FACTORS INFLUENCING ORGANIC CONTAMINANT DIFFUSIVITIES IN SOIL BENTONITE CUTOFF BARRIERS

Quasi-steady-state and transient diffusion experiments indicate that the effective diffusivities of low molecular weight organic contaminants in saturated soil-bentonite media are but several factors less than corresponding free aqueous diffusivities. Evidence suggests that bentonite forms a fractured gel within the pore spaces of the soil matrix, thus contributing little to the diffusive resistance of a barrier. Reduction of the "free" volume for diffusion due to the presence of the solid phase and tortuosity of diffusion paths through the pore system account for observed reductions in contaminant diffusivities. The relationship between effective and "free" aqueous diffusivities can in fact be described satisfactorily by a simple power function of porosity.