GROUNDWATER, WELLS AND PUMPS

Unit-I
Occurrence and movement of ground water; aquifer and its types; classification of wells, fully
penetrating tubewells and open wells, familiarization of various types of bore wells; design of open
wells

➢ Groundwater
Groundwater is broadly defined as the water present in the zone of saturation below the ground.
Groundwater and the Water Cycle

Water perpetually circulates on the earth from the oceans to the atmosphere to land and back to the
oceans; this is called water cycle or hydrologic cycle. Note that the term ‘hydrologic cycle’ literally
means “Water-Science Cycle”, and hence the correct term to describe this cyclic movement of water
in nature is water cycle, which should be used instead of widely-used term ‘hydrologic cycle’. The
major pathways in the water cycle are schematically shown in Fig. Thus, the water cycle describes
how water moves into and out of various domains viz., atmosphere, land surface, subsurface
(underground) and oceans. The main components of water cycle are precipitation, evaporation,
transpiration, infiltration, surface runoff (overland flow and streamflow), and subsurface runoff
(interflow, vadose-water flow and groundwater flow).

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 Importance of Groundwater

The study of groundwater is essential because of several reasons. Of the freshwater readily available
for human use (approximately 1% of the liquid freshwater available on the earth), about 98% is
groundwater and the remaining is surface water. Hence, groundwater serves as a major source of water
supply to life (humans, animals and ecosystems) throughout the world. Because of its physical and
chemical quality, groundwater provides a reliable source of water supply in both humid and arid/semi-
arid regions of the world and during emergencies (e.g., droughts, earthquakes, etc.) as well as it
sustains flow in rivers/streams and lakes during dry periods. Thus, groundwater is one of the most
valuable natural resources of the earth, which supports human health, human livelihoods, socio-
economic development, and ecological diversity.

Besides the above-mentioned vital roles, groundwater also influences the design and construction of
engineering facilities such as dams, open-pit mines, tunnels, deep foundations, and geologic storage
of nuclear wastes or carbon sequestration. Groundwater is also important due to its geologic role by
supporting various geological processes such as the formation of soils and their alternation, the
development of landslides, rock falls, channel networks and karst landscapes, oil formation and
valuable mineral deposits. Thus, groundwater plays a variety of roles on a global scale, which make
this resource so vital for human beings. However, the water resource and engineering aspects of
groundwater hydrology are the major focus of practice, though the groundwater hydrology field has a
rich relationship with other earth sciences.

➢ Occurrence and movement of ground water

Vertical Distribution of Subsurface Water

In order to understand the occurrence of groundwater and its vertical distribution, let’s first consider
the hydrological zones present below the ground Fig. The zone between the ground surface and the
top of capillary fringe is called unsaturated zone (or, zone of aeration) which consists of voids (pores
or interstices) partially filled with water and partially with air. Water is held at a pressure less than the
atmospheric pressure in the unsaturated zone. The zone between bottom of the unsaturated zone and
top of the water table is called capillary zone, wherein most voids are filled with water but the water
is held at a pressure less than the atmospheric pressure. Finally, the zone extending from the water
table to an impermeable layer is called saturated zone (or, zone of saturation), wherein all voids are
completely filled with water. In this zone, water is held at a pressure greater than the atmospheric
pressure, and hence it moves in a direction based on the contiguous hydraulic situation.

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 Fig. Hydrologic zones below the ground.

The unsaturated zone can be further sub-divided into ‘soil-water zone’ and ‘intermediate zone’ (Todd,
1980). The zone between the ground surface and the top of water table is known as the vadose zone.
Thus, the vadose zone consists of unsaturated zone and capillary zone (also known as ‘capillary
fringe’). The water present in the vadose zone is called vadose water which is held at a pressure less
than the atmospheric pressure. Hence, while this water is still able to move within the vadose zone due
to matric potential and gravity, it cannot move out of the zone into wells, pits, or other water collection
systems that are exposed to the atmospheric pressure. Note that the term vadose zone is technically
more appropriate than the conventional term unsaturated zone. This is because portions of the vadose
zone may actually be saturated, even though the pressure of water is below the atmospheric pressure.
Hence, the term vadose zone has become popular and is widely used these days in the fields of
groundwater hydrology and soil physics.

Broadly speaking, the water stored in the zone of saturation is called groundwater. Not all underground
water is groundwater, rather only free water or gravitational water (the water that moves freely under
the force of gravity into wells) constitutes the groundwater. Therefore, a precise and practical
definition of groundwater is (Bouwer, 1978): “Groundwater is that portion of the water beneath the
earth’s surface, which can be collected through wells, tunnels, or drainage galleries, or which flows
naturally to the earth’s surface via seeps or springs”. Depths to groundwater may range from 1 m or
less to 1000 m or more. There are also places where groundwater does not exist at all.

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 Occurrence of Groundwater
The occurrence of groundwater is influenced by the following factors:

Topography: A water table is not horizontal, but follows the topography, or upward and downward
tilts, of the land above them. The water table is higher near the hilltops and lower near the valleys,
because water seepage into streams, swamps and lakes cause lowering of water table near the
valleys.
In an aquifer, the water table is mostly vertical and reflects the surface relief of the area above.
This is due to the capillary effect of the soil, sediments and other porous material. In the aquifer,
the flow of groundwater is in both horizontal and vertical directions and from points of higher
pressure to lower pressure. The slope of the water table is known as the hydraulic gradient. This
depends on first, the rate at which water adds and subtracts from the aquifer and second, the
permeability of the material. At times, the water table of an area is not dependent on the
topography. This may be due to variations in the underlying geological structure (e.g., folds, faults,
fractures in the bedrock). Sometimes, a water table intersects with the land surface.

The fluctuation of the water table is due to the fact that in areas which experience monsoon climate
like as in the Indian subcontinent, especially during dry seasons, the water table flattens and gradually
the high water table beneath the hills decreases to the
level of valleys.
In addition to topography, water table is influenced by many factors, including climate,
land use, geology, etc.
Climate: In humid regions, recharge areas through which water percolates underground are found
everywhere except streams, and adjacent floodplains. On the other hand, in arid regions recharge
areas encompass only the mountains and adjoining alluvial fans and the streams below which porous
alluvium soil is there through which water can percolate and recharge the groundwater. Groundwater
is available at great depth in dry or arid regions whereas it exists at shallow depth in humid areas.
Seasonally, water table rises during the rainy season and sinks in the dry season. Change in climate, for
example, major increases in frequency and intensity of exceptional rainfall events in groundwater
recharge areas can result in water tables rising to levels higher than previously recorded maxima,
causing extensive ‘groundwater flooding’ with damage to property and crops.
Land use can also influence an area's water table. Most groundwater is formed from excess rainfall
percolating the land surface. Urban areas often have impervious surfaces, such as roads etc. These
prevent water from seeping into the ground below. Instead of entering the zone of saturation, water
becomes runoff and the water table dips. Lands having irrigation from surface water source has the
greatest impact on groundwater, both quantitatively and qualitatively as excess water infiltrates into
the shallow aquifers.
Geology often determines the quantity of water that filters below the zone of saturation, making
the water table easy to measure. Porous rocks can hold more water than dense rocks. For example,

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 an area underlain with pumice which is a light and porous rock can retain a fuller aquifer, is easier
to assess than the water table of an area underlain with hard granite or marble.

Properties of Materials: The degree to which a body of rock or sediments will function as a
groundwater resource depends on many properties. The two important physical properties are
porosity and hydraulic conductivity. Transmissivity is also an important concept in knowing an aquifer’s
ability to yield groundwater.
1. Porosity of the Rock
Porosity is determined by studying the shape and arrangement of soil particles. It is the amount of air
space or void between soil particles. Infiltration, groundwater movement, and storage occur in these
spaces. The porosity of soil is the ratio of the volume of pore space in a unit of material to its total
volume. The total amount of water that can be contained in the rock depends on the proportion of the
gaps in a given volume of rock, and this is called as porosity of the rock.
It is expressed as a decimal fraction or percentage. It is a measure of the amount of groundwater that
is stored in the geological material.
It can be defined mathematically by the equation:

n= Vv ÷ V×100%

Where,
n = Porosity, expressed as percentage
Vv = Volume of void space in a unit volume of geological materials, written as L3, cm3 or m3
V = Unit volume of earth material, including both voids and materials, read as L3, cm3 or m3
Porosity ranges from zero to around 60%.
Porosity is dependent on the type of rock which contains the water. In other words, the porosity
depends upon the spacing, pattern of cracks and fractures of the rocks. In sediments, the porosity
of the rock depends on the grain size, shape of the grains, and the degree of sorting and
cementation. The sorting or packing arrangement is most important in these rocks. The porosity of
well-rounded sorted sediments is significant as they are all almost of the same size. Poorly sorted
sediments generally have low porosity because the fine-grained particles tend to fill the void
spaces.

➢ Aquifer and its types

Aquiclude is defined as a geologic formation that can store significant amount of water but does not
have the capability to transmit a significant amount of water. Clay is an ideal example of aquiclude.
Aquitard is defined as a geologic formation that can store some water as well as can transmit water at
a relatively low rate compared to aquifers. Although an aquitard may not yield water economically, it
can hold appreciable amounts of water. Sandy clay is an ideal example of aquitard. On the other hand,
aquifuge is defined as a geologic formation that can neither store nor transmit water. Solid granite is
an ideal example of aquifuge. Thus, aquifuge is essentially a non-leaky confining layer, whereas

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aquitards and aquicludes are essentially leaky confining layers. In practice, however, aquiclude is often
considered as a non-leaky confining layer because leakage through aquicludes is generally very small
which can be considered practically insignificant.

Types of Aquifers

Aquifer can be basically classified into three types: (i) unconfined aquifer, (ii) confined aquifer, and
(iii) leaky aquifer. Sometimes, fourth type of the aquifer is known as ‘perched aquifer’, but it is not
the focus of any groundwater exploration. Fig. 3.1 illustrates the types of aquifers available below the
ground.

Fig. 3.1. Types of aquifers for groundwater development.

1 Unconfined Aquifers
Aquifers which are bounded by a free surface (known as ‘water table’) at the upper boundary and a
confining layer at the lower boundary are called unconfined aquifers (Aquifer 1 in Fig. 3.1). At the
water table, water is at the atmospheric pressure, and hence unconfined aquifers are also called ‘water-
table aquifers’ or ‘phreatic aquifers’.

Unconfined aquifers receive recharge directly from the overlying surface through rainfall infiltration
or percolation from surface water bodies. They usually exhibit a shallow water level. A typical
indicator of an unconfined aquifer is that the water level in a well tapping this aquifer is equal to the
water table position at that location of the aquifer. In other words, water level doesn’t rise above the
water table.
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2 Confined Aquifers

Aquifers which are bounded both above and below by impervious or semi-pervious layers are called
confined aquifers and the water present in these aquifers are under pressure (Aquifers 2 and 3 in Fig.
3.1). Confined aquifers are sometimes also called ‘pressure aquifers’ or ‘artesian aquifers’; the latter
term is gradually becoming obsolete. Since the water present in a confined aquifer is at a pressure
greater than the atmospheric pressure, the water level in a borewell penetrating a confined aquifer will
always rise above the top confining layer of the aquifer. The term ‘piezometric level’ is used to denote
this water level. Thus, ‘Piezometric level’ is an imaginary position to which the water level will rise
in a borewell tapping a confined aquifer. Piezometric level in two dimensions is called ‘piezometric
surface’.

Unlike unconfined aquifers, confined aquifers don’t receive significant amounts of recharge from the
overlying surface. The groundwater within a confined aquifer is under a pressure equal to the sum of
the weight of the atmosphere and the overburden. As mentioned above, the groundwater level in a
well penetrating a confined aquifer is usually above the upper boundary of the confined aquifer.
However, there may be cases where the piezometric level of a confined aquifer is above the ground
surface. The well tapping such a confined aquifer yields water like a spring, and hence it is called a
‘flowing well’ and such a confined aquifer is known as an ‘artesian aquifer’. Note that the word
‘artesian’ comes from the name of a place in France where flowing wells were seen for the first time
in the world. Hence, this word is now widely understood to refer only to the hydraulic condition in a
confined aquifer due to which flowing wells exist (i.e., groundwater flows naturally beyond the ground
surface). Unfortunately, some books on groundwater still use the term ‘artesian aquifer’ synonymous
with ‘confined aquifer’.

Moreover, most confined aquifers are unconfined at their exposed edges in the upstream portion of
the aquifer, which is called ‘outcrop’ (Fig. 3.1). They receive significant recharge through the outcrop
by direct rainfall infiltration into this unconfined portion. Confined aquifers also receive recharge
through their upper and lower leaky confining layers under natural conditions or when pressure
changes are artificially induced by pumping or injection. Groundwater flux to and from an aquifer
through a confining layer is termed ‘leakage’ (Fig. 3.1) and the confining layer is called a ‘leaky
confining layer’ or ‘aquitard’.

3 Leaky Aquifers
If an aquifer (confined aquifer or unconfined aquifer) loses or gains water through adjacent semi-
permeable layers, it is called a ‘leaky aquifer’ (Fig. 3.1). Therefore, the terms ‘leaky confined aquifer’
and ‘leaky unconfined aquifer’ are widely used depending on whether the leaky aquifer is confined or
unconfined. However, the case of ‘leaky confined aquifer’ has been mostly dealt with by the
groundwater experts. This is why, the term ‘semi-confined aquifer’ is sometimes used to denote a
‘leaky aquifer’. The term ‘nonleaky’ is also used to describe the status of a confined or unconfined
aquifer, such as ‘nonleaky unconfined aquifers’ and ‘nonleaky confined aquifers’. In reality, ideal
confined aquifers or ideal unconfined aquifers occur less frequently than do leaky aquifers.

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 4 Perched Aquifers

A perched aquifer is a special type of an unconfined aquifer, in which water exists under water-table
conditions. Therefore, the upper boundary of this aquifer is called ‘perched water table’ (Fig. 3.1).
Perched aquifer always exists in the vadose zone above an unconfined aquifer or a confined aquifer
when a low-permeability layer impedes the downward movement of water above it. Perched aquifers
have generally very limited areal extent and they may not have sufficient storage to support significant
well production. Therefore, perched aquifers are not the target of a groundwater exploration. However,
perched aquifers can support shallow dug wells, thereby can provide water supply to a small
community for a limited time period.

It should be noted that hydraulically single aquifers seldom exist in nature. An aquifer is generally part
of a system of two or more aquifers, which is more complex. Aquifer thickness, hydraulic head, Darcy
velocity, seepage velocity, hydraulic conductivity, transmissivity, intrinsic permeability, storage
coefficient (specific storage), specific yield, and specific retention are the important hydraulic and
hydrogeologic parameters which are used to characterize an aquifer system.

➢ Classification of wells
Diverse geological formations require different types of wells for tapping ground water for irrigation
and water supply. The choice of the type of well for irrigation is influenced by the size of farm holdings
and the relative preference given to private, cooperative and public wells. Broadly, water wells may
be divided into three categories, namely, dug wells, dug-cum-bore wells, and tube wells. Tube wells
may be deep or shallow
1. Dug Well- Dug wells comprise of open surface wells of varying dimensions dug or sunk from the
ground surface into the water-bearing stratum. They may be circular or rectangular in cross-
section. Usually, two types of wells are constructed: masonry (lined) wells and unlined wells in
the hard rock. A typical masonry well, usually constructed in alluvial or semi-consolidated
formations, has a masonry steining wall sunk in sub-soil by applying static weight with sand bags
and simultaneously scooping out earth from inside. A typical dug well constructed in a hard-rock
formation is usually an open excavated pit through the top soil and weathered rock. The top
portion along the soil mantle is usually lined with bricks or stones. The well taps water from the
sides as well as from the bottom, and is generally large to provide for storage of water.

2. Dug-cum-Bore Wells-Dug wells are frequently bored through the bottom to augment their yield.
These are referred to as dug-cum-bore wells. In sedimentary formations, boring consists
essentially of drilling a small bore of diameter usually ranging from 7.5 cm to 15 cm, through the
bottom of the well, and extending the bore down to a layer of good water-bearing formation to
tap that aquifer. Bores are made by percussion or calyx rigs or down-the-hole rigs.

3. Tube Well- A tube well consists essentially of a bore hole drilled into the ground for tapping
ground water from the pervious zone. Tube wells constructed in India may be broadly divided
into three categories: (i) shallow tube wells; (ii) deep tube wells; and (iii) bore wells.

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 I. Shallow tube Well- A tube well in a sedimentary formation, not exceeding 60 to 70 m in
depth, is called a shallow tube well. They are lined with pipes usually made of mild steel,
rigid PVC or other types of pipes. Shallow tube wells are usually privately owned by
individual farmers.
II. Deep tube Well- Deep tube wells usually extend to depths of 100 m or more and are
designed to give a discharge of 100 to 200 m3/h. These are usually drilled by direct rotary
or reverse circulation rigs, except in boulder formations where percussion or rotary-cum-
percussion rigs have to be used. They operate round the clock during the irrigation season.
Usually, deep tube wells in India are constructed as artificially gravel packed wells. Slotted
steel tubes are mainly used as screens and it has hitherto not been possible to cut
satisfactory slots of width less than 1.6 mm on these tubes. Deep tube wells in India are
usually owned by the Government or public sector organizations like state tube well
corporations.
III. Bore Well-Tube wells in hard rock areas are called bore wells because the bore hole is
able to hold on its own in the bottom portion and a tube is pushed only in the upper
weathered zone. These wells usually depend on joints, fissures and fractures in rock
formations for their water supply. Even with a heavy drawdown of 20 to 30 m, such wells
are usually not able to yield more than 5 to 10 m3/h, except when they tap some embedded
water bearing strata.

➢ Fully penetrating tubewells and open wells

1. Tubewells-
Tubewells are wells consisting of pipes ranging in size from 6 to 45 cm in diameter and sunk
into an aquifer (Sarma, 2009). Tubewells are constructed by installing a pipe below the ground
surface passing through different geological formations comprising water-bearing and non-
water-bearing strata. Blind pipes (casing pipes) are placed in the non-water-bearing layers and
well screens are placed in the water-bearing layers (Fig.). Several tubewells have been and are
being installed worldwide for meeting water demands in domestic, agricultural and industrial
sectors. The type of tubewell to be constructed

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 Fig. Dug-cum-bore well with a screened vertical bore.

depends on the type of geological formation, intended use of the well and the availability of
fund. Tubewells are also classified based on the depth, method of construction, entry of water
into the wells and the type/nature of the aquifer (Michael et al., 2008; Sarma, 2009). As
mentioned above, based on the depth of the well, tubewells

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 Fig.. A typical tubewell in an unconsolidated formation.
are classified as shallow tubewells or deep tubewells. Shallow tubewells are of low capacity
and their average depth is normally less than 35 m. They mostly tap one aquifer. Deep
tubewells are of high capacity and their depth usually ranges from 60 to 300 m (Michael et al.,
2008). They often tap two or more aquifers. Based on the method of construction, tubewells
are classified as bored tubewells, drilled tubewells, driven tubewells and jetted tubewells; they
are described in Lesson 17. Tubewells in unconsolidated formations generally consist of blind
pipes, strainers and gravel pack (if necessary). However, tubewells in hard-rock formations are
known as borewells, because the borehole remains stable for most of its depth and the tube is
placed only in the upper weathered soil zone (Fig.). No strainer/screen or gravel pack is
required for borewells.

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 Fig. Schematic of a borewell tapping fissured zone in a hard-rock area
Moreover, tubewells are also classified as fully penetrating tubewells or partially penetrating
tubewells depending on whether the well screen penetrates the saturated thickness of the
aquifer fully or partially. In some special hydrogeologic situations, the drilled hole is
terminated at the top of the confined aquifer without penetrating it, and hence no strainer is
required; such wells are called cavity wells or non-penetrating wells which are described
below. In coastal areas, partially penetrating wells with controlled rate of pumping are used
expediently to ‘skim’ the upper layer of fresh water overlying the saline water. Such tubewells
are popularly known as skimming wells (Michael et al., 2008; Sarma, 2009).

2. Open wells & Design of Open Wells-

Open wells, also known as dug wells, are popular since ancient times and are the most
convenient and cost-effective way of harnessing groundwater present in shallow and low-
yielding unconfined aquifers for small-scale water supply (e.g., domestic and small-scale
irrigation purposes). They can be constructed both in consolidated formations (e.g., alluvial
plains and river deltas) and in unconsolidated formations (e.g., weathered and fractured hard-
rock formations). Open wells may be either circular or rectangular in shape. Generally, the
circular shape is adopted for open wells in alluvial and other such formations because of its
greater structural strength. Open wells are of large size with the diameter usually ranging from
2 to 5 m, though the diameter may be as large as 20 m under special circumstances. The open
wells of larger size and rectangular in shape are preferred in hard-rock formations to facilitate

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larger amount of groundwater inflow into the well. The depth of open wells varies from a few
meters to about 50 m.

Open wells can be of four types

(a) unlined open wells,
(b) open wells with pervious lining,
(c) open wells with impervious lining, and
(d) dug-cum-bore wells. They are briefly described in subsequent sub-sections.

Unlined and Lined Open Wells

Open wells dug for purely temporary purposes are normally not protected by lining of their
walls (Fig. 8.1). The depth of unlined open wells is limited to about 6.5 m in order to ensure
stability. However, open wells dug for permanent purposes in loose and unconsolidated
formations require lining (steining) to prevent the collapse of side walls and are usually lined
with dry bricks or stone masonry (Fig. 8.1). Pervious lining is suitable when the water-bearing
formation consists of coarse sand and/or gravel. It is economical and more lasting where
aquifer and subsoil conditions are favorable and when the rate of withdrawal is not excessive
(Michael et al., 2008).

Fig. Unlined open well and an open well lined with pervious lining.

Impervious lining such as permanent masonry lining (laid in cement mortar) are normally used
in the open wells constructed in alluvial formations (Fig). The depth of open wells with
impervious linings is generally larger than the two types described above, but the depth usually
does not exceed 30 m because of excessive construction cost beyond the 30-m depth. Open
wells with reinforced cement concrete (RCC) lining are also sometimes used, especially for
greater depths. RCC collar wells (also called ‘ring wells’) are used in some shallow water-table
regions mainly for domestic water supply.

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 Fig. Open well lined with permanent masonry lining.

On the other hand, the open wells in hard-rock areas are excavated pits through the rock and
are lined only a couple of meters from top (Fig.) because the rocky formation ensures the
stability of side walls.

Fig. Open well in a hard-rock formation.

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