Solute Breakthrough Curves for Processed Kaolin at Low Flow Rates

Solute breakthrough curves for a 0.01-M NaCl solution permeated through two compacted test specimens of processed kaolin soil are measured at two volumetric flow rates (2.65 × 10 ⁻⁴ cm ³ /s and 2.65 × 10 ⁻⁵ cm ³ /s) using a flow pump system. Regression analyses of the effluent solute concentrations with two analytical models resulted in hydrodynamic dispersion coefficients D ranging from 1.49 × 10 ⁻⁶ cm ² /s to 3.95 × 10 ⁻⁶ cm ² /s for chloride and from 2.11 × 10 ⁻⁶ cm ² /s to 8.74 × 10 ⁻⁶ cm ² /s for sodium indicating that diffusion dominated the transport process in the two column tests. The effluent electrical conductance values measured immediately after sampling also tend to reflect the dominance of diffusion on the solute migration process. An observed decrease in effluent pH from between 4.8 and 5.5 during permeation with distilled water to about 4.5 during subsequent permeation with the NaCl solution is consistent with Na ⁺ for H ⁺ exchange at relatively low pH previously reported for studies involving kaolinite soils. The results of this laboratory study tend to confirm previous field studies that indicate diffusion-dominated transport at the low flow rates common in fine-grained barrier materials.