 Welcome to lecture number 34 of this groundwater hydrology course. In this lecture number 34, we will cover saline water intrusion in aquifers. The techniques to be covered are geochemical investigations, control of saltwater intrusion and practical modeling of saltwater intrusion. In geochemical investigations, we can use certain parameters to identify whether the saltwater intrusion has happened in a particular aquifer or not. So sea water has got sea water as a uniform chemistry due to the long residence time of the major constituents. So main point is that predominance of chloride and sodium with molar ratio of 0.86. If the aquifer is not anthropogenically polluted, the fundamental if the aquifer is not anthropogenetically polluted, the fundamental cations calcium and magnesium and to a lesser extent the alkali ions sodium and potassium, the fundamental cations are mostly this calcium, magnesium and to a lesser extent they are alkali ions sodium and potassium. So to identify the saltwater intrusion, different indices or parameters can be defined. The first one is salinity. So time series of steadily increasing chloride concentration can indicate the early evolution of the early evolution of a salinity breakthrough from sea water. Next parameter is chloride to BR ratio. C L or chloride BR ratio can be used as a reliable tracer because both C L and BR are usually behave conservatively. So C L and BR just behave conservatively and in case of sea water, this C L to BR ratio this weight ratio is approximately 297. For anthropogenic sources like wastewater effluent or sewage effluents, this C L by BR ratio is up to up to 800. The third parameter is sodium by chloride ratios. Sodium chloride ratios of saltwater intrusion are usually lower than the marine values which is less than 0.86 molar ratio. The fourth parameter is calcium, magnesium or you can have calcium with bicarbonate and sulfate. So these two ratios in sea water indicates with a high saltwater intrusion if it has got value greater than 1. So high calcium, magnesium ratio or calcium bicarbonate plus sulfate ratio both are greater than 1 indicates sea water intrusion. Other than this you have oxygen and hydrogen isotopes, stable oxygen and as isotopes can be used to describe the mixing process between sea water, sea water and fresh water. Fresh groundwater aquifer is generally depleted in both and deuterium relative to sea water. Mixing of fresh water, fresh and sea water should result in linear correlation. The last one is boron isotopes. One of the process modify the chemistry of sea water intrusion is the adsorption of potassium. Adorption of potassium, boron and lithium onto clay minerals in the host aquifer. These elements are relatively depleted in saline associated, saline water associated with sea water intrusion. So elements are relatively depleted in saline water associated sea water intrusion. Thus this boron isotopic composition can be used for identification of salinization sources in particular to distinguish from anthropogenic contamination such as wastewater. So boron isotopic composition of groundwater can be used as a tool to discern the salinization sources. So these are the parameters with which salt water intrusion can be identified. Next thing is that in density dependent flow the density depends on, density depends on the reference density, reference density, temperature and concentration. Similarly, dynamic viscosity is also a function of density or dynamic reference dynamic viscosity, temperature and concentration. So these two can be treated as equation of state for solving the flow and transport equations. So for the governing to solve this salt water intrusion or saline water intrusion process we need to use density dependent flow equations and flow equation is density dependent that means there is a relationship between this hydraulic head with density and these equations can be solved in a coupled sense. So the first equation is the flow equation. So this is a normal density, rho 0 is the reference density, F is the storage coefficient, H is the hydraulic head or total head, T is time so this is time dependent term. Then we have this is permeability or hydraulic conductivity tensor, this defines the axis direction because in z direction we need to consider one extra component in case of Darcy's law. This q star, this rho star is the density of injected or extracted source or sink and q is the discharge rate of source or sink and this is the transport equation. So this density is related to this concentration with the relationship that defines that this density is basically rho 0 plus 1 plus your epsilon or alpha divided by C s where this alpha is basically 1 rho s minus rho 0 divided by rho 0 and this is the density of water, ground water, this is reference density, this is density ratio or relative density difference. So if we take usual values then this rho 0 is 1000 kg per meter cube, 1000 kg per meter cube then rho s is 1025 kg per meter cube thus this alpha or relative density difference ratio that becomes 0 to 5 and this C s which is usually concentration of sea water that determines the whole thing. Now in this transport equation, this theta is moisture content, this C is concentration, this is again the temporal or time dependent term, this is addictive term and this is diffusive term, this is also alpha prime is the modified compressibility term, theta is again moisture content, concentration, this is total head, q is the discharge rate for source and sink and C in is the concentration of incoming or outgoing that means injection or extraction rate. Similarly, the q star, this rho star is the density of the injecting or extraction related fluid. So these are the terms which are there in the transport equation now as we have seen that the density is related to the reference density, density is related to reference density with the relationship rho and rho naught with 1 plus alpha and this is concentration divided by C s. So we can easily see these two equations are interrelated and it can be solved only in coupled sense. So as because there will be salt water intrusion in coastal aquifers, so we need to take some kind of counter measures. So one thing we can do is that we can manage the demand, by managing the demand itself we can counter, this is one kind of counter measure for the salt water intrusion management. So then non-potable water reuse, so non-potable water reuse is the second measure. Third one is the modified pumping rates, so we can have modification related to pumping rates and with the modified pumping rates only we can control the sea water intrusion. Then pumping caps, with the pumping caps also we can control the salt water intrusion, well relocation. Let us say that we have a well which is near to sea shore or near to your shoreline. Then it is obvious that there will be extra pumping from those wells and due to those pumping there will be intrusion of sea water and at faster rate and if we relocate it in the inward direction then at least we can check that sea water intrusion in the coastal aquifers. Then conjunctive use, conjunctive use is that we can use different combination of surface water, ground water so that people can use a percentage of ground water from their coastal aquifers and a percentage from surface water source or nearby rivers. So that way people will not be that much dependent on the ground water aquifers or coastal aquifers and we can check the salt water intrusion. And the first one and the last one is the aquifers storage and recovery. So aquifer storage and recovery is the most important one because we can store water in a certain aquifers and we can extract that water in future that way if that is connected to the sea water face then it can be it can act as injection barrier also if it is not hydraulically connected with the aquifers so we can store good quality of water in the aquifers and we can reuse it in future. So tapping alternate aquifers let us say that there is saline water intrusion, saline water intrusion in this particular aquifer then one can try to tap a lower aquifer which is not hydraulically connected or not that much affected due to saline water intrusion. So that way we can control the sea water intrusion basically it is by reducing the demand from this particular aquifer and we are transferring that extraction from a different aquifer or we are transferring that extraction thing to a different aquifer. Next is extraction barrier let us say that we have a sea here then saline ground water this is our fresh water and this is our production well. So and this is another barrier well which is near to your coast and this is our initial ground water table. So if we start pumping there will be a movement of this saline ground water towards the fresh water fresh ground water or fresh water aquifer and this barrier well will act as some kind of semi barrier thing and it will reduce the sea water intrusion effect by pumping saline water from the pumping wells and we can safely use our production well for some amount of extraction but there should be sufficient limits or there should be caps so that we can check the salt water intrusion. So this is the original interface and now this is the final interface after the sea water intrusion. Then scavenger wells so these wells are deep wells and these wells are meant so that we can arrest the total saline ground water and it goes up to full depth. So this is our final ground water table this was our initial one so we can safely extract water from the production well from our fresh water aquifer. Injection barrier with the injection barrier in case of barrier well we have already seen that there is extraction barrier. Now we can use the same barrier well as injection barrier we can inject water into these barrier wells so that water will spread into this aquifer and it will reduce the effect although this movement is shown towards sea but this is in a scaled up this is a scaled up view there will be a smaller effect or smaller retreat towards the sea. Now another point is land reclamation with the land reclamation as this is our ground water table and this is our sea level we can have a different interface position with the land reclamation. So that may reduce some kind of sea water intrusion effect. And next one is the impermeable barrier so impermeable barrier is basically made by injecting bentonite slurry or highly some kind of material so that the porosity the ports are filled with that material and the hydraulic conductivity will be reduced drastically. So with this kind of impermeable barrier in position there will be reduced effect and there will be slow movement towards this production well. So by this method also we can check the salt water intrusion towards coastal aquifers then another one is the surging pit. So surging pit this wall is basically this wall is basically impermeable and these two walls are permeable walls and water is filled in this surging pit and it is just parallel to the coast. So with high velocity if we pass the water then the saline water intrusion effect can be reduced. This is the production well and there will be reduced effect. So this is basically one kind of recharge effect and impermeable boundary effect. So our goal is to maintain the quality and quantity. So we need some kind of careful planning of withdrawal strategies for control and remediation. Remediation not in true sense but we can easily control the salt water intrusion with different type of countermeasures. So now I will discuss one case study of Nellore district in coastal Andhra. So coastal aquifer of Andhra Pradesh that is in Nellore district. So these are two mandals Allure and Vidavalure situated in the Nellore district and we have taken these two mandals for salt water intrusion modeling and control. So objectives where collection of all head concentration pumping data for the study area along with boundary condition and parameter estimates. Next is implementation of a three dimensional finite element based numerical salt water intrusion model for the coastal aquifer study area and calibration of the developed model for the study area. And predicting the future salt water intrusion scenarios and evaluating the management strategies for possible control of salt water intrusion. So this is a study area and total area is 355 kilometer square. This is the boundary between Allure and Vidavalure. So to model this kind of coastal aquifer regions basically these are having one coastal boundary that is Bay of Bengal, one is Penner River boundary, another two boundaries are political boundaries that is Allure-Mondale boundary and Vidavalure-Mondale boundary. And this black dots these indicates the location of pumping wells. Generally we have allocated these pumping wells in such a way that it represents the pumping scenario of each of the villages. These are the villages which are situated in these two mandals. So in a particular village there can be more than one pumping well but these are basically representative pumping wells for the study area. So number of villages in Allure-Mondale is 15, Vidavalure is 10. So area-wise is Allure is bigger and normal rainfall is almost same and the major crop is paddy. But recent few years there has been shift in the cultivation aspect. So this is a basic static graphy of the study area. With the RL values from different sources this model was prepared. These are the pumping wells. These are basically strainers and it has been found that bedrock was not encountered even at drilling depths ranging from 250 to 500 meters. So we have considered the top portion only for modeling. So first thing is that we have considered 12 meter thick sand layer then second one is 3 meter thick sandy clay and the last one is sand layer with 15 meter thickness. So assumptions that were made because of non-availability of some of the data sets. Recharge rate in the form of infiltration is taken as 15% of the normal rainfall in that particular period. The river boundary is assumed as no flow as Penner River remains dry most part of the year in this region. Although there is some amount of discharge but we have assumed it is a no flow boundary or not contributing to the study area. Seasonal fluctuations are not considered tidal effect is not considered. Then the pumping and recharge rates are averaged over the year. The total value is the total pumping occurring over the year and you already know this is the equation in case of epsilon there was alpha. So this is a 3D view of the study area and this is with finite element mesh and you can see that for draft values the maximum draft value assigned for a particular well is this much and minimum is this much and in case of with a value it has been found that drafts are very high. So you have increased the draft based on the population and other data so calibration and validation the calibration process is carried out for the time period between 2000 July 2002 July 2002 and simulated and observed head and concentration values are compared with the July 2001 July 2002. The model is then validated for July 2003 2004 mainly in the head in terms of head values and as estimated data is very scanty for 2003 and non-existent for 2004. Longest general dispersivity is considered as 50 meter and lateral dispersivity is 15 meter. So calibration trials with sand layer with particular hydraulic conductivity values it has been found that the upper region and the lower region it is not matching and also in upper region there is some kind of deviation but not that much but in the lower part it is not completely matching it may be due to the parameter values or due to the boundary conditions. So ground water modeling is basically it is either you can adjust the boundary condition or depending on the actual values or if you do not have that you can try to adjust the parameters to get the actual value. So calibration trial 2 in that one also it has been found that there is deviation in the lower portion or with a value region. And third trial also it is not matching. So in case of 2001 it is almost matching with the actual one. So it has been found that simulated and observed values are well in this 45 degree line and simulation for 2001. So simulation difference, so concentration scenario difference is only here in the internal portion. This is the observed one and this is a simulated one and simulation for the 2002 also it has been found that observed and simulated the middle portion only there is some kind of deviation. And the simulation for 2003 also it is found that there is some kind of deviation for that same region. And simulation for 2009 with the existing pumping pattern this is the scenario. But it has been found that internal region we have high concentration values. This is due to the fact that there can be some kind of inland salinity for this region. So three scenarios are considered for control of salt water intrusion. So in with the value region the five wells are considered near to well near to seashore. Then it has been found that there is decrease in concentration for a reference point one with that scenario again near the allure bundle if five wells are installed as extraction barrier wells then for the reference point also there is increase in concentration level. But the rate is much slower as compared to the existing pattern. So scenario three if three five wells are installed within that inland salinity portion then also there will be increase there will be decrease in concentration values for that region. So this is a comparison of concentration for 2009 scenario one and three the scenario four also same thing has been experienced. We can have two objectives for any salt water intrusion management model that is production well extraction barrier well. This is the ground water flow transport equation which relates the concentration with the flow transport relationship. There should be some kind of some kind of limit on concentration that means the concentration in the region should not exceed some value and the pumping there should be cap on the pumping values. And this is basically maximizing the pumping from production well minimizing the pumping from extraction barrier wells and both pumping from production well and pumping from barrier wells these are bounded variables. So that way one can formulate the management model for salt water intrusion and it can be solved for any particular aquifer. So this is the end of salt water intrusion saline water intrusion modeling and basically geochemical investigation control of salt water intrusion practical modeling of salt water intrusion and last we have discussed this management aspect of salt water intrusion. Thank you.