FAQ on GroundWater
Dr
Mihir Kumar Maitra develops a simple question and answer format Frequently
Asked Questions (FAQs) on Groundwater.
We get so much water from
underground rocks. Are there natural streams flowing underground?
Not really! Groundwater moves through porous rock formations
similar to the way water flows through a sponge with inter-connected pores. In
nature, no space remains empty. Therefore, the pore space within the
underground rock formations, no matter how small, remains filled either with
air or water (sometimes oil and gas in deeper formations). Given a continuous
supply, water enters a porous rock formation replacing the air and gradually
saturates all the pore spaces. As the process continues, excess water tends to
move through the saturated formation under gravitational force. Water can even
seep through poorly cemented house walls and concrete basements, particularly
during rainy season.
Therefore, when a storage space such as a dug well is constructed
within a saturated rock formation, a part of the water from within the
formation flows out as free water (specific yield) and gradually accumulates to
fill in the well till it reaches to a level equal to the water level in the
formation. Water also flows through joints, fractures and contact zones between
two hard rock formations. Sometimes, one can even see water flowing out through
a fracture or a contact zone in a dug well constructed in hard rock formation
tapping such zones giving credence to the erroneous notion that groundwater
moves underground as a subterranean stream.
Carbonate rocks are also known to develop due to the action of
water, large sized cavities and inter-connected solution channels which contain
and transmit large quantities of groundwater. Sometimes, in coal mines,
groundwater accumulated in large quantity in spaces created due to removal of
coal over the years can cause accidents thus creating the effect of a flowing
underground river. Coal mines can also get flooded due to direct inflow of
water from surface water bodies.
Which rock formations are
good for transmitting groundwater?
From the hydrogeology point of view, rock formations are
categorized conveniently as unconsolidated (loose), consolidated (hard) and
semi-consolidated. Recent and older alluviums are unconsolidated sedimentary
formations, which occur usually as alternate beds of sand and clay (or shale)
with varying thickness and proportion. Sand formation is a natural carrier of
water; coarser the grain size and lesser the compaction of the sand, better the
water content and its flow. Clay and shale on the other hand being impervious
are natural barriers to groundwater flow.
Water is also unable to pass through compact rocks like granite,
basalt, quartzite etc., which are usually devoid of any primary porosity.
However, in course of time the top portions of these hard rocks when exposed to
extensive weathering can develop numerous fractures and get weathered into
loose formation with granular consistency. Such weathered and fractured
portions can transmit groundwater commensurate to the degree of secondary
porosity developed due to weathering.
Although, formations like sandstone are compact in nature, they
are prone to quick weathering and develop extensive joints and fractures and
turn out to be good aquifers. Similarly, limestone is prone to develop cavities
and solution channels. Both sandstone and limestone yield moderate to good
quantity of groundwater and may be referred to as semi-consolidated formations.
What exactly is an aquifer?
A rock formation capable of holding and transmitting water through
its inter-connected pore spaces (or through joints and fractures) is called an
aquifer. Unconfined aquifers are formed by formations which are exposed and
pervious at the top surface but impervious at the bottom at some depth. Thus
water entering from the top of such formations percolates downwards till it
reaches the impervious level and starts building up a saturated zone within the
formation. The water occurring within the weathered zone of a hard rock
formation is a typical example of unconfined aquifer. For an unconfined hard
rock aquifer, more the thickness of the weathered zone and the degree of
weathering, more is its water content subject to its receiving the necessary
recharge. Water from unconfined aquifer is essentially tapped through open dug
wells (Figure 1).
Figure 1:
Dug wells in an unconfined hard rock aquifer
Dug well C in Figure 1 is likely to yield more water than wells B
and A as it is located in a depression and taps greater thickness of weathered
zone. Dug well A is also likely to yield substantial quantity of water due to
tapping of the fractures even though situated at a higher elevation.
An aquifer that occurs sandwiched between two impervious
formations is a confined aquifer. In alluvial region, when a sand layer
(aquifer) occurs in between two clay layers (impervious formations), the sand
layer becomes a confined aquifer. Water in a confined aquifer enters from a far
away location where a part of the aquifer is exposed to the surface or to other
sources of water for its recharge. Water in a confined aquifer occurs under
hydrostatic pressure as the recharge area is essentially located at a higher
elevation. Water level in a bore well drilled in a confined aquifer moves up
much above the depth of the tapped aquifer to a level referred as the
piezometric surface which corresponds to the hydrostatic pressure in the
aquifer. An over-flowing or artesian well is an example of tapping a confined
aquifer whose recharge area is located at a level higher than the surface
elevation of the bore well site.
Constructed properly, yield from tube wells in alluvial formations
vary as per the nature and thickness of the aquifer tapped as indicated in
figure 2.
Figure 2: Tube wells at different locations tapping aquifers to
different depth in alluvial formations
While tube well B in the figure above taps a confined aquifer with
maximum rise in water level (piezometric surface) within the well, water level
in the other two tube wells also rise to some extent as they may be said to
have tapped semi-confined aquifers. Tube well A will have the lowest yield as
it has ended in a clay formation.
Where does this water which
eventually supplies us with so much groundwater come from?
Rainfall (precipitation), an integral part of the hydrological
cycle, is the source of all fresh water in the earth. We tend to misjudge the
enormous quantity of water brought down from rainfall in our neighbourhood. For
example, the volume of 1 metre (1000 mm) rainfall that falls over 1
hectare (10000 sq. m) of land surface is equal to 10000 cubic metre or 10
million litres. With 1170 mm average annual rainfall and 328 million hectare of
land area, India receives nearly 4000 billion cubic metre of water annually
from rainfall.
Rainfall however occurs in bits and pieces with varying duration,
distribution and intensity. After reaching the surface of the earth, a part of
rainwater goes as infiltration, a part as run-off while the remaining water
goes back to the atmosphere as evaporation and evapo-transpiration through a
complicated inter-related process. Again, all the run-off does not end up in
the rivers and the sea, a substantial part of it remains stored in natural and
artificial storage structures, parts of which continue to infiltrate and
evaporate.
As per the official estimate, out of the total annual
precipitation of 4000 billion cubic metres of water in the country, nearly 1780
billion cubic metres (45 per cent) goes away as surface run-off, 1320 billion
cubic metres (30 per cent) as evaporation and 900 billion cubic metres (25 per
cent) as sub-surface infiltration. Considering topographical, technical,
socio-political and other constraints, utilizable quantity of fresh water has
been estimated as 1123 billion cubic metres comprising of both surface (690
billion cubic metres) and groundwater (433 billion cubic metres).
In case of an unconfined granular aquifer with deep water table,
all the water that infiltrates however does not necessarily contribute to
groundwater recharge. After saturating the entire soil profile and the
intervening unsaturated zone, the excess water known as gravitational water
finally percolates down to reach the groundwater table resulting in rise of the
same. The process is commonly referred to as groundwater recharge (Figure 3).
It is apparent that deeper the water table, larger would be the
quantity of water required to saturate the intervening zone before actual
recharge starts taking place. There would be no recharge to the groundwater
storage in case the quantity of infiltrated water is not of sufficient quantity
to saturate the intervening unsaturated zone first and then supply the additional
quantity of water for recharge.
Figure 3:
Schematic diagram depicting the process of groundwater recharge in an
unconfined granular aquifer with deep water table
In case of a weathered and fractured unconfined hard rock aquifer
there however would be much less limitation posed by any unsaturated
intervening zone as water would flow directly through fractures to recharge and
build up the groundwater storage.
If the terms water table
and water level mean more or less the same then why do we have two different
terms?
Sometimes they mean the same, but not always. While the upper most
level (top) of the saturated zone in an unconfined aquifer is the water table,
the level of water seen in a well is commonly referred to as water level. When
a well is dug in an unconfined aquifer, water from the formation seeps in to
the well and gradually builds up a water column. The top surface of this water
column finally stops rising any further and remains static when it reaches the
level of the water table. The depth of static water level as seen in a well is
also the depth of the water table in the aquifer. Any water level seen in a
well which is not the static water level is called the dynamic water level or
simply the water level. The depth of dynamic water level always lies below the
water table.
When a well is pumped, water level in the well starts falling.
Initially, the rate of decline of water level is faster which gradually slows
down as the aquifer starts releasing water into the well simultaneously. Water
level will continue to fall as long as the rate of pumping remains higher than
the rate of release of water from the aquifer in to the well. However, in a
pumping well, if at any point, the quantity of water being pumped out becomes
equal to the quantity of water being released by the aquifer, water level in
the well stabilizes and attains a steady state condition. When pumping is
stopped, the water level starts rising again, to begin with at a faster rate
which slows down as the dynamic water level approaches the static water level.
Geohydrological studies, amongst others, require collection of
water level data from numerous wells throughout the year to obtain the extent
of fluctuation in groundwater table. The water level measured for collection of
water level data must essentially be the static water level and not any other
dynamic water level. For this reason, any functional well that is pumped
regularly cannot be used for water level monitoring and data collection.
Separate observation wells are to be drilled or selected exclusively for
collection of water level data.
Is it not true that wells
showing shallower water level yield more water?
Regionally, the top surface of the saturated zone is visualized as
flat as the top of a table and hence this surface is referred to as the water
table. In reality, regional water table is not as flat as we presume. Water
table tends to follow the shape of the topography. Therefore, in an undulating
area, water table is not quite flat but would tend to take a shape akin to the
topography.
For example, if the static water level in a well is found to be at
10 ft below ground level (bgl), the water level in another nearby well located
at a ground elevation of 20 ft higher than the other, will not show water level
at 10 ft + 20 ft = 30 ft bgl, but considerably higher. Had the water table been
flat then the water table in the second well should have been 30 ft bgl. For
this reason, it is not true that shallower the depth of water table more is the
yield of the well. In other words, the depth of water table is not always a
good indication of yield of the well.
It is the property of the aquifer called permeability (ease of
flow) that determines the yield of a well. The shape of water table also gets
influenced by the permeability of the aquifer. Water table would tend to rise
higher in that part of the aquifer where aquifer permeability is relatively low
and vice versa. As mentioned earlier, shallow water table encountered in a dug
well is not necessarily an indication of higher yield. Another well in the
vicinity with deeper water table can yield higher quantity of water due to
better permeability of the aquifer.
Seasonal fluctuation of water table is also relatively high within
an aquifer of poor permeability than in a more permeable aquifer. It is true
that the better yielding dug wells are usually found at depressions and valleys
rather than on a ridge or high ground. This is because, the formation at a
depression or valley is more weathered (permeable) and receives more recharge.
What are the factors that
control groundwater flow through an aquifer?
Water first enters an aquifer through gravity induced downward
movement (percolation) but within an aquifer it also flows in a lateral
direction. This flow can occur due to the dip (slope) of the aquifer or
artificial conditions induced within the aquifer such as pumping or recharging.
Natural groundwater flow through an aquifer is rather slow and is about a few
centimeters a day.
The flow of water through a granular formation is governed by
Darcys law which states that the flow through a porous media (Q) is directly
proportional to the hydraulic gradient (L/H), where the permeability (K) of the
media is the constant of proportionality (Fig 4). In hard rock areas, however,
flow of water through fracture zones does not follow Darcys law as the flow is
similar to the flow through an open pipe.
Figure 4:
Flow of water through porous media
When water from a surface water body like a reservoir or a river
contains water at higher level than the groundwater table, some water flows
down as per the permeability of the intervening formation to join the aquifer
and the process is known as influent seepage. Conversely, if any part of the
water table is located at an elevation higher than a nearby surface water body
or a stream, water flows from that part of the aquifer in to the water body and
the process is referred to as effluent seepage. Many seasonal streams and
rivers are known to carry a base flow even long after the rainy season as a result
of effluent seepage.
Hilly regions are known to have poor groundwater storage. This
happens because water from rainfall and other surface water bodies that seep
underground tend to flow out when exposed to steep mountain sides. In hilly
regions, it is quite a common sight to find that water is seeping or spurting
out as a spring from along an exposed side of a hill.
Sometimes we find small dug wells at the top of a hill temple that
contains water round the year attributing the phenomenon as a miracle. This
happens because the well at the hill top eventually receives groundwater
recharge through fractures or delayed seepage from a recharge area located at
an even higher elevation in the adjoining hills. Besides, in most cases, such
wells do not get dry as only very little quantity of water is extracted for
regular use. Nature indeed is a miracle by itself.
Why do we get sufficient
groundwater in one place and not much in another?
Occurrence and distribution of groundwater is controlled primarily
by the geology of the area and the quantum of recharge received by the existing
aquifer formations. Thus the alluvial tracts of river valleys and the coastal
plains containing alternative deposits of sand and clay of varying thickness
are rich in groundwater. Regions underlain by hard rocks such as a large part
of south India is poor in groundwater occurrence. In fact from compactness
point of view, regions underlain by hard rock formations like basalt and
granite are not expected to yield any significant quantity of groundwater. But
nature has its own way.
The top portion of hard rocks in many areas develops extensive
fractures and secondary porosity to a considerable depth due to weathering. The
highly weathered granular hard rock derivative known locally as murrum functions as a good shallow
water table aquifer. Presence of deep sheeted intensive fracture system
in granite, contact zones between two similar or dissimilar hard rock
formations, sets of joints in sandstone, solution channels in limestone are also
known to contain and transmit sufficient quantity of water under confined and
semi-confined conditions.
One might wonder as to why a place like Rajasthan despite having
such thick layers of sand formations is still poor in groundwater occurrence.
The reason being that the recharge received from rainfall by these thick sand
formations is never adequate to build up a water table. The entire infiltration
gets disseminated and lost within the unsaturated zone before reaching an
impervious layer at depth for building up a water table. However, shallow dug
wells in sand formations underlain by hard rock or a hard pan at shallow depth,
yield moderate quantity of groundwater depending upon the recharge received.
Rajasthan and Gujarat also have some typical deep confined aquifers comprising
of sandstone but with time the yield of these aquifers have reduced
considerably due to over extraction and lack of recharge.
Which type of well yields
more water: a dug well or a tube well?
It is the nature of the aquifer tapped and not the type of well
which is responsible for yield. A well is merely an extraction structure.
Choice of the type of a well will depend on factors like depth of the aquifer,
availability of technology for its construction, cost and convenience. Dug wells
are ideal groundwater extraction structure for shallow unconfined aquifers.
These structures are preferred by farmers not only because these have
considerable storage capacity but also can be constructed and deepened in
phases using local expertise.
Dug wells are most suitable for hard rock regions of south Indian
States where the top weathered and fractured formation serve as the aquifer
containing groundwater under shallow water table condition. Depending upon its
depth below the water table, a dug well can store considerable quantity of
water for extraction as per convenience. Common horizontal centrifugal pumps
which are relatively less expensive can be used conveniently to lift water from
a shallow dug well. However as the regional water table in most parts of the
country has gone down substantially over the past few decades due to
over-exploitation, millions of existing dug wells in our country have either
gone dry or are yielding only seasonally that too after extensive deepening.
A dug cum bore well is ideal when a confined aquifer occurs within
a reasonable depth below the water table aquifer which is already being tapped
through a dug well. When the existing confined aquifer is tapped through a
vertical bore drilled at the bottom of the dug well, the chances are that the
water level from the confined aquifer which occurs under hydrostatic pressure
will rise and flow into the existing dug well. Water can then be pumped out
conveniently from the dug well.
Tube wells are most ideal for tapping high yielding confined
granular aquifers occurring at considerable depths. Tube wells are also
convenient for tapping groundwater from a thick unconfined granular aquifer
with deep water table. Tube wells are also known to yield considerable quantity
of water from sandstone and limestone aquifers occurring within multilayer
formations and deep seated fractures in granitic rocks under favourable
geological conditions.
How do we select a well
site that will yield sufficient quantity of groundwater?
The Central Ground Water Board (CGWB) has prepared the
geohydrological map of India presenting a regional picture of groundwater
occurrence in the country. Although such macro level map is useful for regional
level groundwater development programmes but it is not adequate for selecting
an individual well site.
Individual well sites are best selected by Geohydrologists. To
select a good well site in an area, the Geohydrologist would need to collect
data from the existing wells in the vicinity. The survey known as well
inventory would reveal the rock type, degree of weathering, depth to water
table, thickness, inclination, yield potential etc., of the sub-surface
formations observed in different wells. The information could be further
supplemented if strata chart, yielding zones, final yield etc., of one or more
existing bore wells in the neighbourhood are available for reference. Based on
the assessment about the various strata likely to occur at a given site and
their yield potential, the Geohydrologist gives his recommendations.
In an area with no existing dug wells, the Geohydrologist could
also carry out a special survey known as Vertical Electrical Sounding (VES) in
which, the resistivity of different sub-surface formations are measured along
depth. Based on the measured and interpreted values of sub-surface
resistivities and other local information, the Geohydrologist is able to
predict the suitability of a site for water well.
Resistivity survey is an indirect method and is as good as the
data available and interpretation of the available data made by the surveyor.
This survey requires a piece of land with at least 100 m open space in both
directions from the point of measurement and requires an hour or two to
complete. This method is applied extensively for regional survey to delineate
different zones. It is the most popular, low cost survey technique available
for groundwater well site selection. However, in a hard rock region even a well
trained professional Geohydrologist cannot always guarantee adequate yield from
a bore well drilled at the recommended site.
As professional Geohydrologists are not available everywhere,
water well sites are also selected by water diviners. Water diviners use a
variety of props like pendulum, compass, tree twigs etc., to locate a water
well site but in a sense are only as rational as a palmist or an astrologer.
These prediction makers become famous when some of their predictions turn
right. Bad predictions are forgotten and the search for a still more authentic
one continues. Many water diviners in the past had been fairly successful as
many of them were informal natural Geohydrologist as they could guess
groundwater occurrence based on experience and observations of the topography.
How do I decide the depth
of my tube well?
In an alluvial area with multi aquifer system, the depth of a tube
well should be such that it taps sufficient thickness of the aquifer(s)
encountered at site so as to get sufficient yield. Many a times, a bore is
drilled much deeper than the planned depth just to explore if any additional
aquifer occurs at further depth. Actual depth and design of a tube well is made
on the spot based on the decision about which formation is to be tapped and
which one is to be blocked. For this reason, it is necessary to maintain a
record of the strata encountered at a regular interval of depth while drilling.
Such a record known as the strata chart or litho-log is useful not only in
deciding the final depth and other parameters of the tube well but also remains
as a data base for future groundwater development in the area.
In a monolithic hard rock formation comprising of granitic rocks,
the only chance of encountering groundwater beyond the weathered zone is a
chance encounter of deep seated fractures. The services of a Geohydrologist are
essential in deciding the limiting depth of a bore well in hard rock area. If
one cannot afford the services of a professional Geohydrologist, one may resort
to the services of the local drilling agencies. Many a times, the local drilling
agencies possess a fairly good idea about the aquifer layout, expected depth of
productive zones, their likely yield, quality of water etc., of an
area through the data collected by them from bore wells drilled earlier in the
vicinity. Services of local drilling agencies work out very well in alluvial
areas where the aquifer system is known to occur at a certain depth over a
large area. For hard rock areas, where higher yield is expected from deep
seated fractures, use of resistivity survey is highly recommended.
Precautions should be taken in deploying the services of a
Geohydrologist. There may be a dishonest service provider who would pretend to
carry out a resistivity survey completed in a few minutes and report his
findings based not on the actual survey but on their local knowledge of the
area acquired over a long period. Sometimes the recommendations of such
unscrupulous experts serve the convenience and interest of the drilling agency
they are associated with. There may also be unscrupulous drilling agencies that
would advice the clients for a drilling depth more suitable to the capacity of
their drilling rigs than an optimum depth to which groundwater is expected to
occur. In this regard, the services of an honest and experienced drilling agency
become very useful.
What are the common options
available for drilling water well in an alluvial area?
Drilling a bore well essentially involves a) cutting a hole in the
ground b) removing the loose material to the surface and c) keeping the hole
straight and preventing its collapse. The drilling rig used in an alluvial area
is referred to as a rotary rig. These machines use pieces of hollow metal
pipes joined together to form the drill rod with the drilling bit attached to
the lower end. The drill rod is rotated vertically downwards by a
powerful truck or tractor mounted diesel engine and other support systems. The
hole is cut using a sharp edged cutter, the diameter of which is kept
deliberately a few sizes larger than the drill rod. The loose material
deposited within the hole is flushed out using a continuous circulation of mud
formed by mixing water with clay.
In Direct Rotary method, muddy water is pumped into the bore
through the hollow of the drill pipes allowing the mud to rise up to the ground
surface along the outer surface of the drill rod carrying the drill cuttings
with the flow. Water is used in controlled quantity to maintain viscosity of
the mud so that it can carry sufficient drill cuttings (mostly sand) during its
upward circulation and also prevent the bore from collapsing temporarily.
Conversely, in Reverse Rotary method, water is allowed to enter the bore hole
along the outer surface of the drill rod and is sucked out to the surface
through the central hollow of the drill rod (pipes) carrying the drill
cuttings. Reverse rotary method provides a more accurate strata chart.
Normally, a standard truck mounted rotary rig can easily drill a bore well with
diameter of 12 to 18 inches up to a depth of 500 feet or more within a
few days.
Hand boring is a simpler version of rotary technique which uses
tripod, cutter, bailer, strings, tackles etc., and the process is operated
manually. The drill rods with the cutter are held in vertical position by a
tripod and are rotated manually to cut the hole. To remove the drill cuttings,
the drill rods are first taken out of the hole and then an indigenously
designed bailer is used. The bailer is a slightly conical heavy hollow pipe of
5-6 feet in length. The bailer is lifted and dropped a few times within the
bore hole to gather the drill cuttings within the hollow space of the bailer. A
round plate welded with a hinge at the bottom end of the bailer works as a trap
door such that it allows entry of drill cuttings inside the bailer pipe but
closes when it is lifted up. The bailer is then raised to the surface and the
drill cuttings are removed.
Some times to avoid the use of a bailer which is a time consuming
process, a hand operated twin piston reciprocating mud pump (denki type) is
used to simulate the action of direct rotary method to flush out the drill
cuttings from the bore. Though difficult but not impossible to construct a hand
bore beyond a depth of 200 feet with diameter more than 6 inches. The
limitation emanates from the fact that rotating and raising frequently the
numerous heavy pipes forming the drill rod beyond a depth 200 feet by manual
means become quite difficult.
For drilling a very shallow hand bore in a relatively soft
formation, a length of pipe usually 2 inch in diameter with a sharp bottom end
(sharpened pipe socket) is rammed repeatedly up and down in a shallow hole
drilled initially using an auger. The driller creates a little pressure within
the pipe by placing his palm on the top of the pipe as it starts descending and
removes the palm as it lands within the bore allowing in the process some muddy
water to eject out of the pipe carrying some drill cuttings which usually are
fine sand.
How are water wells drilled
in hard rock formations?
Predictably, the drilling techniques in an alluvial area and that
in a hard rock area would differ drastically. Most common type of drilling
technique used in hard rock drilling is referred to as the Down the Hole
Hammer (DTH) method. The drilling bit tipped with very hard cutting material
(Tungsten Carbide or industrial diamond) is attached to the drill rod through a
hammer mechanism in between. Compressed air is used both for rotating the drill
rod as well as generating a constant hammering action on the attached drill
bit.
Hard rock is cut to small chips or crushed to powder due to
rotating and hammering action of the drilling bit. The drill cuttings are
flushed out from the bore using a stream of pressurized air released through
the hollow of the drill rod that escapes through a hole located in the centre
of the drilling bit. The whole operation is carried out by using a high powered
air compressor generating more than 120 pounds per square inch (psi) of air
pressure. DTH rigs can drill a 4 to 6 inch diameter borehole up to 200 feet
depth within 24 48 hours of operation.
DTH rigs do not use mud circulation and hence are not suitable for
drilling through soft formations beyond a certain depth as the bore hole would
keep collapsing without the mud. The drilling machine used specifically to
construct water wells in an area containing soft (loose) formation at the top
to a considerable thickness followed by hard rocks below are known commonly as
the Combination Rig. These are high powered machines which have both the mud
rotary and down the hole hammer arrangements. The bore drilled within the loose
formation is first cased to prevent from collapsing by inserting a casing pipe,
after which the DTH drilling is continued within the hard rock below.
Other hard rock drilling techniques are the diamond drilling and
calyx drilling. In case of former, a hollow diamond studded bit (industrial
variety) is rotated using a diesel engine to cut and collect core samples
within the drill pipe. In the case of later, the cutting is achieved by rotating
the drill rod along with a few hard metal balls dropped inside the bore hole as
an additional cutting tool. These drilling techniques are very slow, used
mostly in the mining sector and not used any longer for drilling water wells.
Why are some bore wells
fully cased while others hardly use any casing pipe?
In alluvial area, the entire depth of a bore hole needs to be
cased to prevent it from collapsing. In order to allow water to enter into the
well from the aquifers, small openings usually in the shape of thin slots are
provided at appropriate locations in the casing pipe. The casing is assembled
by joining blank and slotted pipes in desired lengths and sequences in such a
way that on insertion, the slotted pipes face the aquifers to be tapped and the
blank portions stay against non-productive formations. The bottom end of the
casing pipe is also closed using a bottom plug to prevent entry of sand into
the well during pumping of water from the well. Since, the entire well in loose
alluvial formations is provided with a casing pipe, such wells are referred
commonly as a tube well.
The annular space between the casing pipe and the bore is usually
filled up with pea sized gravel of uniform size till the top of the first
aquifer. Gravel filling is not extended up to the ground level so as to prevent
entry of polluted water in to the formation from surface run-off through the
gravel filled annular space. The remaining depth above the gravel pack is
usually filled up with impervious clay.
A newly constructed tube well needs to be cleaned and developed
properly. This is done by pumping the well with a high capacity pump for many
hours till all the muddy water with fine sand entering through the slots are
removed enabling the well yield clean water. Although, not impossible, but once
the complete length of a casing pipe is inserted and packed properly in a
finished bore well, removal of the casing pipe without damaging the same
is a difficult task.
In hard rock areas, a drilled bore hole on the contrary does not collapse.
Hence there is no need to insert a casing pipe to keep the bore hole standing.
However, as the top portion in hard rock areas usually becomes soft and loose
due to weathering, only this weathered portion needs to be cased. This top
weathered portion referred usually as the overburden is cased leaving the
rest of the hole uncased to allow water to flow into the well from the
formations encountered. As these bores stand by themselves without the support
of any casing pipe, these are usually referred to as bore wells.
What are the important
factors to be kept in mind while designing the casing pipe?
A casing pipe comprises of sections of slotted pipes (filter) and
blank pipes. In a multi-aquifer system of granular formation, the slotted pipes
are to be placed across the aquifers to be tapped and the remaining length of
the casing should be of blank pipe. Blank pipe is also placed across an aquifer
containing brackish or poor quality of water in a multi-aquifer system to
prevent entry of poor quality water into the well. Important aspects that
require special attention for proper design of a casing pipe are its length,
diameter, composition, size and locations of the slots. Selection of gravel
size for the outer envelop is also important for efficient functioning of the
tube well.
Usually, a bore is drilled a little deeper than the planned depth
so as to tap as much potential aquifer formations as possible. However, since
casing pipe constitutes the major cost of a tube well, there is no need to case
the entire drilled depth. The length of casing needs to be minimized by tapping
up to the productive zones while leaving the remaining unproductive zones below
uncased.
Cost of a casing pipe also increases in geometric progression with
its diameter. It would therefore be prudent to select a diameter which is not
too large and at the same time provides sufficient space for lowering the pump
of desired capacity and diameter. In order to save on the cost of casing, the
diameter can also be reduced by a size or two below the depth up to which the
pump will be inserted making the casing telescopic in shape. Thus a bore
drilled with say 12 inch diameter up to 400 feet depth may have a finished
dimension of 300 feet casing length with 8 inch diameter at the top and 6 inch
diameter at the bottom.
Mild Steel (MS) or Cast Iron (CI) pipes are used commonly as
casing pipes as these are strong, can be cut and joined easily at site and are
heavy enough for easy insertion into the bore. The MS/CI casing pipes however
are prone to severe rusting resulting in an effective life not more than 15 to
20 years depending on the site conditions. High Density Polyethylene (HDPE)
casing pipes are more popular these days as these are also easy to cut and join
and available in large diameter with requisite thickness and hardness.
Resistance to rusting provides a longer life to HDPE casing and hence to the
tube well.
The slots are cut in standard length, thickness and pattern on the
pipe in the factory itself. Based on the knowledge of the strata, the casing
assembly is designed joining slotted and blank pipes in desired sequence and
length, so than on insertion into the bore, the slotted portion(s) lies against
the aquifer(s) to be tapped. The purpose of a slotted pipe is to allow entry of
water from the aquifer into the well and at the same time prevent entry of
undesirable sand particles. The blank part of the casing does not allow entry
of any water into the well from the adjacent formations.
After insertion of the casing pipe, the annular space between the
casing and the bore is filled with pea sized gravels of uniform size. The
gravel packing serves the purpose of acting as a filter to prevent entry of
sand from aquifer into the well and also act as a support to hold the casing
pipe in place.
What are the common types
of pumps available for lifting water?
The most common type of pump used for lifting water is a
centrifugal pump. A centrifugal pump is necessarily comprised of a set of
impellers or blades placed within a close fitting circular housing with suction
and a delivery end for entry and exit of water respectively. When the impellers
are rotated at a high speed with the help of an electric motor (or a diesel
engine), the rotating impellers create vacuum sucking water within the pump
through the suction pipe which is kept submerged in water. The sucked water is
thrown out of the delivery pipe due to the centrifugal force created by the
impellors.
Since a centrifugal pump uses vacuum to lift water, theoretically,
it can lift water maximum from a depth of 33 ft (10.03 m) which is equal to one
atmospheric pressure. There is no such limit for lift on the delivery side.
Water can be lifted to the desired height by increasing the number of impellers
(stages) placed serially within the pump casing. A centrifugal pump is used
very conveniently for pumping water from a shallow dug well or other open water
bodies for overland conveyance through pipes.
A submersible pump is essentially a centrifugal pump designed in
such a way that both its motor and the pump assembly coupled together can be
submerged within water without damaging the electric motor. The pump looks like
a long cylinder, the lower half of which is the motor and the upper half is the
pump body within which a number of impellers (stages) are placed in series. The
motor and the pump are coupled together through a common shaft with a small gap
in between for entry of water in to the pump. The power cable from the motor
runs upwards to the surface for running the pump (motor). When the impellers
are rotated at a high speed, water rises through the rising pipe attached to
the pump due to the centrifugal force created. There is no need for a suction
pipe in the system as the entire pump assembly is lowered directly into the
water. Submersible pumps are most suitable for lifting water from a bore well
for domestic use.
Another variation of centrifugal pump used for lifting water from
relatively large diameter bore wells (>12 inch diameter) is a Vertical
Turbine Pump. In this system, while the pump is kept submerged below water
level, the motor is placed at ground level above the casing pipe. The motor and
the pump impellors are connected with a vertical shaft (rod) running down the
bore for rotating the impellors. The shaft is encased within a riser pipe
through which water reaches to the surface. These pumps are suitable for low
head, high discharge use installed commonly by urban water supply agencies in
high yielding tube wells.
Twin pipe ejector or Jet pump, is sometimes used in low yielding
bore wells. Water is first pumped into the bore through an inlet pipe allowing
it to come out through an attached rising pipe of lesser diameter. The process
known as ventury effect allows additional quantity of water from the bore
well to come out through the rising pipe due to the created pressure
difference. Hydram is a pumping device used sometimes to lift water in hilly
regions where the pulses created by the impact of falling water on a diaphragm
located within the pump body is used as the prime mover to lift water to a
higher elevation. Deep well hand pump (reciprocating pump) is used for lifting
water from a bore well with poor yield.
What are the important
factors one should look at while selecting a pump for use in a water well?
Selection of a pump depends primarily upon the structure of the
water source, availability of power source, operating conditions and discharge
requirement. Common horizontal centrifugal pumps which are relatively less
expensive are used conveniently to lift water from shallow dug wells using
either an electric motor or a diesel engine as the prime mover. Small diameter,
high speed submersible pumps are most appropriate for lifting water from a tube
well where electric power is available.
From performance point of view, the most important factor to be
kept in mind while selecting the Horse Power (HP) of a pump is the operating
condition i.e., what is the discharge requirement at the given operating head
including all of suction, delivery and friction heads. Horse Power requirement
varies directly with both discharge (Q) and head (H) and is calculated by using
the formula HP = 1000 QH/75 where Q and H are in metric system. Additional
margin of about 20 per cent is usually added with the calculated HP to provide
a margin for minor changes in operating condition due to the falling water
level.
Figure 5:
Performance characteristic curve of a centrifugal pump
It is desirable that a pump should operate at least at 65 per cent
efficiency level under the operating condition (Figure 5). Pump operating at
lower efficiency level will consume more power. The most efficient functioning
of the pump with performance characteristics shown in the figure above is when
the pump discharges about 16 lps at an operating head of 35 m.
Pump manufacturers usually provide the characteristic curve for
high capacity pumps so that the users operate the pump at the optimum operating
condition. For smaller pumps, the manufactures provide a table based on the
characteristic curves, listing what would be the discharge (capacity) of the
pump at different operational heads. It can be seen clearly from such tables
that as the head of the operation increases, the discharge decreases and vice
versa. It is recommended that the pump be so chosen that it operates at its
optimum efficiency level at the given Head-Discharge requirement. Pumping
systems are also categorized as low head-high discharge, medium head- medium
discharge and high head-low discharge systems.
Another important factor of centrifugal pump is the rated speed of
the pump (impellers). Discharge varies directly with speed, head varies
directly with the square of speed, power consumption varies directly as the
cube of speed. Also, the required speed of a pump for a given performance
varies inversely with the diameter of the impeller. In other words, to obtain
the same performance from a given pump, if the speed is increased, the diameter
of the impeller can be reduced and vice versa. The speed should however be
contained within the practical limits other wise there would be high wear and
tear. Fluctuation in electric power and adverse quality of water can reduce the
effective life of a submersible pump considerably.
Why is it not advisable to
construct too many wells too close to each other?
When water is pumped out from a water well, the water level begins
to fall and the aquifer starts releasing water from the aquifer into the well.
The area dewatered from within the aquifer surrounding the well is known as the
radius of influence (Figure 6).
Water table will continue to fall if the rate of water being
extracted from the well is higher than that of the water being released by the
aquifer into the well. However, water table becomes steady and stops falling
when the quantity of water being pumped out remains less than or equal to the
quantity of water being released into the well. Water table stabilizes at any
depth depending on the balance between the extraction and the aquifer
replenishment rate. Water table however starts recovering (rising) as soon as
pumping (extraction) is stopped and gradually stabilizes back at its static
water level.
When two wells too close to each other are pumped simultaneously,
the two radiuses of influences get superimposed with each other resulting in a
cumulative drawdown in each well. When a number of wells close to each other
are pumped simultaneously, the fall of water level in all the pumping wells
starts accelerating which may result in pumping failure due to shortage of
water in some of the wells that are shallower than others.
Figure 6:
Formation of a cone of depression in a pumping well
Also, a localized large scale extraction of groundwater is likely
to cause inordinate delay in the recovery of the water levels in all the wells
when the aquifer is of poor permeability. Regionally such aquifers tend to form
a trough of depressed water table, which in turn attracts groundwater flow
towards this sink from other parts of the aquifer so as to smoothen out this
depression. This eventually leads to the lowering of the water table at a
regional scale.
How to obtain
sustainable/maximum/optimum yield from a bore well?
The concept of sustainable yield applies to an aquifer only and
not to a well. Sustainable yield of an aquifer refers to the total
quantity of water it can yield i.e. total number of wells it can support
without causing any unacceptable lowering of the regional water table over a
long period. The yield of an aquifer eventually depends upon its size,
permeability and annual recharge received by the same. Ideally, the quantity of
water extracted annually from an aquifer should be less than or equal to the
quantity of water received by the aquifer annually through recharge. If more
water is extracted every year, naturally, the water table would start falling
eventually turning the yield of the aquifer unsustainable and thus reducing the
yields of the existing wells.
The question of maximum yield applies to a particular well. How
much water a well can yield depends on its location, size, extent and permeability
of the aquifer tapped. The maximum yield will be produced when the rate of
pumping will be such that it allows stabilization of the water level very close
to the bottom of the well. At this stage the aquifer releases water equal to
the pumping rate which is the maximum yield of the well.
It should be noted that a pump always requires some margin of
submergence for its safe operation. Most users therefore tend to lower the
submersible pump into the well as much as possible. Lowering the pump deeper
helps in achieving greater water submergence and yield but consumes more power.
Water level must not be lowered inadvertently at any time to expose the pump
which could cause over-heating due to dry running leading to burn out of the
electrical wirings. However, every bore well even after proper construction and
development still tend to accumulate some mud and sand at its bottom.
Therefore, to prevent pumping of muddy and sandy water and thus damaging the
pump, necessary margin should be provided between the pump and the bottom of
the bore well. Such a gap is also useful for circulation of water to dissipate
the heat generated by the pump.
One should therefore lower a pump in a bore well at a depth where
it has sufficient bottom clearance and at the same time has adequate depth of
submergence. If the user desires to draw maximum possible quantity of water
irrespective of cost of pumping, then the discharge rate of the pump should be
so adjusted that water level stabilizes at a few feet above the pump level. It
should be noted that in an attempt to obtain maximum yield, the cost of pumping
is not considered at all by most users. Deeper a pump is lowered, lesser is its
discharge and higher is the consumption of electric power.
Many farmers wait for over-night (or longer period) for a well to
recover fully and pump out the stored water conveniently within few hours. The
practice is convenient but do not necessarily produce maximum yield. As the
initial recovery in a well is faster in the initial stage which gradually slows
down with time, It would be prudent to start pumping again when nearly 90% of
water level has recovered instead of waiting for 100% recovery. Ideally, a
farmer needs to pump at a rate at which the water level stabilizes in the dug
well so that the pump can be run for 24 hours and produce maximum yield.
What exactly then is the
meaning of optimum yield of a well?
Optimum yield of a well is the concept which takes into account
the pumping cost also. We are aware that as the head increases, the discharge
reduces. But in order to maintain the requisite discharge a user may be
encouraged to deploy a pump of higher Horse Power. Using more power than
required no doubt is wastage.
Optimum yield of a well is arrived at by conducting a step
drawdown test in the well. In step drawdown test a bore is first pumped at a
very low and constant discharge rate for a fixed period of time say 1 hour in
such a manner that the water level stabilizes at a shallow depth. The
pump discharge is then increased to a medium rate of pumping so that water
level falls further down but eventually stabilized at an intermediate depth.
The next step is to increase the pump discharge further higher so that the
water level again stabilizes at a lower level further down. More such steps can
be conducted if possible. Each step is carried out for a fixed period of time
and discharge during each step is kept constant. When the discharge versus
steady state drawdown of the different steps is plotted, we get a parabolic
curve as shown in Figure 7.
Figure 7:
Optimum discharge of a tube well as obtained from step drawdown test
The curve indicates that the rate of fall of water level increases
exponentially with the increase in discharge. The point of tangent in the curve
is the optimum yield of the well. The optimum yield of the well would therefore
be such that the pump discharges about 20 meter cube per hour of water so that
the water level stabilizes at about 10 meters of drawdown.
To obtain optimum yield from a bore well, the depth and discharge
of the submersible pump within the bore are so chosen that the water level
stabilizes at the optimum drawdown level at which the pump is capable of
running for 24 hours a day with highest efficiency and at the lowest operating
cost. It is therefore recommended that in order to obtain the optimum yield
which is different from the maximum yield, a pump need not be lowered to the
maximum possible depth.
As mentioned earlier, local drilling agencies usually possess a
fairly good idea about the layout of the aquifer system, their yield potential
and quality of water in the area of their operation. Based on their experience,
a drilling agency can and usually provides many advisory services including the
selection of the site, diameter and depth of the bore and also the horse power
and stages of the pump to be deployed. However, when no such services are
available, the right way to go about is to carry out a step drawdown test using
a service pump to decide the requisite capacity of the actual pump to be used
in the tube well under consideration.
If water table fluctuates
seasonally then what is meant by falling water table?
Water table undergoes a seasonal fluctuation following a natural
cycle. Water table rises during rainy season due to recharge received from
rainfall and falls back during summer season due to lack of recharge and
continuing extraction of groundwater. In geohydrology, the highest water level
is referred to as the post-monsoon water level and the lowest level in summer
as pre-monsoon water level. Quantum of water being received by an aquifer from
annual recharge can be computed from the annual ground water fluctuation in
conjunction with estimated groundwater extraction in that area.
Water table is said to be falling when the post-monsoon (and also
the pre-monsoon) water table do not remain the same every year but keep going
lower by a considerable extent. Rainfall (recharge) remaining more or less the
same, this fall in water table takes place due to increasing groundwater
extraction in every successive year. In other words, groundwater in an area is
said to be falling (receding or declining) when annual groundwater extraction
exceeds the annual groundwater recharge. The excess water extracted in such a
situation comes from the groundwater dead storage built up over many years and
hence the over-extraction is known as groundwater mining. It is a well known
fact that water table in most parts of the country has been falling at an
alarming rate particularly over the past few decades.
The prime cause of over-exploitation is the rising demand for
groundwater. An aquifer is said to be over-exploited when the annual
groundwater extraction is higher than the annual recharge. For sustainable
utilization of groundwater, only that much water should be extracted which is
replenished every year by rainfall through groundwater recharge. To achieve
this, we must either reduce our groundwater extraction (demand side management)
or increase recharge to the extent possible by adapting artificial recharge
measures (supply side management). Else, the groundwater table will continue to
fall leading to an unprecedented situation.
Apart from posing an increasing difficulty in accessing
groundwater and increasing pumping cost, falling water table is also a matter
of great environmental concern. Lowering of water table has rendered millions
of existing shallow dug wells in the country go dry. This in turn has led to
the rat race for deepening the existing dug wells but more so for the
construction of deeper and deeper bore wells increasing construction cost. As
the cost of accessing groundwater has been increasing progressively, in near
future groundwater will be accessible only to the resourceful depriving the
poorer lot.
How does one estimate
groundwater recharge in an area?
As mentioned earlier, groundwater table undergoes a seasonal
fluctuation. It rises during rainy season due to recharge and falls during
summer season due to lack of recharge and groundwater extraction. From recharge
estimation point of view, a year is divided in two parts, one is of rising
water table during and after rainy season (post monsoon period) and the other
of falling water table during summer (pre monsoon period). The total annual
rise of water table from its lowest level at the end of summer to the highest
water level at the end of rainy season and vice versa is known as total annual
water table fluctuation.
For a given geographical area, the total seasonal rise of water
table is a direct measure of the annual recharge. It should however be
remembered that water table fluctuation also depends inversely upon the
permeability of the aquifer. For a given amount of recharge, an aquifer with
poor permeability will show higher rise than an aquifer with higher
permeability.
Technically, the quantum of recharge is estimated by the
fluctuation method using the following formula.
R = FAS
Where,
R = Annual recharge
F = Annual fluctuation of water table in metre
A = Area in hectare
S = Specific yield of the aquifer
Specific Yield is the capacity of an aquifer to release percentage
of water from within its pores under gravity. For field calculation, specific
yield is taken to be between 0.1 to 0.3 depending upon the permeability of the
aquifer system under consideration.
Since, groundwater extraction also continues simultaneously during
the post-monsoon season as the water table rises, the water table rise would
have been considerably higher had there been no simultaneous extraction. The
quantum of post-monsoon groundwater extraction is therefore added with the
quantity of recharge computed from FAS method to obtain the effective seasonal
groundwater recharge mainly from rainfall including the recharge from existing
surface water bodies, if any.
Groundwater extraction or draft from an aquifer is computed
directly by adding the total discharge of each of the wells tapping the aquifer
in the area. A rough estimation of groundwater extraction can also be obtained
by assessing the extent and nature of crops grown using groundwater in the
area. The amount of groundwater extraction that took place simultaneously
during the rising phase of water table is therefore added and the amount of
recharge received from sources other than rainfall, if any is subtracted from
the total recharge estimated by the fluctuation method to obtain recharge
received from rainfall alone.
Groundwater recharge can also be estimated empirically by taking a
percentage of rainfall as recharge in a given area where the geological
conditions are known. Such estimation however needs to be fine-tuned based on
the actual rainfall pattern of the year and its percentage factor arrived at
based on long term experience.
How is groundwater over-exploitation
monitored?
Both the Central and State Ground Water Board keep track of
groundwater situation in the country through regular monitoring of water level
using a network of observation wells (hydrographs). For some reason, these
water level data are not placed in public domain.
Total groundwater resource of a region is taken as the total
estimated annual groundwater recharge of the water table aquifer(s). Total
annual (or seasonal) groundwater extraction (draft) of an individual well is
computed by multiplying its average discharge and annual working hours. Number
of hours if not recorded properly can also be computed from hourly consumption
of electric power or diesel. Total annual extraction from an assessment area is
thus arrived at by adding the extraction being made through all individual
wells. Groundwater draft is also calculated indirectly from the
irrigation water requirement of all crops grown within the assessment
area.
Based on
the status of groundwater development, NABARD has accordingly categorized the
country as Dark, Red and Green zones as presented in the table below
|
Category of areas |
Stage of groundwater development in per cent |
|
White |
Less than 65 per cent |
|
Gray |
Between 65 85 per cent |
|
Dark |
Between 85 100 per cent |
In dark areas, micro-level surveys are required to evaluate the
groundwater resources more precisely for taking up further groundwater
development.
Annual
replenishable groundwater resource of the country