MODELING CONTAMINANT TRANSPORT THROUGH POROUS MEDIA USING ASYMPTOTIC DISPERSIVITY

Abstract

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. Most of the pathogenic bacteria and virus in ground water originate from human and animal sewage from municipal wastewater discharges, septic tanks, sanitary landfills and agricultural processes. The wastewater infiltrates through the vadose zone and, upon reaching the water table, continues to travel for large distances through the subsurface environment. When this water is drawn by wells and consumed without any treatment, it may be hazardous to human health. Hence, it is necessary to study the transport mechanism of reactive chemicals through porous media. Most of the early studies on contaminant transport through porous media considered the contaminant to be either non-reactive or to have instantaneous reaction with the porous matrix. In such cases, the transport process could be described by advection (including retardation for reactive contaminants), diffusion and dispersion. Advection is governed by the movement of contaminants along with the flowing groundwater at the seepage velocity in porous media. Diffusion is a molecular mass transport process in which contaminants move from area of higher concentration to the area of lower concentration. Dispersion is governed by spatial variability of groundwater velocity in porous media caused by the heterogeneity of hydraulic properties of the porous media. Transport of non-reactive solute through homogeneous and heterogeneous porous media has been investigated experimentally, theoretically, and numerically by a number of researchers. A common approach to study the transport behaviour subjected to the seemingly irregular variation of hydraulic properties in porous media is that based on the stochastic theory. The resulting equations are, however, quite complicated and difficult to solve analytically except for a few simple cases. ii 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. Physical non-equilibrium (PNE) is accounted by a diffusive mass transfer between the advective and non-advective partitioning within porous medium. Sorption non-equilibrium (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 asymptotic 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 constant concentration type input. To describe the features of MPNE transport model, results of breakthrough curves were simulated using constant and asymptotic distance-dependent dispersion models. An experimental investigation on large heterogeneous soil column is performed for which a 1500 cm long heterogeneous soil column was constructed in the lab using different types of materials. Chloride and Fluoride were used in the experiments which represent conservative and nonconservative 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 was observed when mass transfer between advective and nonadvection region is considered. It was also observed that asymptotic distance-dependent dispersion model gives a good fit to the observed breakthrough curve as compared to constant dispersion model. It is found that physical non-equilibrium significantly affects the breakthrough curves of both non-reactive and reactive solutes through porous media. The mass transfer from advective to non-advective region influence the behaviour of distribution of BTC’s obtained at various distances. Further the, experimental investigation of solute transport through long soil column experiments is studied. Batch sorption study is performed to estimate the linear and nonlinear iii sorption coefficients for different types of soil materials. Linear and nonlinear sorption models were used to simulate experimental breakthrough curves through porous media. Analysis and simultion of the observed data of fluoride suggested that nonlinear sorption i.e., Freundlich sorption model simulate better as compared to linear sorption model. It is also shown that the MIMA model gives the best fit curve of experimental breakthrough curves in long heterogeneous soil column experiment as compared to both MIMC and MIML models. Estimated value of dispersivity is smaller in case of MIMA model as compared to both MIMC and MIML models. Thus MIMA model is efficient to capture the evolution distance-dependent dispersion behavior. Accurate prediction of mass transfer coefficient is also essential and significant for transport of contaminant through porous media. Hence, asymptotic dispersivity including variable mass transfer coefficient can be useful for describing solute transport in long heterogeneous porous media Finally, the behavior of concenration profiles and spatial moments for reactive transport through triple-permeability porous medium was studied. For this, numerical model has been developed for transport equations using both FDM and FVM methods. A detailed analysis of triple-permeability transport model has been carried out to study its performance for advection and dispersion dominant cases. For an advection dominant flow, FDM model produces oscillation in presence of small and higher values of of mass transfer coefficients. Hence, FVM can be used for both the simulation of solute transport through porous media for any value of Peclet number. The results of mean travel distance and spreading behavior of solutes remain the same in the presence of higher values of mass transfer coefficients.