Contaminant transport through an unsaturated soil liner beneath a landfill

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.