# Mod-01 Lec-38 Lecture-38 Good morning and welcome to this post graduate
video course on advanced hydrology. We are into the ground water hydrology module, the
last lad of this course. Yesterday, we looked at the moment of ground water in the saturated
zone and essentially we looked at the Darcy s law, its applicability and validity. And
then we also looked at a numerical example in which we saw that how we can calculate
the hydraulic conductivity and calculate certain other parameters like Darcy is flux and actual
velocity and so on. Today, what I will like to do is; like to go back a little bit, look
at some more aquifer property is which we actually skipped. So, we will start looking at specific retention
and the specific yield, right. So, how we define these parameters for these aquifer
properties, which are important to understand as we will move along this course and look
at the movement of the ground water in the saturated zone. This specific retention is
denoted as S subscript r. And let me give you the definition and then give you the expression
of a soil or rock and we are taking aquifer. Here is the ratio
of volume of water; it retains after saturation,
against the force of gravity to its own volume or to its total volume. So, what it is this? Basically, what we are
seeing is that it is the ratio of the water retained by the soil, against the force of
gravity means what? When the gravity forces are not there. So, water being retained by
suction or some other forces, right. After these sample has been thoroughly drain due
to the gravity forces and to its total volume. So, if we defined it, it will be S subscript
r; will be let say V r over V t. And using the equation numbers, let me number this as
15 which we are in following. Where V r is the volume occupied by retained water. And
thing to remember is that it cannot be drained by gravity
or pumped out even. When we pump out, we cannot
pump out the detained water. And V t of course, we had defined earlier it is the total or
the bulk volume of the soil. So, this basically, this parameter gives us some idea about how
much water cannot be extracted and this retention power or the retention capacity of different
types of soils will be different. So, depending upon that if we get have some
idea, we know how much water we are actually going to play win; if we have an aquifer of
certain volume available to us. The next one is the specific yield. This is very important
and this is denoted as S subscript y of an aquifer or a soil or rock is the ratio
of the volume of water
that after saturation
that after saturation can be drained by gravity to its own volume; that is the ratio. And,
this we can then write as V y over V t that is the ratio; let me number this equation
as 16. And then V y is the volume of water drained or pumped out. So, this basically
this specific yield gives us an idea about how much water is actually available. There
are there is a total force space in the or the porosity, out of this certain fraction
actually; you know if it is saturated certain fraction cannot be available for any useful
purposes. And, that is your specific retention and the specific yield is something which
can be a pumped out or we can use. Let us see that or note that what will be
the sum of this V r and V y; that is the sum of the water retained and the sum of the water
drained from a sample or a saturated sample volume of the voids, the total voids. And
similarly, because if you divide this equation by V t, then you can say that the specific
retention plus the specific yield will be equal to what? This obviously has to be equal
to your porosity eta; that is let say 18. Now, this S y is something important. It depends
on what? Let me just list a few factors on which it will depend. It will depend on the
grained size and shape
of the soil on which your eta actually depends, right. And also the distribution of the pores,
how the voids or the pores are distributed; distribution
of the pores. How they distributed with the
specter space? How about the compassion? You may have an undisturbed sample and you may
have a compacted sample; there porosity will be different and this specific yield obviously
would be different. So, you have the compassion of the stratum.
Also, the time of drainage, if we keep pumping water the specific yield will be changing
as a function of time; because the compassion characteristic may be different, as a function
of time. The next factor is the course or the fine grains; basically, the soil type
or the size of the soil basically. So, if we have the fine grains; means you will have
little yield. Just find a points we are looking at an if we have the course grain soil would
mean what? You will have significant yield. Now, this yield will decrease with depth as
we have said earlier that these specific yield is will depend upon the compassion and as
we go down into the ground, the compassion due to the over burden will be increasing
and the pores spaces will be decreasing. So, that specific yield also will decrease; so
as we go down the yield will be the function of the depth also. This S y is about 27 percent for course sent
and
it is about 3 present for clay type of strata or an aquifer which consists of mainly clay
and this is about 44 percent for peat. So, you see that looking at the kind of aquifer,
the soil properties; so, we can carry out the whole investigation, we know what kind
of soils are present in the aquifer. So, once we have that, we get some pretty good idea
about how much water can be taped in the ground. For example, if we find that there is core
sand in the aquifer around particular area, then we know the maximum yield we can have
is about 25 percent or so. But if it is a clays strata, there is hardly any yield we
will have. So, there is no point in spending money in exploring that particular area in
which clay is present. So, what I am going to do next is, I would like to look at an
example. Third example in this chapter, in which we
will look at this specific yield, how it can be useful in computing certain important things.
And this is how it goes, estimate the average draw down, estimate the average draw down
over an area where 25 million meter cube of water was pumped
from many uniformly distributed watts, distributed
watts. The area which is exacted is about 150 square kilometers and S y the specific
yield of that aquifer is given to about 25 percent. So, looking at this what kind of
soil is this, in some course sand; from the unconfined aquifer. This is an unconfined
aquifer and we are going to look at little bit more closely; what is this unconfined
and confined this aquifers. One thing which is you know to be noted here is what is given
is a towards is that, we have an area areal extent of the aquifer; the areal extent of
the aquifer is about 150 square kilometers. So, it is a large area in which there are
many you know tube wells which are dug and we are pumping water. And what he said is what is given is that
these dug wells or the pumping wells are distributed uniformly with the specter space. And this
is an important assumption why because we can assume that the draw down is uniform;
it is same everywhere. So, that is what we actually try to do; so that we do the pumping
the draw down are not very high at a concentrated location. So, these are the data given and
what we have to find is the average drawdown. So, if we have pumped out 25 million meter
cubed of water, in some certain duration; what will be the lowering of the water table,
the ground water table in this unconfined aquifer? Specific yield is given and the amount
of water which is a pumped out, that is also given. So, let see how will do that. What
is given to us is, the volume of your water pumped out or drained, what is this? This
is V y, we have just seen volume of water which is pumped is V y and this is given to
us as 25 million meter cube; so I can say 25, 10, 6 meter cube. What is also given to you is the specific
yield. S y is 25 percent; so it is 0.25. What is S y? Well, we have just seen it is V y
over V t. So, this will give you the total volume of the aquifer. V t would be what?
It will be nothing but your V y over S y, is it not. So, it will be 25, 10, 6 over 0.25,
which will be 1, 10, 8 meter cube. So, the total volume of your aquifer is 10 to the
power 8 meter cube. Now, the average water drop, average draw down or the average water
drop delta h over the whole area, entire area will be equal to what? Will be equal to this
total volume divided by the aerial extent, is it not. So, that will be equal to 1, 10
to the power 8 over the area is 150 square kilometers. So, you multiply this by 10 to
the power 6, this is meter cube, this is square meters. So, it will be about 0.67 meters; so you say
that the drawdown is about 0.67 meters or 67 centimeters. So, you see that we can calculate
the average drawdown but we have to find out first the total volume of the aquifer. So,
when we find the drawdown, we have to make sure that we are using the total area, not
the volume of water. Because that total volume of water actually is coming out from where?
It is coming out of the whole volume. So, water is actually stored only within the pores
and where are all those pores? Those pores are in the whole volume. So, you cannot just
use the V y, you have to use V t. So, this is an important point in this problem. So,
let us move on; the next thing we are going to look at is the concept of these different
types of aquifer. The types of aquifers. We had very briefly
said this, that they mainly two types; basically confined and unconfined aquifers, but will
say first we will look at unconfined, un confined; then we will look at confined. And then we
will look at what is called leaky aquifers; so we will look at these 3 types. So, what is an un confined aquifers? I think
you are provably aware of these things. What is an unconfined aquifer? Well, it is the
one in which the ground water table is free or it is not confined between confining layers.
So, you have a bottom layer which is an impermeable layer and then you have the water surface
or the ground water table at atmospheric pressure and these are shallow mainly in nature. So,
you can defined these as, is one in which the ground water table is undulating; it may
vary from one place to the other in the form and slope, depending on recharge. Because
an unconfined aquifer can be recharged from the ground; directly through the rain water
or the agricultural water or any other type of pond. And discharge and discharge pumpage
from wells and permeability. And unconfined aquifer is one in which the ground water table
is undulating in form and slope depending on the recharge, the discharge, pumpage etcetera
and its permeability. The main thing is ground water table is at atmospheric pressure in
an unconfined aquifer, it is an important characteristic. The other one is that the rise and fall in
the ground water table correspond to correspond to change in storage change in storage in
the aquifer. This is important; why because when you have a unconfined aquifer, as we
said it can be recharge from the top, from the ground. So, if there is any changes in
the ground water table, if there is a rise; that means, there is a change in storage or
the storage is increasing. And if there is a fall in the ground water table, that is
a result of the change in storage. So, the changes in storage are or the rise in fall
of the ground water table is corresponding or related to be change in storage in a unconfined
aquifer. And we will see later that this is not the case for the confined aquifer; that
is why it is important to understand the difference. Now, the other thing is the contour maps or
profile of the ground water table in the whole aquifer which is basically your water table
in the wells, observed wells. We can find such maps; so these maps actually help in
determining the quantities of water available and its distribution and movement and its
distribution and movement. So, the important thing here, what we are seeing is that; the
contour map. If you want to develop a contour map in a unconfined aquifer or the profile
of the ground water table, then what do we do? Well, we just put some observation wells,
we dig some wells in you know at some spacing and then we observe the water table. So, if
you note down that water table the elevation and we develop some contour, what is a contour?
Contour is basically line joining the equal elevation. So, once we do this exercise then
that contour map is very useful in what? In, let me see in determine the quantities of
water available because that will tell us how much water is available in certain area
and its distribution, where it is concentrated and how it can move. So, let say if you are a develop a contour
map, it may have a certain contour interval; 1 meter here 2 meters here and so on. So,
if you see the concentration of those contour lines which are very close in one area, means
it is steeply sloping down. So, the movement will be very faster there and if you have
a very flat radiant, then that is where we want to tap the water. Anyways, so this contour
map is nothing but the water table in the observation wells, in a unconfined aquifer;
which actually will not be the case. In the confined aquifer it is a piezometric surface,
it is slightly different, will come to that little later. Before we move to that I would
like to briefly give you a concept of what is a called a perched water bodies or perched
aquifer. What is it? It is a special case of an unconfined aquifer. What I will do is
I will just give you schematic of this, before I give you the details of this. So, this is
the impervious bed town at the bottom; impervious rocks eta and then you have the ground water
table somewhere here. So, this is your unconfined aquifer and this is your ground water table. So, what may happen is you may have some material
which is impervious and there may be some water at a shallow depth lying in here. And
these are called the perched water table, this one and this one. This we say is the
perched water table and these are some clay lances, some impervious material which is
lying above the groundwater table. And, these are I would say impermeable strata, this one
and this one. So, this perched water body is basically, they exist between what? The
ground water table and the ground level, this is the ground level. So, this is your ground
level. So, it exit between ground water table and the ground level in zone of aeration.
What is the zone of aeration? It is basically the unsaturated zone above the ground water
table. Due to relatively, how it is occurring? Due
to relatively impermeable strata, impermeable strata of small areal extend. So, it is very
small, you see that you know some water may be may be available here; but it will not
be much. It will usually shallow as I said the perched water body and if you dig a well
or the wells in a perched water body or aquifer; such wells will have what? Will have very
small and temporary yields. You know some times what happens is, we start digging a
pumping well; we start digging a pumping well. And then what happen is we encounter certain
water; so we feel that we reached the ground water table and then we start pumping the
water out. But after few months may be or after some time, there is no water because
this for the perched water bodies and it is not the actual ground water table. And if
you dig deep it is very difficult to go through that clays strata; so, that is when we realize.
So, whenever we are doing any investigation for water we have to go sufficiently deep. Then, continuing further the example of such
perched water bodies is the basically your clay lances, as I just mentioned; clay lances
in the sedimentary deposits. So, this was about the unconfined aquifer. Now, we will
move on and look at confined aquifers. They are also known as artesian wells; I am sure
you may have heard these terms or pressure aquifers. These confined aquifers are, you
know called in many different names. Few of them is artesian wells; artesian is basically
coming from France, some French. Hydraulic engineers they carried out many investigation
in the earlier 19 century and they found or they after they went very deep in to the ground,
they found the water. And the deep well there are called Artois and origin of this word
is from that Artois, which is a very deep well; so it is an artesian wells. And, also
it is known as the pressure well. Why because the water which is there in the confined aquifers
is at higher pressure than atmospheric; it is at a very high pressure. That is why if
you deep, you know dig into it water can come out very easily. So, let us look at this the characteristic
of a confined aquifer. As I said the water is at pressure greater than atmospheric pressure.
Second one is the water is sandwiched, sandwiched between two confining or impermeable
rock or consolidated strata. The water level
in any well which is dig into it, will rise above the upper confining layer. Try to understand
this. If we dig a well into a confined aquifer, the water level will rise above the upper
confining layer. Why because water is at higher pressure than atmospheric; it is just that
this confined, alright in a well penetrating a confined aquifer. There are few properties we are looking at
of the confined aquifers. The water in the confines aquifers enters at regions, where
does the water come from in this confined aquifers? Well, it comes or it enters the
regions where the confining bed that the confining bed rises to the ground level. Because the
water is sandwiched between two layers; so how does it enter into it? So, this confining
layer some, somewhere have to meet the ground. So, when you have these two confining layers,
the some you know very far distances in certain region it has to you know meet the ground
and that is where the recharge area will come from. Normally, may be the hill area also.
Many of the confined aquifers in Uttar Pradesh and the planes, the recharge area is coming
from the Himalayas for Indian conditions. So, this is an important characteristic. When the confining that ends inside the earth.
The earlier point what we said is the confining bed is going all the way up to the ground
level. In the second one we are saying, we confining that when it ends within the earth;
then what happens? That means, the confined aquifer actually becomes what? Unconfined.
Try to understand this you have these two confining layers, the recharge area we are
seeing is very far of the regions where this confining layer will meet the ground. However,
if you have this upper confining layer, let say it goes and then it disappears. There
is no strata or the confining strata end here. After that what happens? Well it becomes the
unconfined aquifers so there can be all different types of possibilities. The recharge is from the recharge areas
which are for off or leakage through, leakage
through the confining bed as we have just said the recharge in the confined aquifer
is where from very far of areas as compare to the unconfined aquifers because in the
unconfined aquifers the recharge can be from the top. Now, the important point I would
like to make here is rise and fall the rise and fall of the water level in penetrating
wells in the penetrating wells results in a confined aquifers we are taking about from
changes in
pressure rather than rather than changes in storage volume. So, you have a confined aquifer, if you tap
into it if you have a well inside the confined aquifers you are drawing the water out. Where
is this water coming out from it is actually not the storage you have a recharge which
is a continuous conduit so this changes in the pressure which will take place. So, they
will be very little changes in the storage volume in a confined aquifer so if there is
a rise in fall in the piezometric surface then they correspond to not the change in
storage in the confine aquifers, but what change the change in the pressure the pressure
will go down. So, let me reiterate that only small very small changes in storage and that
basically means, that this confined aquifers serve primarily as primarily as conduits for
transmitting conduits for transmitting the water from the recharge areas
the recharge areas to natural or artificial
discharge location discharge locations. So, I am going to say that this rise in fall
this point is important to understand also this last one. So, what we have said is that
this confined aquifers basically serve as a conduit for transmitting the waters how
they have a recharge area which is very far of. So, there water enters if you pump the
water out there is hardly any change in storage, but you have the water getting started to
move from the recharge area and what is a discharge area discharge locations are what
either it is artificial in which we are pumping the water out or it may be natural. From the confine aquifers we have the phenomena
what is called the springs or springs and so on. So, the confined aquifers are very
much different as compare to the unconfined aquifers as for as there operation is concerned
because water is at a pressure which is higher than atmosphere. So, we have seen that the
unconfined aquifer we the ground water table at the atmospheric pressure now in the case
of the confined aquifers if you dig a well observation well into a confined aquifers
water will rise quite high. And if you join the if you have many such observation wells
and if you join the points of the water levels in different observation well what is that
surface called what is that line called the potential meter surface or the piezometric
surface rather. So, that is what we will look at the characteristic
of a piezometric surface in a confine aquifer. Piezometric surface we do not call it down
water table we call it is not as atmospheric pressure. It is also known as potentio-metric,
potentio-metric is a one word and they separating it surface of a confined aquifer
of a confined aquifer is an imaginary surface.
Remember, it is an imaginary surface does not exists coinciding with coinciding with
the hydrostatic pressure
the hydrostatic pressure level of the water in the confined aquifer. So, it is an imaginary
surface which confined with the hydrostatic pressure as we said in a confined aquifer
the water is at a higher pressure so the hydrostatic pressure may be higher that the atmosphere
atmospheric pressure so whatever it is if you dig a observation well the water will
rise quite high and if you join that surface that is called the piezometric surface. The water level in a well penetrating confined
aquifer confined aquifer is what is equal to the elevation of your piezometric surface
that is what we are saying. The piezometric surface
the piezometric surface may be below or above
the below or above the ground level can you imagine that you have a confined aquifer in
which the water is at a high pressure if you dig a well what may happen is that the water
may rise above the ground. Water will actually just huge out it is called a flowing well
and we will look at these phenomena in a schematic diagram very shortly. If it is above the ground
level means what do we have the phenomena is called the flowing wells we do not have
to actually pump the water if your piezometric surface is above the ground level, and we
have springs the contour maps slash profiles of the piezometric surface
can be prepared
from what water level data from the wells
in a similar manner as I just said. Because the water level in the well represent
the pressure conditions so if we can join them we can develop what is called contour
maps in terms of the piezometer surface. If the piezometric surface falls below the upper
confining layer would mean what would mean that the water level will be at what at atmospheric
pressure, because it is connected to the recharge area that would mean you confined aquifer
actually becomes unconfined. Try to understand this what we are saying
is that if you have a confined aquifer in which the piezometric surface if it is above
the ground level you have a flowing well or the springs. But if the piezometric surface
drops below the upper confining layer this is the upper confining layer and if the piezometric
surface drops below that. Then what happens? Then this confined aquifers is no longer confined
because the water here is at the atmospheric pressure, because it is related to the atmosphere
somewhere from the recharge area. So, then the confined aquifers actually becomes
unconfined if the piezometric surface falls below the upper confining layer. The last
point about them is that usually the unconfined aquifer occurs above a confined we have a
confined aquifer which are deep at the bottom and then on top of that you will have an unconfined
aquifer. The next thing I am what I am going to look at is I look at overall system which
I will show you all of these things. So, let say
this is your bottom confining layer, this
is your impermeable strata and then you have this is your ground this is your ground level
and then you have let say well very deep into this confined aquifer there may be another
one down here
and then you have some upper confining layer like this, so this
is your some confining layer at the top. And
I am not drawing this above this is continuing here. So, this is your upper confining layer
and let me say that this is your confining bed, so what is this is your confined aquifer
so if your pumping water out from this we have dig this well from the top then what
will happen the water will travel towards it further distances. And then you say that you have the ground
water table there is a unconfined aquifer somewhere down there this is your ground water
table so what is this? This is unconfined aquifer this is unconfined aquifer what is
this
is obviously your recharge area very far of which is open to atmosphere. So, water can
actually infiltrate into this and travel from long distances into these confined aquifer.
And then it will go into this tube well and then if I draw the piezometric surface let
say it is somewhere down here it will be let say horizontal here I am saying it will cut
here like this. So, this is your piezometric surface what is a piezometric surface
if we have dig this well into the confined
aquifer so water level will rise up to this piezometric surface. So, this is your water
level in this pumping well now this piezometric surface at this location in this particular
well is higher than the ground this is the ground you see. So, what will happen here because there is
higher pressure then water will naturally start coming out, so this is your what is
called the flowing well for a spring. It may be natural it may be artificial this is the
way I have shown this is artificial then I dug a well there and then water will start
boozing continuously we can also tap into the
unconfined aquifer. So, this is a tube well
dug into unconfined aquifer water will travel into this you may also have some perched water
bodies here some clay lances this is our perched. So, this way you see that we have a looked
at a schematic diagram in which we have try to demonstrate. How the confined aquifer will occur what could
be the possible ways of that recharge area? We can dig wells into a confined aquifer and
then we may have either a flowing well in which water may be coming out automatically
because the piezometric surface is higher. And then also we may have a penetration well
in which the water table is not above the ground rise in that case we will have to pump
the water out. None the less, if the observation well or the pumping well is deep enough than
the water will come from the confined aquifers we can also pump water from the unconfined
aquifers. So, this is the different ways of looking at let me say that this speaker is
basically as catch which is showing what unconfined and confined aquifers. And how they occur, how they recharge and
so on and obviously this unconfined aquifer how can it be recharge through rain fall water
will go down and then recharge that area. Let us quickly move on and look at the what
is called the leaky aquifer? They are also known as semi confined aquifers. The confined
and unconfined aquifers actually occurs less frequently then the leakier so leaky aquifers
are in fact more frequent so this is important to understand. When do they occur well they
occur when a permeable strata permeable strata is over length or under length either above
or below under length by a what by a semi pervious aquitard or semi confining layer. So, let us look at that so you have let say
this is your impervious strata down at the bottom and then this is your ground level,
let say you have a well very deep inside this confining layer. And then you have this is
there is a semi confining layer it is not exactly confining semi confining layer so
this is what is called your leaky aquifer. Because it is leaking at the upper confining
layer so it may receive water from the aquitard above. So, you have an aquitard that say sitting
above this there is the water table which is down here this is your ground water table
so it can receive water from the aquitard like this. And one can tap into the leaky
aquifer and then you will have water flowing like this now this is important to understand
that the pumping from a leaky aquifer
pumping from leaky aquifer removes water in two ways. How? One horizontal flow asIhave
shown within the leaky aquifer
which is what which is actually this one? And two or b the vertical flow through aquitard
vertical flow through aquitard into the leaky I have said here aquitard, but there can be
an unconfined aquifer here also which will contribute water vertically down into the
leaky aquifer because of this existence of this semi confining layer. These are the three
different types of aquifers. Now, what we will do is we will look at what is called
an idealized aquifers so these are aquifers which actually occurs naturally in the nature,
but when we are doing the modeling when we want to model this it is extremely compact
so what we do is we try to idealize this or we assume that here homogeneous isotropic
and so on their lots of type of you know words I am going to through actually. So, let us look at how we are actually or
how we actually handle this situation? How we actually model this? Because it is a very
yeah heterogeneous system it will be very difficult to model so let us look at what
is called the idealized aquifer. So, we assign aquifers are heterogeneous and anisotropic
what is anisotropic? An anisotropic aquifer is one in which the aquifer property is bearing
in different direction. So, the mathematical modeling
its difficult so what we do is we idealize
or we use the idealized aquifer. As what as homogeneous and isotropic so we
are kind this regarding those in most of the time it is important to note that this such
idealized aquifers that is homogeneous isotropic etcetera do not exists there nothing like
a idealized aquifer which will find in nature. However, good or practical a quantitative approximations quantitative approximation can be obtained
under such assumptions. So, we see that what we have done today is
we have looked at some other aquifer properties in terms of the specific yield and the specific
retention and then we looked up at an example. And then we looked at different types of aquifers
and mainly confined and unconfined there various entry cases the properties you know and how
they behave when they are subjected to either pumping or different types of force in functions?
So, all of this is important to understand and towards the end we have seen what is an
idealize aquifer. So, I think I will like to stop at this point of time and in the next
class we will look at how we can actually handle this anisotropic aquifer in which we
may have different layer and how we can bought it that I would like to stop at this point
of time.