Mod-01 Lec-06 Zones of Aeration and Saturation; Aquifers and their characteristics/classification

Mod-01 Lec-06 Zones of Aeration and Saturation; Aquifers and their characteristics/classification


Welcome to this lecture number six on zone
of aeration and saturation followed by the aquifers and the characteristics classification
so in the previous previous lecture we discussed about the ground water column and specifically
of course we will briefly dealt about the zone of saturation which is below water table
and zone of aeration which is above the water table and we also discussed about how the
in the zone of aeration how the water is held which is basically in the in the intermediate
zone which is vadose water and here so this water is held by
hydroscopic and capillary action
so here because of this the attraction between the the soil or the rock particles as well
as the water so the water is held the water which is not moving is held by these two actions
and the capillary rise is the we also saw the expression for capillary rise which is
a given by 2 sigma into cos theta divided by r into let it be the specific gravity so
here now let us continue before this one i would like to mention so
essentially so this vadose water water held by hydroscopic and capillary action
and here so in this ground water so water is held by gravitational action okay and continuing
with this the expressions for this capillary rise so here suppose the this is a capillary
tube which is basically a narrow tube this is center line and
the diameter of tube which is given by 2r and
so this is the surface tension force whichis measured as force unit length and this is
the angle theta and then this is the water surface this is meniscus and so this is the
that is the liquid this case it is the water with specific weight gamma and this theta
is the angle of contact which i have mentioned here and say for if we further simply so this
so surface tension which is mentioned as the force per unit length generally it has a value
of say 074 so this is a grams per centimeter ofcourse so this multiplied by the acceleration
due to gravity that is 981 and then this specific weight of water again this is one gram per
cubic centimeter again it has to be multiplied by the so this multiplied by g so this g is
the gravitational acceleration this is 981 meter per second square and this theta approximately
=0 degree for water and clean glass same thing can be assumed in there for water in
soil also same can be assumed for water and soil also therefore the expression for capillary
rise h so it reduces to 5 divided by r okay so as can see from this one the capillary
rise h which determines the height of the capillary zone which inversely proportional
to the size of the capillary pores that is the the radius of the capillary force that
is basically here we are approximating the pores in to the requirement circle or shape
and then taking its radius so this h as a this the equivalent the circular pore radius
gets reduced the height of the capillary rise increases and here this mentioned that say
for different samples that is the say the material grain size in millimeter then the
capillary rise in centimeter so here on the lowest level were the capillary rise is as
low as just 25 centimeters we have this fine gravel whose grain size is that is 05 to 02 ok further as we go so this
is a next is a coarse and of course there are intermediate formations are there so here
coarse and for which it is a one the grain size is one that is a 1 to 5 millimeter and
it has a capillary rise of 135 centimeter and then i am sorry i made a mistake here
so this says this just like correct this one so this is grain size is 2 to 5 and coarse
and the gain size is 5 to 1 let me re write this again so this is the material grain size
in millimeter then capillary rise in centimeter see on the coarse test we have say find gravel
the grain size is said 2 to 5 millimeter when the capillary rise because this is too large
grain size so therefore the pore size is also too large so therefore the capillary rise
is also as say 25 centimeter followed by vary coarse sand the grain size is 1 to 2 millimeter
and the capillary rise is say 65 centimeter still let us go to still smaller this one
that is a coarse sand where the grain size is 5 to 1 millimeter and then the capillary
size is getting reduced and correspondingly the pore size also gets reduced so this is
135 centimeters then the medium sand as a grain size of 2 to 5 and the capillary rise
of say 246 millimeter centimeter am sorry then followed by say fine sand the grain size
is say 1 to 2 and the capillary rise gets further increased to say 428 centimeter then
it is the silk which has a grain size of 05 to 1 millimeter the capillary rise increases
to 1055 centimeter and lastly it is fine slit it has a grain size of say 02 to 205 and the
capillary rise is as high as 200 hundred centimeter so you can imagine so how the grain size the
material gets ahh finer and finer in terms of its grain size so the capillary rise increases
and this has been so sources ahhh study by lachman okay and now let me also represent
here the distribution of water in a coarse sand water distribution in a coarse sand above
water table after draining so here suppose this is the so this is the
moisture so this is the soil moisture content as percentage and this is a height above water
table in centimeter and here say let us say this is say this is 20 and this is 40 and
here let us say this is 20, 40 so here this variation is it will be something like this so this is the so this represents porosity
and here this represents capillary zone and above this this represents the intermediate
zone so in the capillary zone just at the water table level so the moisture content
is exactly equal to porosity and as we go above the water table the moisture content
goes on increasing as we can see here and so when we reach this intermediate zone so
this moisture content it represents only the hydroscopic water so whereas so here we can
say this is the hygroscopic component and then this the moisture content held by capillary
action and this moisture content help a hygroscopic action so therefore so in the intermediate
zone it is entirely the water is held by water is held by hygroscopic action whereas
in the capillary zone water is held by hygroscopic and capillary action so you can see how it
varies and say so we just at the water table level just above the water table so almost
all the pores are filled with this moisture soil moisture and then as we go higher and
higher it is only those force which is only continuous so they will be filled with as
we reach this intermediate zone so it is only the hydroscopic water which is the water held
by the force of attraction between the in the the spaces the void spaces as well as
the air, as well as the rock particle and then the water so so this is how for for in
the zone of saturation the water is held entirely by gravity and that is for all the zone below
the water table and as we go above at the water table so this is held by hydroscopic
as well as capillary action and as we reach the intermediate zone so the number of force
which contains soil moisture gets reduced only those continuous pores ah they first
of all for the capillary action they must have a very small size and then so through
which the capillary rise takes place due to the force of attraction between the soil or
rock particle and water due to surface tension force and above that in the intermediate zone
it is entirely by the molecular the inter molecular attraction between the soil moisture
the air void as well as or rock particles now let us consider the and this water content
which is held by various actions whether it is the hygroscopic action in the intermediate
vadose zone or hygroscopic as well as capillary action in the capillary zone both in the zone
of aeration and further below in the zone of saturation by gravitational actually acceleration
so this moisture can be measured by various methods which is a gravimetric method as well
as the other the tensiometers and so on and the same thing here we can mention here by
a tensiometer basically it made it just pulls so this water which is held in the
that is the measurement of soil moisture so this is by
gravimetric method and which we take the weight measurement and
also say by tensiometer so in this tensiometer suppose this is the soil column and here
so this is the the tensiometer so this is the poros cup and
this is the unsaturated soil and here so this is the suction head which is essentially
water held by the molecular attraction between water air particles as well as soil particle
so now let us come to what is known as the available water or available moisture content
so here suppose i represent so this is the soil moisture
and here so this is 0 so the maximum soil moisture so this is the soil moisture content
which can be which is possible is which is denoted as field capacity so this moisture
content let me represent this as mc so this is a field capacity moisture content so this
field capacity is essentially the maximum possible water which can be held in this zone
of aeration and also the minimum this is amount of soil moisture corresponds to so this is
the permanent wilting point moisture content and of course there is also an intermediate
this is known as the optimum moisture content content or say omc so here essentially so
this field capacity is the amount of water which is held in a soil after wetting and
after drainage as become negligible so generally it is after say two days of drainage so the
after two days of keeping it for draining so all the water which is generally goes out
through drainage that gets drained out and then the moisture content which remains in
that soil sample or soil or rock sample is the one which represents the field capacity
moisture content and the total amount of moisture is the volume for that is the known as field
capacity similarly so this welting point is the ahhhh ahh the moisture content it represents
the moisture content wherein the plants start welting that means say below this welting
point all the water all the moisture it is held by only molecular attraction between
moisture or water particles as well as air and soil or rock particles so it is not given
away by the soil so therefore plants cannot extract any water by capillary action so the
plant welting or drying so this drying plants so further so they die so this difference
between the field capacity moisture content and then the permanent welting so this is
known as available moisture content and similarly the difference between the field capacity
and optimum moisture content which is slightly higher than the permanent welting point so
this is known as the readily available moisture content so essentially so they all the irrigation
so they depend upon readily available moisture which can be easily extracted by the plants
through capillary action so therefore in this irrigation what is done so as soon as this
soil moisture gets depleted to this optimum moisture content level so the one irrigation
supply is given so then the moisture content increase to field capacity moisture content
level and then further again by the due to a plant metabolic activity due to evapo-transporation
as well as evaporation the soil moisture gets gradually decreased and again when it reaches
this the optimum moisture content then the next irrigation is given so like this so this
is the this how the process of irrigation continues now let us come to the moisture
content in the zone of this one so this available moisture so here so this is in the zone of
aeration ok so now let us come to the zone of saturation which is the which is the now
let us come to available water in the zone of saturation
again here so in the zone of saturation so most of the water is held by is this zone
of this is a gravity action of course very small amount is held by hygroscopic action
so here let us define say two terms which is the specific retention
so if we denote this as sr so this is the ratio of the volume of water which is retained
in soil against gravity after saturation divided by total volume so this vr is the the water
volume retained in soil after saturation against gravity
and this v is the total soil or rock volume and let us also define another terminology
here that is the the specific yield which is denoted by sy so here this is equal to
vy divided by v where again v is the total soil or rock volume and this vy is the volume
of water drained and obviously this is by gravity okay and so this n which is the porosity
of course few hours in the previous class i think i represent this by alpha and few
authors they use the notation n so this is equal to the
specific retention plus specific yield so this is a very important relationship between
porosity and specific yield as well as specific retention and here so essentially so if we
if i represent so this is the water table so water retain above water table is known
as the field capacity water here you can say it is moisture retained
in the zone of aeration here you can say let me the maximum water and here so this is the
and below this it is the water retained so this is what i retain corresponding to
the specific retention water sr maximum water retained in the zone of saturation so essentially
this field capacity as well as the water retained so they represent the same water content while
the field capacity represents the water the maximum water retained in the zone of aeration
wherein all the pors are saturated well the water retained represent the maximum water
retained in the zone of saturation and obviously so this is corresponding to sr so this here
we can say this is and both are against gravity so like this so the essentially this field
capacity as well as water retain so they represent the water which is which can be retained ahh
maximum extent in the zone of aeration as well as zone of saturation so now let me also
represent here so the and as this ground water this one so our objective is to harness or
extract ground water as much as possible so therefore this specific yield is more important
to us and here suppose i represent on a scale the specific yield express as percentage so
here from the lowest level say this a 10, 20, 30, 40 and in the highest this one say
44 we have peat which is basically in the vegetative matter which is decomposed and
then the lowest specific yield is observed in clay which is so peat as 44% and this clay
which is as minimum which is 3% and in between we have say the dune sand 38% and we have
say medium sand which is say 28% and we have say coarse gravel 23% and we have sand stone
21% and we have lime stone 14% and we have silk here with generally the this one okay
so these are some of the typical values of the specific yield in different materials
ranging from the minimum 3% in found in clays and maximum of 44% found in around 44% found
in peat so in between we have silk, limestone, sand stone, coarse gravel, medium sand, dune
sand okay and of course few more so this is the variation of specific yield in different
soil or rock furnishing now let us come to the aquifers their classification
characteristics and classification so so this aquifers are essentially there
is a water bearing layers so here you can say in this so typically we can say say this
above this lime stone all this rock or soil formation they represent aquifers below this
limestone all this soil or rock formations they represent either aqui tards, aqui tudes
and aqui fuges which we discussed in the previous class in the previous lectures so here i would
like to draw you attention to a schematic cross section representing the various aquifers
so this are the impervious so this is the impervious strata so this is the recharge
area so this is the ground level and here i would like to represent the water
table or piezometric surface so this is the water table and this is and this is peizometric
surface which represent the basically the energy
and here up to this recharge through different forms of precipitation so this soil moisture
the water moves and this this is known as the confined aquifer which is essentially
confined at top or as well as bottom confining layer so this is the confining layer
and here suppose we drill a well here which penetrates all the way up to confined aquifer
and here this one we may find we may so this is water table
so this well is penetrating through the all the way and here this is the confined aquifer
and then this is the unconfined aquifer also known as water let me write here so this unconfined
aquifer that is water table aquifer so this unconfined
aquifer represents top most aquifer which has only the one confining layer at the bottom
whereas its top surface represents the water which is subject to which is undulating and
it is depending upon the slope the areas of recharge, discharge and pumping and other
factors and in this case so now let us come to this well which has been dug in the which
has been drilled in the in the area and the land area which is below the peziometric surface
and it penetrates all the way up to the confined aquifer and here the total s this peziometric
surface represents the energy the total energy at that level for this depth all the way for
confined aquifer so therefore here the well starts oozing out water that is corresponding
up to peziometric surface and such a well is known as a flowing well
so here the water gushed out of the well on its own so now artificial pumping is required
and on the other hand suppose we drill a well which penetrates only the unconfined aquifer
in this case the surface the water surface corresponding to the water surface at that
location and so therefore this well which penetrates only the unconfined aquifer is
known as the water table well and on the other hand suppose we drill another well
which penetrates through the unconfined as well as unconfined aquifer so here water table
corresponds to this one and the peziometric surface and this well is known as artesian
well so this confined aquifer is also refer to as artesian aquifer which
is also known as pressure aquifer okay so there are three types of well the
flowing well the water table well as well as artesian well when the flowing well the
water is gushes out on this naturally by because of the the ground surface below the peziometric
surface is there whereas in the water table well as well as the artesian well so the water
surface will be below the corresponding to the water table or the peziometric surface
and this artesian well it draws from both the confined as well as unconfined aquifer
at the bottom as well unconfined aquifer at the top so we will stop here and we will continue
further our we will continue discussion in the next lecture thank you

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