Presentation on theme: "GROUNDWATER CONTAMINANT TRANSPORT MODELING FOR RIVER BASINS"— Presentation transcript:
1GROUNDWATER CONTAMINANT TRANSPORT MODELING FOR RIVER BASINS Suresh A KarthaAssistant ProfessorDepartment of Civil EngineeringIIT Guwahati
2This presentation is an academic exercise, where we intended to generate interest in you on groundwater contaminant transport modeling.I am sure such a presentation should help some students in their future research.
3What is Groundwater?Water present in the subsurface environment of earth is called groundwater.It extends vertically and aerially on the subsurface of the earth.It requires certain type of soils to bear water in the subsurface that are called aquifers.Ground LevelWater Table LineAquiferImpermeable Strata
4You are aware of Hydrologic Cycle Figure taken from the document “Some Critical issues on Groundwater in India” Centre for Water Policy (2005) (www.sandrp.in/groundwater/crisugrdwtr.pdf)Ground water is an integral part of Hydrologic Cycle
5Distribution of water on earth Courtesy: Corbett Foundation
6How Groundwater is formed in the subsurface? Rainfall water percolates into the subsurface.It gets accumulated over impermeable layers forming GW formations.From the National Commission on Integrated Water Resources Development data, 432 billion cubic metre volume of GW is estimated.
7Groundwater as part of River Basin Ground water is part of hydrologic cycle.Similarly one can think of ground water as part of large river basins.For example River Ganga also encompasses a large underground water reservoir along
9Groundwater runoff in some river basins in India Source: (www.sandrp.in/groundwater/crisugrdwtr.pdf)
10In India,Groundwater is used for drinking, irrigation, and other domestic purposes.Even many industries extracts GW for their use.A conjunctive mechanism is therefore required in many regions for sustainable extraction.
11Link of GW with RiversTherefore, groundwater aquifers are closely linked with rivers.Here the groundwater is feeding the river.Ground SurfaceGroundwater TableRiver
12Link of GW with RiversIn this case, the river is feeding the groundwater.Therefore, any effect of quality or quantity on one may affect the other.Ground SurfaceRiverGroundwater Table
13Groundwater can get contaminated? GW is prone to contamination by various means.landfill leaching, radio-nuclide dumping, subsurface chemical reactions, etc.RainfallUnsaturated zonePercolation of leached liquidGround Water TableGround LevelRadio-nucleic wasteWater Table
14What is Landfill Leaching? The process of dissolution of harmful chemicals (or materials) from landfills formed from dumping of wastes is called leaching.The migration of these toxic dissolved chemicals from landfills into the natural resources like river, groundwater, etc. is a major urban environmental problem referred as landfill leaching.
15Landfills are dumps or heaps formed from dumping of wastes. Wastes are of the following typesFoodsPlasticsElectronic goodsIndustrial wastesHospital and medicinal wastesBiological wastes, etc
16Some Photographs of Landfill sites This photograph is taken from the following websiteThis photograph is taken from the following website
17Some Photographs of Landfill sites This photograph is taken from the following website
18These wastes acts as huge reactors of formation of toxic products. Amount of wastes generated in urban areas are large and wide.Therefore, urban areas has unique problems like landfill leachingThese landfills also breeds harmful insects and organisms.Spreads contagious diseasesGroundwater pollution
19How will you then study contaminant transport for subsurface water system? One can use the laboratory experiments and on field data measuring equipments to measure the contaminant concentrations in groundwater. Then plot them and infer.Can use remote sensing equipments to trace the movement of contaminants in the subsurface and map them.Can use mathematical models to express the contaminant movement in subsurface.
20How will you then model contaminant transport for subsurface water system The subsurface water system – i.e. the groundwater is considered as porous medium.Using the continuum mechanics approach, the fundamental principles on mass, momentum, and energy can be applied on the porous continuum.Principles of conservation of mass of the contaminant in the fluidConservation of linear momentum
21Porous MediadxdydzGrey coloured area is liquid or gas domainWhite coloured area is solid domainqPores may be partially or fully filled with water in the subsurface.Groundwater regions below the surface of earth are in fully saturated conditions.
22As contaminant transport in groundwater involves the flow of water andMigration or transport of contaminants through porous media,An integrated approach of fluid flow and solute transport through porous media has to be taken for analysing contamiant transport.
23Flow of liquidsFlows of liquids - analyzed by determining the liquid pressure and liquid saturation in the entire porous media.They are mathematically described using the principles of conservation of liquid mass.This principle suggests - Mass of water is conserved in the porous media (the landfills, unsaturated subsurface below, and the groundwater).
24Flow analysesDarcy’s velocity is function of hydraulic conductivity and hydraulic gradient.In variably saturated (unsaturated) conditions the hydraulic conductivity is related to the intrinsic permeability of soil and the viscosity of the liquid.Again, hydraulic conductivity as well as the liquid pressure in the medium can be related with the liquid saturation in unsaturated conditions.This gives you a partial differential equation (pde) either in terms of pressure or saturation.Solving this pde gives you pressure distribution or saturation distribution.
25Conservation of mass for the contaminant In a porous medium control volume, the conservation of mass of the contaminant can be expressedThe Material DerivativeOrTime Derivative of Mass of ContaminantThe amount of mass of contaminant generated or destroyed inside the CVNet mass outflow ofContaminant massthrough the surfaces of CV=+
26Transport analysesDissolved toxic chemicals or contaminants are transported in porous media by advection, dispersion, sorption, and degradation.Advection of contaminants is the movement caused along with flowing groundwater at the seepage velocity in porous media.Dispersion is a mixing process, in which the solute is seen to spread out from the flow path.Sorption describes the chemical processes in which the contaminant mass is either entrained into soil or leaches out from the soil.
27Mathematical Formulations These phenomena in transport can be described mathematically using again the principles of conservation of mass.Conservation of mass equation is developed for concerned contaminant specie.The transport equation for contaminant transport in the groundwater is:Change in contaminant massChange due to dispersive effectsChange due to advectionChange due to consumptionwhere C is concentration of contaminant in the liquid flowing through the pores of ground water sources,D is hydrodynamic dispersion coefficientR is retardation factor
28Conservation of mass for the contaminant Advective fluxes through the boundary surfaces of the volume =Dispersive flux through boundary surfacesInside the above volume mass may be generated or destroyed. This is given byThis process consist of sorption, degradation, chemical reactions, etc.
29Mathematical Formulations… The transport equations can also be extended to include:Instantaneous sorptionRate-limited sorptionCa is the concentration of pollutant specie in the flowing liquidρb is the bulk density of porous media or the mineSm1 is the concentration of pollutant in the instantaneously sorbed stateSm2 is the concentration of pollutant in the rate-limited sorbed stateF is the fraction of total sorption that is instantaneousk is the rate of sorption or the rate at which the acid extracts the pollutant specie
30Numerical methods for model development Domain discretised into rectangular grids. Here two-dimensional case is describedi,j+1Z-directionY-directioni-1,ji,ji+1,ji,j-1X-directionFor a 2D case, the width or thickness in the y-direction is assumed as unityThe conservation of mass for acid in the flowing liquid is:Divergence theorem application gives:
31Numerical methods for model development Approximating the above equation by implicit finite-difference scheme, we getA residual equation of the above can be obtained as such:
32How will you apply such mathematical models for contaminant transport For example, if there is a river and the banks consist of agricultural plots filled with harmful fertilisers.The fertilisers may dissolve and reach the groundwater first and from their it may flow to the river.RiverAgricultural Plot
33To apply such mathematical models for contaminant transport The boundary condition in such a situation will beConstant hydraulic head on the river boundary. The concentration boundary can be given as zero gradient.RiverAgricultural Plot
34To apply such mathematical models for contaminant transport Different types of boundary conditions you need to incorporate as per the physical situation demands.Such mathematical studies are essential for analysing ground water pollution in river basinsBecause these will help in better understanding of the process.It will also help in better preparation for experimental studies.We can also plan experiments in a better as we are aware of the physical processes.
35CONCLUSIONS Groundwater essential component in hydrologic cycle. Most groundwater aquifers are inter linked with river bodies.Groundwaters are susceptible to contamination from rivers or top soil.Similarly groundwater may also contaminate rivers.Conceptual modeling of contaminant migration in groundwater bodies are essential for better understanding of the processes prior to experimentation.The processes that cause transport or migration of contaminants are advection, dispersion, sorption, etc.Any planning at river basin level should also include groundwater modeling.