Contaminant transport through an unsaturated soil liner beneath a landfill

Abstract

Contaminant transport through soil is often modelled mathematically by means of the dispersion-advection equation. When investigating contaminant migration beneath a landfill, a common assumption is that the soil is saturated; however, it is well known that the soil beneath many landfills is only partially saturated. In this paper, steady-state unsaturated moisture distributions are employed in the dispersion-advection equation to find transient contaminant distributions beneath a landfill. A finite layer formulation is used to simplify the contaminant mass transport equation and account for heterogeneous soil profiles. Various assumptions concerning the flow regime beneath the landfill and the functional relation between volumetric water content and the diffusion coefficient in the transport equation are made to highlight differences between contaminant transport through saturated and unsaturated soils. It is found that diffusive contaminant mass transfer through a partially saturated soil liner is comparatively insensitive to variation of the volumetric moisture content at the top of the soil liner alone. However, accounting for the dependence of the diffusion coefficient on the volumetric moisture content of the soil does have a significant effect on diffusive mass transfer through a partially saturated soil liner. Though it is difficult to measure the dependence of the diffusion coefficient on the volumetric moisture content of the soil experimentally, this information appears necessary for a rational analysis of contaminant transport through partially saturated soils.Résumé : Le transport de contaminants à travers le sol est souvent modélisé mathématiquement au moyen de l'équation de dispersion-advection. Au cours de l'étude de la migration de contaminants sous un enfouissement sanitaire, l'on pose habituellement l'hypothèse que le sol est saturé; cependant, il est bien connu que le sol sous plusieurs enfouissements sanitaires n'est que partiellement saturé. Dans cet article, des distributions non saturées de teneur en eau à l'état permanent sont utilisées dans l'équation de dispersion-advection pour déterminer les distributions transitoires de contaminants sous l'enfouissement sanitaire. Une formulation de couche finie est utilisée pour simplifier l'équation de transport de masse du contaminant et prendre en compte les profils de sol hétérogènes. L'on fait diverses hypothèses concernant le régime d'écoulement sous l'enfouissement et la relation fonctionnelle entre la teneur en eau volumétrique et le coefficient de diffusion dans l'équation de transport pour mettre en lumière les différences entre le transport de contaminant à travers les sols saturés et non saturés. L'on trouve que le transfert de masse des contaminants par diffusion à travers une membrane de sol partiellement saturé est comparativement insensible à la variation de la teneur en eau volumétrique au sommet de la membrane de sol seule. Cependant, le fait de prendre en compte la dépendance du coefficient de diffusion sur la teneur en eau volumé...

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