 Hello everyone, welcome to NPTEL course Rural Water Resource Management week for lecture 2. This week we will be looking at components for understanding groundwater hydrology. More focus will be kept on how to understand the properties to recharge better and store the water in the groundwater aquifers and then what are the losses that happen. So this week the groundwater hydrology started with understanding where the water comes in through the hydrological cycle. We also looked into how the space for a space where water can be stored reduces as you go along the depth. So which is like this when you go down deeper the water would not be stored more because the size of the void spaces the space where water can enter is reduced drastically. So where is groundwater stored? So what is it called? We call it as voids and water gets in right but there is a name given for it so aquifers. So we would be learning about this term when you go more technical into groundwater. We'll call it as groundwater aquifers. Last week when we spoke about groundwater components I said the government of India has a plan on mapping the aquifers which is the boundaries of the water supply schemes and within that where the boundaries of the aquifers the water groundwater aquifers are because then they can supply water to irrigation or domestic water supply. So what is an aquifer? So an aquifer is the medium which stores your water. It can be made up of rocks you can have a consolidated aquifer where purely rocks are there and within the rocks there are spaces where the water can be stored then you have partially weathered rocks which are called semi-consolidated or fractured rocks where water can be stored in the fractures and those are called fractured aquifers. Then when you go up further you have the unconfined aquifers which is not under tremendous pressure of another layer on top of it so it is open and water can infiltrate. So we saw that in different terms also so the zone of aeration zone of saturation is purely depends on the amount of water so please differentiate between saturated and unsaturated like the term says it is a degree of water available in the aquifer so if it is full we call saturated aquifer if it is not full we call unsaturated aquifer even 10 percent 50 percent the same term is there unsaturated only when it is fully with water we call it as saturated aquifer. Moving on how does it store water? As I said there are four spaces and those spaces store water but it also has to stay there for longer time if it is keep on moving down then there's no point of calling it as an aquifer okay it should be slow relatively slow as plants can take it up trees can take it up farmers can take it for their agricultural fields or even your domestic pumps can take it for domestic use drinking water even government schemes are there for drinking water purposes okay so moving on what do we have is how does water get stored so let's start with the presentation you do have a porous medium and multiple porous mediums in this example we have two porous mediums on the top and one porous medium the bottom so three layers okay and in between the layers you have an impermeable layer which is separating the whole chunk on the top from the bottom so now you had three layers and then now you have an impermeable layer which is dividing the whole three into two sets one plus two and then another one on the bottom let's start from the top so all these have porous medium let's understand that all these three have porous medium depending on the water it is called saturated or unsaturated okay so now let's look at where the aquifer the term aquifer comes water gets recharged okay it goes into the porous medium first phase it is still not saturated fully so it is called unsaturated aquifer this part where you can see is an unsaturated aquifer not much water then water comes down here you have a saturated aquifer after the water table is reached and water doesn't flow down more but laterally so you have a saturated aquifer you also have another saturated aquifer on the bottom so depending on the saturation of water you can have an unsaturated aquifer or a saturated now coming back to the terms of confined or unconfined okay so is this aquifer confined yes it is a confined aquifer why because your water is coming in through the porous medium right here it's not confined if you can find this what happens precipitation cannot come in so please understand that confined means it is not like blocked on all sides somewhere it has to reach somewhere water has to come in so that is this part you can leave that part then water comes in through the porous medium and actually recharges saturates the porous medium and then it becomes a confined aquifer because it is confined between two impermeable layers so one layer on the top and one layer at the bottom so it can move laterally or it can get stored in one place it is called a confined aquifer the purged water table or purged aquifers are another aquifer type where it is purged and held somewhere because of a geological process we'll get into that in the next slides but please understand that here we have a confined unit and that's why it's called a confined aquifer another type of aquifer exists on the top which is not confined which means on the top there is no confining unit it is pure land it is it is a permeable land water can come in so it is an unconfined aquifer so aquifer can be called saturated unsaturated aquifer can be called confined or unconfined then there is another aquifer which is totally based on the nature of the rock consolidated unconsolidated or semi-consolidated so let's get into that and understanding here is we understood what is the aquifer we understood how water gets stored in the aquifer it is by porous space and also the potential gradient should not be too dynamic or too high if it is too high water just goes down okay for example water is entering here do we call this is an aquifer no because water doesn't stay there for long it enters and because of the sudden gradient it just moves down but after that after that after the water table a stable water table is reached we call it as an aquifer and then we have a non-regular case where we have a purged water table or a purged aquifer it is formed because suddenly when the weathering was happening or the plates were moving some deposition of an impermeable surface happened intermediately and that stayed it didn't move so water cannot move in and break it so water gets stored there is a purged water it's not common okay so please understand that it is one off you won't see it much common in nature but still it is good to be understanding that such a situation happens let's continue with the different types of aquifer right we discussed the zone of aeration versus zone of saturation so you have a saturated aquifer as saturated then depending on the confining unit presence is it present or not then we had a confined aquifer versus an unconfined aquifer you see this is the confining layer as it's mentioned here and so number two is called the unconfined aquifer for the term aquifer to be used you should have a water table established so you have a water table established and therefore there is an aquifer then you have precipitation coming down and it is being sandwiched between two impermeable layers so it is called a confined or artisan aquifer artisan only when there's a well so if you don't have a well it is normally called as a confined aquifer other terms here that we didn't look previously is the aquilub what is an aquilub for us understanding for this course it is good to understand that aquilub aquitard bedrock all these units all these surfaces are highly impervious water cannot move in so you can just think about these terms as a block for groundwater movement vertically so what happens it won't move vertically but it can move naturally it forms like a pathway for lateral movement so it is aquitard aquilub bedrock these kind of terms or impermeable layer confining layer all these are impervious layers or less permeable layers which means water cannot move in it has to move sideways so different types are confined and unconfined aquifers very commonly used so if you go to groundwater books from the government of India they would mostly talk about confined aquifers and unconfined aquifers how many unconfined can you have only one right so because on the top of the surface you have the only one open surface you have one unconfined aquifer but underneath you can have after you have an impermeable layer it becomes a confined layer another layer can come here if possible because nature is complex you can have many many layers those all would be unconfined aquifers you won't have a confined in between an unconfined aquifer because already this layer is there which is impermeable so you can have an unconfined aquifer and after some time after another impermeable layer you can have another unconfined aquifer so please understand that it's not just one unconfined aquifer you can have within the unconfined aquifer there are different wells even the wells are called confined aquifer wells unconfined aquifer wells and an artesian well is just free flowing water just water comes out because of the potential difference that is also not much common all the wells you see in the field most of them I would say 99 percent of them you still have to put a pump to extract the water only when you put in the bore well and establish the well or develop the well you'll see water gushing out because the initial pressure is to be released once that is released it is a normal well you'll have to put a pump and extract it formation of different types of aquifers now we know what is a different type of aquifers just keeping it unconfined and confined let's see how they are formed confined aquifers are created by alternating aquifers and confining units deposited on a single dip so this is your dip you could see here and as I said there there could be multiple unconfined aquifers so the first aquifer on the top is confined think about a full confined aquifer right here and then suddenly there is a deposition of a confined aquifer or confining unit impermeable layer it pushes down and it is a confining unit then another layer another layer so by movement by plates moving by your land mass moving or by deposition okay so it says here it's a deposition on a dip so this is your dip and on top of it water related movement can deposit or sand can be deposited by wind so by different drivers deposition happens the key driver is water because water takes sediments etc so suppose you have a dip and water is sedimenting and confining layer on top and what happens this this layer becomes your confined unit and then afterwards it is an unconfined unit on the top on top of here and then another layer comes another layer comes so you you are creating more layers by deposition and that deposition would lead to multiple confined units with one unconfined unit on the top so this is your unconfined unit on the top and then underneath it you have a confined unit then you have a impermeable layer and then another unconfined unit impermeable layer so that is one process by deposition okay on a dip a dip is a land which is dipping in and then you have and it could be oceans your waves waves can deposit material and then it can become unconfined and confined those kind of things so confined aquifers the second figure can also be deposited by deposition of alternating layers of permeable sand and gravel and impermeable silt and clay deposits in intermountain basins an intermountain basin is where mountains are there okay i watershed with a lot of mountains what happens on the top of the mountains there's a lot of erosion because it is there high potential weathering is happening rocks break movement comes down so when it moves down and you have an unconfined aquifer okay confined aquifers very very much of alternating layers of permeable sand and and gravel and impermeable silt the silt and clay in the mountain basin like it could be from the rocks it could be from the deposition of rainfall through surface runoff streams etc depositing so you have and i'm confining it but unfortunately your silt clay can be deposited on the top so now what happens this silt and clay layer becomes impervious water cannot move down further it's very hard so it will move this way so the sand and silt underneath it becomes a confined aquifer same way another big storm comes another silt can be formed on top silt and clay on top of your sand so sand deposits and permeable gravel are always coming down right you could see big stones coming down sand coming down but the silt and clay can be deposited by a big event a big flood so after a flood so that's why you don't have these confined unconfined happening multiple times it is only when there's a big hundredth flood or a big event a big movement of rocks then you find these confined units forming so you can visualize a unit which was first unconfined became confined because a layer got across top of it okay so that is this figure explaining the next one is confined to aquifer created by uprapping of beds by intrusion okay you have your sea beds and because of intrusion of waterways waves etc then you have your confined units created okay it's a more complex geological process but the understanding is clear that two different forces have to combine and fly and then it gets deposited okay so this is one way this is another way and then it gets one below the other dipping or deposition etc so confined aquifer created by uprapping of beds by intrusion so intrusion happens and because of that there's uprapping uprapping on top one on top of the other and if the uprapping unit is a impervious layer it becomes a confined unit you can see how units are formed confined aquifers created when aquifers are overlined by confining beds so this is the simplest term to tell how confined aquifers are formed it is formed when aquifers like unconfined aquifers are overlaid by confining beds confining material confining layers so any layer which goes on top of a unconfined aquifer converts the unconfined aquifer into a confine and all this is at a big big time scale don't think that tomorrow i'm going to have a confined aquifer from an unconfined aquifer it won't happen that way if it's a big flood and then sudden movement that is fine earthquake that is fine but those processes don't happen every day that's what i'm trying to say it happens in one location in a big flood comes in the ganges but you don't expect the next year another one would happen okay so please think about it in an earthquake zone like Nepal you will see more often happening but again it is a big movement big force that has to come through the first aquifer story so this is a very interesting as i said it is not that common but as a water hydrologist you should also know that where this can happen so as i said some suddenly there's a clay layer all this could be a porous layer let's say sand silt and suddenly you see a clay layer because it was not it got deposited there and it didn't move so all the clay would have moved but this just stayed there or it didn't weather out all the others weathered because of soil exposed to sand and water but this clay layer never got exposed so what happens is it stays now let's imagine a rainfall happening so you have precipitation happening all these materials would take the water through infiltration gets stored so it becomes an unsaturated aquifer and here you have aquifer which is saturated you because your water table is reached but on top of it you have an unconfined aquifer and inside that water can get stored what can be stored not all the water goes off because there's a big void space water just goes and gets stored in the pocket and that is going to purge water table and when a farmer unfortunately hits it within a couple of distance he or she can pick the water but then in the water drill the boat drill cannot go further so the water all the water can be used they will be happy for some time but then it runs out faster but another farmer has to go deeper and then get all the water from here another thing you could see is because when the water table just gets up when the water volume gets up up added up suddenly it has to get released and how does it get released through springs so when you drive through mountains especially in Maharashtra through the gorges and hilly regions you'd see that because they broke the hills to put the roads you'll see water gushing out from the hills and those are because of the water recharging and coming out and there could be some purged water table and it sees some some openings some openings or weaker sections in the land where water can come out and that is how water comes out what is it called when all the springs combine together as a big spring it becomes a waterfall okay so all these are related through the groundwater not just rainfall but groundwater is the key so all springs are fed by groundwater so when water comes on the top and comes down it is a river or is or overland flow stream etc only when water goes down under and comes out it's called spring it springs out so that is why the name comes spring and then it mixes into your river as as normally okay first I go from above the main water table on a lower permeability layer in the unsaturated zone we've seen these where it is weak and then you have properties of aquifers and water table of the aquifers as I said the water table is established for a particular region based on the level of water in the aquifers okay so you have a level and the water table is reached that is your potential and water moves from high potential to low potential so if you want to see the gradient or the direction of water moving it will be from your left to the right to the screen so the properties of aquifers are water table of the well is here an artisan well is right on the surface on the surface you will see water so that is an artisan well some water would flow but most importantly water is at the brim at the ground okay so you easily take the water out because the water level is there and a free flowing well is some well which is lower at a lower elevation or at a higher pressure so the potential is high compared to the atmosphere so it just goes from high potential to low potential so all of these wells are tapping on the aquifer but this opening of the well is below the potential of potential surface so that is why water comes up okay again it is not that common but it is good to understand here what do you see a confining unit on the top a confining unit on the bottom and you have your aquifer so you understood that all these are based on the rock type and that's where the geology is very important so let's go back to the geology and the Sunderground Water Report has done this beautiful map of mapping the key geologic material across India and from that knowledge from knowing about the rocks because every rock has its own porous space has its own fractures of volume how much water it can store we could easily estimate what is consolidated and consolidated aquifers okay so let's look at this legend groundwater potential yield per liters per second yield is given as liters per second if it is unconsolidated formations where formations are not you know confined okay it's not consolidated so you have more potential for water to be taken up consolidated to semi-consolidated some broken parts are there but most of the rock is still consolidated okay so it is not broken but still the rock is tight so which means the pore space is very limited that is consolidated if it is semi-consolidated some weathering is there so it's a little bit more pore space is there all these would yield a very low yield compared to the unconsolidated formations the unconsolidated formations are most like soil silt clay alluvial like so it's broken so we have more space in the hilly areas hilly areas are very very less because there's nothing much of pore space it is just hills and water hits and flows so and the elevation is also there the slope is there so water hits and then flows okay so this would also give you an idea where to do agriculture right if you know the yield you do related agriculture so here you put rice sugarcane etc but here you have to be very careful on what type of crops you use because the yield is very less so that is the cautious that we can get from these kind of hydrological maps the other thing I would like to also touch upon is it is not uniform so suddenly you have a high yielding aquifer and then you move down you have a low yielding aquifer and then wherever the rivers and streams are you have a high yielding aquifer okay and then there could be automated unconsolidated consolidated etc and hilly areas can become consolidated after it is a little bit weathered so a little bit weathering you have also weathering gives you more space for space for water to be stored think about a rock if you crush it there is more space inside the rock water can go inside the rock but if you just have a rock it cannot go much because it is on the surface it might get wet but it doesn't get in so that is the understanding from hydrological maps and using that we can also name aquifers as unconsolidated aquifers consolidated aquifers etc etc with this the aquifer types formations have been covered I'll see you in the next class thank you