STUDY ON REACTIVE SOLUTE TRANSPORT THROUGH POROUS MEDIUM

Abstract

Since last few decades, due to anthropogenic activities, ground water contamination has been one of the major topics of concern. Toxic chemicals, viruses, bacteria and other pathogens present in industrial effluents, sewage, sludge, septic tank are often transported into subsurface via infiltration or recharge. The quality of subsurface water can be affected by natural processes, nonpoint agricultural and urban runoff, waste-disposal practices and industrial discharges etc. The most challenging task for groundwater hydrologist is to make accurate prediction of arrival times and spatial patterns of toxic levels of a waste substance below the ground. The difficulty in prediction increases with the heterogeneity, and chemical properties of solute and porous medium. There are a number of studies that use mathematical modeling and experimental techniques to study and understand the behaviour of contaminants in a heterogeneous porous medium. It has been widely accepted in the literature that the non-equilibrium conditions significantly affect the solute transport at the field scale. Therefore, the present study focuses on the development of a generalized model, which can incorporate physical and sorption related non-equilibrium in heterogeneous porous media with constant and distance dependent dispersion coefficient. Physical nonequilibrium (PNE) is accounted by a diffusive mass transfer between the advective and nonadvective partitioning within porous medium. Sorption nonequilibrium (SNE) is accounted by using a two-site conceptualization for both advective and nonadvective regions in porous media, where at the first site, the sorption is assumed to be governed by an instantaneous equilibrium adsorption isotherm and at the second site; the sorption is described by a first order rate-limited process. In this study, semi-analytical solution of multiprocess non-equilibrium (MPNE) transport model with linear and exponential distance dependent dispersion is developed. Semi-analytical solution was developed in Laplace domain which was then inverted numerically to obtain time domain concentration. Semi-analytical solution was developed for flexible boundary conditions such as pulse, continuous and instantaneous type input. To describe the features of MPNE transport model, results of breakthrough curves and temporal moments were compared between constant and distance dependent dispersion models. ii From the analysis of zeroth, first and second temporal moments, it was observed that at small travel distance; constant dispersion, linear and exponential distance dependent dispersion models behaves identically. As the travel distance increases, lesser spreading and therefore, higher peak concentration is observed in case of linear distance dependent dispersivity model. For the same value of maximum dispersion coefficient, the constant dispersion model BTC exhibits higher spreading. An experimental investigation on large heterogeneous soil column is performed for which a 15m long heterogeneous soil column was constructed in the lab using different type of materials. Cl- (Chloride) and Fl- (Fluoride) were used in experiments which represent nonreactive and reactive solutes respectively. The developed model is then used to simulate the laboratory experimental data of chloride and fluoride, through heterogeneous soil column. It was observed that a better fit to the observed BTC is observed when mass transfer between advective and non-advection region is considered. It was also observed that both linear and exponential distance dependent dispersion models give a good fit to the observed BTC; however, exponential distance dependent dispersion model gives a much better fit. Further the, experimental investigation of solute transport through stratified porous medium is studied. Several tests were performed on two different types of stratifications prepared using the combination of materials of higher and lower hydraulic conductivity. Batch sorption study is performed to estimate the equilibrium sorption coefficients for different types of material. The experiments on stratified porous medium model were performed and developed semianalytical solution is used to analyse the observed data. Analysis of the observed data suggested that physical nonequilibrium significantly affects the breakthrough curves of both nonreactive and reactive solutes. The mass transfer from advective to nonadvective region influence the behaviour of distribution of BTC’s obtained at various distances. It is found that the mass transfer coefficient is varying with velocity and distances in the flow direction. A poor representation of observed data was simulated using constant mass transfer coefficient. An empirical relationship is developed for obtaining variable mass transfer coefficient with velocity and distance. Significant enhancement in simulation of observed data of nonreactive and reactive solutes were observed using variable mass transfer coefficient. iii The applicability of developed semianalytical solution of MPNE with linear and exponential scale dependent dispersion is explored. Inclusion of the distance and velocity dependent mass transfer coefficient improves the applicability of developed semianalytical solutions where physical nonequilibrium is dominating the solute transport. The distance dependent nature of mass transfer coefficient appears to be interesting and can lead to enhance the capabilities of MIM and MPNE model at field scale.