Laboratory diffusion testing for waste disposal - A review

This paper reviews the state-of-the-art for the measurement in the laboratory of diffusion coefficients of chemical waste constituents in fine-grained soils. The purpose of the review is to present the experimental and analytical methods for determining liquid-phase diffusion coefficients which can be used in practice for the design and evaluation of waste containment barriers. After the appropriate equations describing mass transport in soil are presented, the practical significance of diffusion coefficients in soil (known as 'effective diffusion coefficients') are described. Appropriate analytical solutions required to calculate the effective diffusion coefficient (D) from the measured laboratory data also are presented for several different initial and boundary conditions. The advantages and disadvantages of each method are noted. A summary of effective diffusion coefficients from the literature suggests that the major physical factor affecting the value of the measured diffusion coefficient is the degree of saturation of the soil, with D-values for nonreactive and reactive solutes in saturated soils being as much as 10-20 times higher than the corresponding values in unsaturated soils. Most of the other physical factors only become important in soils which are highly unsaturated. In addition, the diffusive transport rates of reactive solutes subject to reversible sorption reactions can be as much as 5000 times lower than those of nonreactive solutes in saturated soils and from 20 to 630,000 times lower in unsaturated soils.