Dual‐porosity groundwater contaminant transport in the presence of colloids

Abstract

Abstract. Prior studies have shown that colloids can facilitate contaminant migration in unimodal porous media. To investigate the effect of no-flow regions on flow and contaminant transport in dual-porosity soils, we model a porous medium composed of two different homogeneous, superposed, and interacting regions: the mobile region and the immobile region. We assume that the advective-dispersive processes govern the transport of contaminant and colloids in the mobile region, while the diffusion process dominates in the immobile region. The contaminant and colloid mass transfer mechanisms between these two regions are represented by a first-order mass transfer. Colloid deposition on the solid matrix is expressed by a kinetic sorption relationship. The contaminant sorption with the solid matrix and colloidal surfaces is also incorporated into the model. Coupled with mass transfer terms, two sets of governing equations representing the fate and transport of contaminant and colloids in both the mobile and immobile regions are developed and applied to experimental data available in the literature. Numerical solutions are obtained by employing a fully implicit, finite difference scheme. The numerical results indicate that the colloidal facilitation is increased in a dual-porosity porous medium compared to a unimodal medium. The model is validated by comparing the numerical results with the experimental data available in the literature for colloid-facilitated contaminant transport in a single-porosity medium and contaminant transport in a colloid-free, dual-porosity medium. A sensitivity analysis is conducted to deduce the effect of major model parameters on contaminant transport. The analysis demonstrates that, although both the volumetric fraction of the mobile region and the mass transfer rate coefficients between the two regions have effects on the dual-porosity transport, the early breakthrough is affected mainly by the volumetric fraction of the mobile region, while the tailing is affected largely by the mass transfer rate coefficients.

A Simple Analytical Model for Interpretation of Tracer Tests in Two-Domain Subsurface Flow Systems

“…The effects of matrix diffusion on the shapes of BTCs in porous media have been widely described (Maloszewski and Zuber 1993;Carrera et al 1998;Ostensen 1998;Corapcioglu and Wang 1999). They are perhaps most readily appreciated by comparing BTCs from systems without matrix diffusion with those from systems with marked matrix diffusion ( Figure 1).…”

“…Where the latter has pore necks smaller than about 10µm, the pores will not drain under gravity and solutes can only be exchanged with the adjoining rapid flow domain by molecular diffusion (Foster 1975). Natural twodomain subsurface flow systems exhibiting such behaviour have been widely described in the hydrogeological literature (e.g., Foster 1975;Lever et al 1983;Maloszewski and Zuber 1993;Corapcioglu and Wang 1999), in which the solute exchange phenomenon is usually termed matrix diffusion. Because of its importance in pollutant migration in vulnerable aquifers, matrix diffusion has been the subject of numerous mathematical modelling studies over the last 25 years.…”

“…Because of its importance in pollutant migration in vulnerable aquifers, matrix diffusion has been the subject of numerous mathematical modelling studies over the last 25 years. While some analytical solutions have been proposed for two-domain systems with highly specific boundary conditions (Grisak and Pickens 1981;Carrera et al 1998;Ostensen 1998), most workers have opted to use complicated and/or computationally expensive (but highly detailed) numerical solutions of the governing equations (Lever et al 1985;Maloszewski and Zuber 1993;Zuber and Motyka 1994;Carrera et al 1998;Corapcioglu and Wang 1999;Martinez and Wise 2003). As the number of analogous man-made twodomain water treatment systems continues to burgeon, especially in the field of passive mine water treatment (Hedin et al 1994;Kepler and McCleary 1994), it is tempting to apply the same numerical models to these engineered reactors.…”

Colloid-Associated Groundwater Contaminant Transport in Homogeneous Saturated Porous Media: Mathematical and Numerical Modeling

“…With the assumption of (n − σ c ) = θ = const and regarding linear adsorption isotherm, substitution Q cm a , Q cc a , and Q d a from Eqs. (5), (6), and (8) into (7) yields an equation that can be simplified by substituting (9), (16), (19), (22) and (25). Where D * = D B + D MD , the use of constant aqueous phase velocity and rearranging yields:…”

“…Dual-porosity approach for groundwater-contaminant transport in the presence of colloids has been presented by Corapcioglu and Wang [16]. They used fully implicit and Crank-Nicholson schemes to solve the one-dimensional colloid-facilitated contaminant transport.…”