DIFFUSION IN SATURATED SOIL .2. RESULTS FOR COMPACTED CLAY

The effective diffusion coefficients, D*, of three anions (Br-, Cl-, and I-) and three cations (Cd2+, K+, and Zn2+) diffusing in two compacted clay soils, kaolinite and Lufkin clay, are measured. The ions are contained in a simulated waste leachate. The effects of molding water content and method of compaction on the measured D* values are evaluated for kaolinite. The calculated D* values varied between 4 x 10(-10) m2/s and 2 x 10(-9) m2/s and, based on the results for chloride diffusion in kaolinite, are relatively insensitive to molding water content and compaction method. The measured D* values for Cl- and Br- in kaolinite are in excellent agreement with previous studies, but the D* values for the cations are relatively high. High D* values for the cations are attributed to nonlinear adsorption behavior at relatively high concentrations and to the possibility of chemical precipitation of the heavy metal species (Cd2+ and Zn2+). Also, D* values determined from reservior concentrations typically are higher than D* values determined from soil concentration profiles.