Mass Transport in Soil Materials Geomembrane Subgrade

3 Mass Transport in Soil Materials (Geomembrane Subgrade) 

In a pore space only partly saturated with soil water, mass transport in the gaseous phase must additionally be considered, i.e. through the pore space filled with soil air. Temperature differences result in the movement of water and water vapour and thus also in pollutant transport. These effects are not considered here. In the following, only the simplest mathematical model is discussed for the description of the mass transport (van Genuchten and Wagenet 1989) a homogeneous, water-saturated soil material of constant temperature is assumed. All initial values, concentration, hydraulic gradient etc. should only depend on one coordinate x and the coordinate axis is assumed to be perpendicular to the plane of the liner, see 

For further calculation, confining assumptions and simplifications must be made about the relevant aspects of physics of the mass transport processes for the respective pollutant in the respective soil material and a mathematical description, derived from it, can then only be applied as an approximation. In the simplest case the following assumptions are made Darcy s law is assumed for the advective mass transport, i.e.  

For the sorption process linearity and reversibility are assumed, i.e. the concentration of the pollutant adsorbed in the soil material is at every instant proportional to its concentration in the soil water  

Finally, the diffusion coefficient De of the pollutant in soil water of the soil material, or the so-ealled effeetive diffusion eoeffieient, may be traced back to the diffusion coefficient Do of the pollutant in free water, whieh is usually known, with the help of the so-called tortuosity factor 7 (Bear 1988)  

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