred to as a cone of depression. The drawdown and resulting cone of
depression in an unconfined aquifer is the result of
gravity drainage and desaturation of part of the
aquifer in the vicinity of the well (Figure I.6.9 (left)).
In a confined aquifer, the cone of depression is
manifested as a decline in the potentiometric
(piezometric) surface, but does not represent a
desaturation of the aquifer (Figure I.6.9 (right)). The
relation between pumping rate, water-level decline
and distance from the well is a function of the
prevailing permeability of the aquifer material and
the availability of sources of recharge.

Figure I.6.7. Example of a potentiometric cross-section indicating the vertical head relation between
several aquifers (Buxton and Smolensky, 1999)


6.5 AQUIFER AND CONFINING-BED
HYDRAULIC PROPERTIES
Quantitative analysis of groundwater fl ow involves
understanding the range and variability of key
hydraulic parameters. Many data-collection
networks and surveys are organized to collect data
for the purpose of determining aquifer and confi ning bed properties.
6.5.1 Hydraulic parameters
The movement of groundwater is controlled by
certain hydraulic properties, the most important
being the permeability. For the study of the
movement of water in earth materials the parameter
for permeability is calculated assuming the physical
properties (viscosity, etc.) of water and is termed
hydraulic conductivity. Hydraulic conductivity is
defi ned as the volume of water that will move in a
unit time under a unit hydraulic gradient through a
unit area, which results in units of velocity (distance
per time). The typical ranges of hydraulic
conductivity for common rock and sediment types
are shown on Figure I.6.10. A related term is
transmissivity, which is defined as the hydraulic
conductivity multiplied by the aquifer thickness.
The difference between the two are that hydraulic
conductivity is a unit property, whereas transmissivity pertains to the entire aquifer.
Storage coefficient is defined as the volume of water
that an aquifer releases from or takes into storage the ability to interface with electronic data logging
equipment. The appropriate instructions from the
manufacturer should be followed to ensure an accurate measurement. Flow meter readings can be
sensitive to the presence of turbulence in the flow.
Operational instructions may require that a
prescribed length of straight pipe precede the meter
to minimize turbulence effects. Additionally, a fullpipe condition is required for most meters. When a
relatively large-diameter pipe serves as a conduit for
a relatively small discharge rate, the pipe may not
be entirely filled with water. To maintain a full-pipe
condition, a valve positioned downstream of the
meter can be partially closed. Entrained air or sediment in the flow can possibly affect the accuracy of
the reading and in the case of sediment, can possibly damage the sensing equipment.
6.4.1.6.3 Orifice discharge
Another common method for measuring the
discharge from a pumped well is the use of a free
discharge pipe orifice. An orifi ce is an opening in a
plate of specified diameter and beveled-edge
configuration that is fi xed, usually by a flange,
over the end of a horizontal discharge pipe
(Figure I.6.8). The diameter of the orifice should be
smaller than the diameter of the pipe. The water
flowing through the discharge pipe is allowed to
freely exit through the orifice. As the orifice somewhat restricts the flow, a back pressure results that
is proportional to the flow. This pressure is measured, usually by direct measurement of a
manometer tube, located about three pipe diameters upstream of the orifice and at the centre line
of the pipe. The measured pressure value, the
discharge pipe diameter and the orifice diameter
are used to enter an “orifice table” to determine
the flow. These tables and the specific requirements for the design of the discharge pipe and
orifice can be found in ISO 5167-2 (2003b).
6.4.1.6.4 Specifi c capacity
A useful index to facilitate a comparison of water level drawdown and discharge rates among wells is
specifi c capacity. This parameter is defined as the
well’s steady-state discharge rate divided by the
drawdown in the pumped well from its non pumping state to the steady-state pumping level
(m3 s–1m–1).
6.4.1.7 Drawdown from a pumped well;
cone of depression
The movement of water from an aquifer into a
pumped well is impeded by frictional resistance
with the aquifer matrix. This resistance results in a
lowering or decline in the water level in a well being
pumped and in the adjacent parts of the aquifer.
This decline is referred to as drawdown. Drawdown
is defined as the change in water level from a static
pre-pumping level to a pumping level. Water-level
decline resulting from pumpage diminishes nonlinearly with distance away from the pumped well.

Figure I.6.10. Hydraulic conductivity of common rock and sediment types (Heath, 1983)


The resulting shape is referred to as a cone of depression. The drawdown and resulting cone of depression in an unconfined aquifer is the result of gravity drainage and desaturation of part of the
aquifer in the vicinity of the well (Figure I.6.9 (left)).


In a confined aquifer, the cone of depression is
manifested as a decline in the potentiometric
(piezometric) surface, but does not represent a
desaturation of the aquifer (Figure I.6.9 (right)). The
relation between pumping rate, water-level decline
and distance from the well is a function of the
prevailing permeability of the aquifer material and
the availability of sources of recharge.

Figure I.6.7. Example of a potentiometric cross-section indicating the vertical head relation between several aquifers (Buxton and Smolensky, 1999)


6.5 AQUIFER AND CONFINING-BED
HYDRAULIC PROPERTIES
Quantitative analysis of groundwater fl ow involves
understanding the range and variability of key
hydraulic parameters. Many data-collection
networks and surveys are organized to collect data
for the purpose of determining aquifer and confining bed properties.


6.5.1 Hydraulic parameters
The movement of groundwater is controlled by
certain hydraulic properties, the most important
being the permeability. For the study of the
movement of water in earth materials the parameter
for permeability is calculated assuming the physical
properties (viscosity, etc.) of water and is termed
hydraulic conductivity. Hydraulic conductivity is
defi ned as the volume of water that will move in a
unit time under a unit hydraulic gradient through a
unit area, which results in units of velocity (distance
per time). The typical ranges of hydraulic
conductivity for common rock and sediment types
are shown on Figure I.6.10. A related term is
transmissivity, which is defined as the hydraulic
conductivity multiplied by the aquifer thickness.
The difference between the two are that hydraulic
conductivity is a unit property, whereas transmissivity pertains to the entire aquifer.

Figure I.6.9. Drawdown from a pumped well in (left) an unconfined aquifer and (right) in a confined aquifer (Heath, 1983)


Storage coefficient is defined as the volume of water
that an aquifer releases from or takes into storage per unit surface area of the aquifer per unit change in head. Storage coefficient is a dimensionless parameter. For an unconfined aquifer, the storage
coefficient is essentially derived from the yield by
gravity drainage of a unit volume of aquifer, and
typically ranges from 0.1 to 0.3 in value. For a
confined aquifer, where saturation remains full, the
storage results from the expansion of water and
from the compression of the aquifer. The storage
coefficient for a confined aquifer is therefore usually
several orders of magnitude smaller than for an
unconfined aquifer, typically ranging from 0.00001
to 0.001 in value.


Hydraulic conductivity and storage coefficient can
be determined for confining units as well as aquifers. The differentiation of an aquifer from a
confining unit is relative. For a given area, aquifers
are considered to have hydraulic conductivities that
are several orders of magnitude larger than confining units.
6.5.2 Overview of common field
methods to determine hydraulic
parameters
The determination of the hydraulic conductivity
and storage coefficient specific to a particular aquifer or confi ning unit is generally accomplished
through tests conducted in the fi eld, referred to as
aquifer or pumping tests. These aquifer tests are
devised to measure the drawdown resulting from
pumping or a similar hydrological stress and then
to calculate the hydraulic parameters. The magnitude and timing of drawdown related to a specific
test is directly related to the hydraulic conductivity
and storage coeffi cient, respectively.
6.5.2.1 Aquifer (pumping) tests
The general aim of an aquifer test is to determine
hydraulic parameters where pumping is controlled
and generally held constant and water levels in the
pumped well and nearby observation wells are
measured. Figure I.6.11 shows a schematic diagram
of the set-up of a typical test of a confi ned aquifer
of thickness, b. Three observation wells, labelled A,
B and C, are located at various radii (r at well B)
from the pumped well. The pumping, of known
discharge, causes a cone of depression in the aquifers potentiometric (piezometric) surface to form
which results in a drawdown, s, measured at well B,
which is the difference between the initial head, h0,
and the pumping head, h. Water-level data in each
well including the pumping well are collected prior
to the start of pumping to establish the pre-test
static water level, and thereafter throughout the
test. The pump discharge is also monitored.
Aquifer tests typically are run from 8 hours to a
month or longer, depending on the time required
to achieve a steady pumping water level. When the
pump is turned on, water levels will drop. The largest drawdown will be in the pumped well with
drawdown decreasing non-linearly with distance
away from the pumped well and increasing non-linearly with time. The observed values are the
changing drawdown with time. This is referred to
as a transient test, because of the changing drawdown with time. The data are generally plotted in
two ways, as either log-log or semi-log graphs of
distance and drawdown, or time and drawdown.
The distance drawdown plot is for data from all
wells for a particular instant in time, whereas the
time-drawdown plot is for all data collected at one
well. Generally, the analysis of the test data proceeds
by either a manual, graphically based method or
through the use of a computer program for aquifertest analysis. The manual, graphically based method
relies on an approach where the data plots are overlaid and matched to established “type curves” to
calculate a solution for hydraulic conductivity and
storage coefficient. The multiple plots are analysed
individually and an average or consensus value for
the test is determined.


It is beyond the scope of this Guide to explain in
detail the exact data collection and analysis
approach because there are many variants of the
procedure. The variants result from a wide range of
factors that can substantially affect the operation of
the test and the specific analysis procedure, such as
whether the test is transient or steady state, run
with many observation wells or one, whether fl ow
(leakage) to the tested aquifer will be considered
from adjacent aquifers or confining units, and
whether the aquifer is confined or unconfined. The
practitioner is directed to Walton (1996), Kruseman
and others (1994) and Reed (1980), for both an
overview of the many common methods and a
detailed description of the analysis techniques.
Additionally, standards for conducting and analysing aquifer tests have been developed by the
International Organization for Standardization (ISO
14686, 2003a) and the American Society for Testing
and Materials International (ASTM D4106-96,
2002). An example of spreadsheet-formulated aquifer test analysis is presented by Halford and
Kuniansky (2002).


6.6 RECHARGE AND DISCHARGE,
SOURCES AND SINKS IN A
GROUNDWATER SYSTEM
Recharge and discharge are the pathways by which
water enters and leaves the groundwater system.
Understanding and quantifying these pathways
are key to understanding the nature of the whole
groundwater system and being able to predict
potential changes. The significant sources of
recharge are from precipitation and leakage from
surface water bodies, such as streams, rivers, ponds
and lakes. The significant sources of discharge are
leakage to surface water bodies, such as streams,
rivers, ponds, lakes and oceans; well pumpage; and
evapotranspiration.

FigureI. 6.11. Schematic diagram of a typical aquifer test showing the various measurements
(Heath, 1983)

6.6.1 Recharge from precipitation
Precipitation that percolates through the soil ultimately can recharge the groundwater system. This
typically occurs in areas of relatively high topographic level and is controlled by the permeability
of the soils. Individual recharge events can be identified as rises in water table hydrographs. If the
porosity of the aquifer material is known, generally
ranging from 5 to 40 per cent, an estimate of the
recharge volume can be made for a unit area of
aquifer, as water-level rise x porosity (as a fraction)
x area.
6.6.2 Groundwater–surface water
relationships
In many areas, the groundwater system is directly
linked to the surface water system in such a way
that even a large volume of water can fl ow from
one to the other. It is important to understand this
relationship.
6.6.2.1 Gaining and losing streams
The elevation of water levels in a stream relative to
the adjacent water level in a surficial or water table
the aquifer will control the direction of flow between
these two parts of the hydrological system. The
situation where the stream level is below the water
table in the underlying aquifer that drives flow
upward to the stream is referred to as a gaining
stream (Figure I.6.12, top). The reverse situation
where the stream level is above the water table in
the underlying aquifer that drives flow downward
to the aquifer is referred to as a losing stream
Figure I.6.12, middle). In some cases, especially in
an arid setting, the aquifer may not have a saturated
connection with the stream. This case is also a
losing stream (Figure I.6.12, bottom).
The groundwater recharge from a losing stream or
groundwater discharge to a gaining stream can be
quantifi ed or measured in several ways:
(a) For a gaining stream, examination of a longterm hydrograph record can indicate the
base flow. The base-flow portion of a stream
hydrograph (Volume II, 6.3.2.2.2) is likely
to include the groundwater discharge. Other
constant discharges from reservoirs or sewage
treatment plants, for example, can also contribute to base flow;
(b) For either a losing or gaining stream, differential streamflow ow discharge measurements taken
at an upstream and a downstream point on
a reach will show the loss or gain within the
uncertainty associated with the measurement
(Chapter 5). The selected stream reach should
not have any other inputs or outputs, such
as tributaries, sewage treatment plants, water
intake plants or irrigation returns;
(c) A direct measurement of the discharge to or
from a stream can be made with seepage meters.
These are instruments that are placed in the
stream bed and that retain the volume of water
seeping through the stream bed for subsequent
measurement (Carr and Winter, 1980). Some
of these devices may only be able to work in
a gaining stream condition. These instruments
are sensitive to, and may not be able to work in, relatively high stream velocities because of
scour.

Figure I.6.12. Relative configuration of groundwater level and stream level for gaining and losing streams (Winter and others, 1998)


6.6.2.2 Springs and seeps
Discharge from springs and seeps, which represent
localized groundwater discharge, can be measured
using standard stream discharge measurement
procedures (Chapter 5).
6.6.2.3 Effects of evapotranspiration on
groundwater system
Deep-rooted plants and plants in general in areas of
a shallow water table can derive water from the
groundwater system. The standard methods to
determine potential evapotranspiration rates can
be implemented for areas where groundwater is
likely to be involved (Chapter 4).
6.6.3 Well pumpage
Pumpage from individual wells and the cumulative
effects of pumpage of many wells in an area can
have a very significant impact on groundwater
levels and the groundwater system in general. It is a
common occurrence for the drawdown from a
pumped well to cause a nearby stream to change
from a gaining to a losing configuration, underscoring the importance of tracking the location and
effect of well pumpage. In particular, wells for
public supply, industrial or commercial use, and for
irrigation pump the largest amounts. Quantification of the pumpage amounts requires a tallying of
reports by the well-owners, or lacking those reports,
an effort to measure the significant users. The procedures detailed in 6.4 can be used to make those
measurements. As pumping can change with the
demands of the well users, keeping track of those
changes can require much effort. It is possible to
develop a relation between the pump discharge and
the pump’s electrical or fuel usage. If such data are
available, this can ease the burden of compiling or
collecting pumpage data for a large number of
wells.


6.7 USE OF DATA IN GROUNDWATER
MODELS
A primary role of a model is the integration of
hydrogeological framework information, water level data, pumpage, and recharge and discharge
information in order to understand the relative
importance of the various processes of the
groundwater system, and to appraise the capacity or capability of the groundwater system to meet
general or specific (usually water supply) goals. The
commonly used modelling options range from
development of a simple water budget to the
development of a complex digital groundwater flow model. It is beyond the scope of this Guide to
provide the detailed background for the
development, calibration and use of groundwater
models; however the methods and approaches for
the collection of data outlined in this chapter and
in Table I.6.3 provide the necessary foundation to
the development of models. Further discussion on
the subject of groundwater modelling as well as
references on the subject are provided in Volume II,
6.3.5.2.


6.8 REMOTE-SENSING [HOMS D]
Currently, there are no direct remote-sensing techniques to map areas of groundwater. However,
indirect information can be obtained from remote sensing sources.
Remote-sensing techniques used to map areas of
groundwater include aerial and satellite imagery in
the visible, infra-red and microwave regions of EMS.
In particular, satellite imagery enables a view of
very large areas and achieves a perspective not
possible from ground surveys or even low-level aerial photography. Although remote-sensing is
only one element of any hydrogeological study, it is
a very cost-effective approach in prospecting and in
preliminary surveys. Owing to the intervening
unsaturated zone of soil, most remote-sensing data
cannot be used directly, but require substantial
interpretation. As a result, inference of location of
aquifers is made from surface features. Important
surface features include topography, morphology
and vegetation. Groundwater information can be
inferred from landforms, drainage patterns, vegetation characteristics, land-use patterns, linear and
curvilinear features, and image tones and textures.
Structural features such as faults, fracture traces and
other linear features can indicate the possible presence of groundwater. Furthermore, other features,
such as sedimentary strata or certain rock outcrops,
may indicate potential aquifers. Shallow groundwater can be inferred by soil moisture measurements,
changes in vegetation types and patterns, and
changes in temperature. Groundwater recharge and
discharge areas within drainage basins can be
inferred from soils, vegetation and shallow or
perched groundwater (Engman and Gurney, 1991).
Airborne exploration for groundwater has recently
been conducted using electromag net ic prospecting
sensors developed for the mineral industry (Engman
and Gurney, 1991). This type of equipment has
been used to map aquifers at depths greater than
200 m (Paterson and Bosschart, 1987).
Aerial photography supplemented by satellite data
from Landsat or SPOT are widely used for groundwater inventories, primarily for locating potential
sources of groundwater. This technique permits
inferences to be made about rock types, structure
and stratigraphy. IR images are valuable for mapping
soil type and vegetative surface features used in
groundwater exploration. Springs can best be
detected using IR and thermal imagery. Underwater
springs can be detected by this method (Guglielminetti
and others, 1982). Furthermore, through temperature differences, thermal IR imagery has the potential
to deduce information on subsurface moisture and
perched water tables at shallow depths (Heilman and
Moore, 1981a and 1981b; Salomonson, 1983; van de
Griend and others, 1985).
Passive microwave radiometry can be used to measure shallow groundwater tables. A dual frequency
radiometer has been used on an aircraft to measure
water table depths of 2 m in humid areas and 4 m
in arid areas (Shutko, 1982; 1985; 1987).
Radar has an all-weather capability and can be used
to detect subtle geomorphic features even over
forested terrain (Parry and Piper, 1981). Radar is
also capable of penetrating dry sand sheets to
disclose abandoned drainage channels (McCauley
and others; 1982; 1986), and can also provide information on soil moisture (Harris and others, 1984).
Radar images may be used to detect water which is
several decimetres below the ground surface in arid
areas, owing to the increase in soil moisture near
the surface. Near-viewing short pulse radars installed
on mobile ground or aircraft platforms provide
information on the depth to a shallow water table
down to 5–50 m (Finkelstein and others, 1987).
Radar imagery has the potential to penetrate the
dense tropical rainforest and rainfall, and yield
information that can be used to produce a geological map for use in groundwater exploration (Engman
and Gurney, 1991). Radar imagery has been used
successfully to reveal previously unchartered
network of valleys and smaller channels buried by
the desert sands (McCauley and others, 1986).
A comprehensive state-of-the-art review of
remote-sensing applications to groundwater
(Meijerink in Schultz and Engman, 2000) as well
as references to a number of specific applications
are included in the list of references and further
reading below.
References and further reading
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American Society for Testing and Materials International,
2000: ASTM D5737-95 (2000) Standard Guide for
Methods for Measuring Well Discharge. American
Society for Testing and Materials International, West
Conshohocken, Pennsylvania.
American Society for Testing and Materials International,
2002: ASTM D4106-96 (2002) Standard Test Method
(Analytical Procedure) for Determining Transmissivity
and Storage Coeffi cient of Non-leaky Confi ned aquifers by the Theis Non-equilibrium Method. American
Society for Testing and Materials International, West
Conshohocken, Pennsylvania.
Buxton, H.T. and D.A. Smolensky, 1999: Simulation
of the Effects of Development of the Ground Water
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States Geological Survey Water-Resources
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Carr, M.R. and T.C. Winter, 1980: An Annotated
Bibliography of Devices Developed for Direct
Measurement of Seepage. United States Geological
Survey Open-File Report 80-344.

Dutartre, P., J.M. Coudert and G. Delpont, 1993:
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El-Baz, F., 1993: TM reveals Arabian Desert secrets.
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Engman, E.T. and R.J. Gurney, 1991: Remote Sensing in
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