Diffusion in Saturated Soil. II: Results for Compacted Clay

The effective diffusion coefficients, $D^{*}\text{,}$ of three anions (${\mathrm{Br}}^{-}\text{,}$ ${\mathrm{Cl}}^{-}\text{,}$ and $I^{-})$ three cations ${\mathrm{Cd}}^{2+}\text{,}$ $K^{+}\text{,}$ and ${\mathrm{Zn}}^{2+}$ 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 $4\times {10}^{-10}{m}^2/s$ and $2\times {10}^{-9}{m}^2/s$ 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 ${\mathrm{Cl}}^{-}$ and ${\mathrm{Br}}^{-}$ 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 (${\mathrm{Cd}}^{2+}$ and ${\mathrm{Zn}}^{2+})\text{.}$ Also, $D^{*}$ values determined from reservoir concentrations typically are higher than $D^{*}$ values determined from soil concentration profiles.