MESHFREE MODELLING OF CONTAMINANT MIGRATION

Abstract

Transport of contaminant in the subsurface regions has been a most crucial and important topics as the quality of groundwater is depleting due to the addition of contaminants that are transported through flowing water. Development of numerical methods for simulating groundwater contamination is important because numerical models can handle complex geometries with ease. Conventional mesh based numerical methods face difficulties when dealing with contaminant transport problems related to high advective velocities and low or high dispersivities. Advection-dominant transport problems often are corrupted by numerical oscillations when they are solved using Galerkin finite element method. These can be improvised by severe mesh and time step refinement. This has motivated the development of alternatives to the standard Galerkin methods which prevent oscillations without requiring mesh or time step refinement. The present study is focussed on modelling one and two dimensional contaminant transport through saturated stratified porous media with regular and irregular node spacing. Formulation is extended to consider different aspects like geomembrane as a landfill liner, transport through fractured porous media and transport through unsaturated media. The meshfree method considered is the Element Free Galerkin Method (EFGM). In the meshfree method, approximation of the solution is assembled entirely in terms of set of nodes. The EFGM makes use of moving least squares approximants to approximate the function and uses the Lagrange multiplier method for imposing the essential boundary conditions. A generalised formulation is presented for EFGM. Based on the formulation a numerical code is developed in FORTRAN. The developed code is used for modelling one and two dimensional contaminant transport modelling through stratified media in saturated zone with regular and irregular node spacing. Contaminant transport through geomembrane overlying soil is modelled by coupling one dimensional model for geomembrane with two dimensional model for soil. Contaminant transport through fractured porous media is modelled by employing coupled equation for flux transfer in fracture and matrix. One dimensional analysis for contaminant transport through unsaturated media in stratified soils is performed. van Genuchten model is used for describing the hydrodynamic properties of the unsaturated porous media. The computer programs developed for EFGM are validated with the analytical solutions, experimental ii results and field investigation data available in the literature. Further a detailed parametric study is performed to demonstrate the effect of change in different parameters like seepage velocity, dispersivity, diffusion coefficient, distribution coefficient, aperture size of fracture, height of the clay liner. Reduction in contaminant transport is observed with increase of dispersivity, and retardation factor. Lowering seepage velocity results in the reduction of contaminant transport. Reduction in contaminant transport is observed with increase in the thickness of clay liner. Contaminant transport is reduced after the use of geomembrane with soil. The location of geomembrane in the soil media has an effect on the contaminant transport. Lesser the aperture size, velocity and dispersivity of the fracture, less is the contaminant transport through the fracture. It requires more time to attain a particular concentration level for smaller values of these parameters. Lower the saturation, lower is the contaminant transport and more is the time taken to reach the maximum normalized concentration. Case study of a landfill situated in China has been considered to verify the EFGM model for stratified strata.