SOLUTE BREAKTHROUGH CURVES FOR PROCESSED KAOLIN AT LOW-FLOW RATES

Solute breakthrough curves for a 0.01-M NaCl solution permeated throug h two compacted test specimens of processed kaolin soil are measured a t two volumetric flow rates (2.65 x 10(-4) cm3/s and 2.65 x 10(-5) cm3 /s) using a flow pump system. Regression analyses of the effluent solu te concentrations with two analytical models resulted in hydrodynamic dispersion coefficients D ranging from 1.49 x 10(-6) cm2/s to 3.95 x 1 0(-6) cm2/s for chloride and from 2.11 x 10(-6) cm2/s to 8.74 x 10(-6) cm2/s for sodium indicating that diffusion dominated the transport pr ocess in the two column tests. The effluent electrical conductance val ues measured immediately after sampling also tend to reflect the domin ance of diffusion on the solute migration process. An observed decreas e in effluent pH from between 4.8 and 5.5 during permeation with disti lled water to about 4.5 during subsequent permeation with the NaCl sol ution is consistent with Na+ for H+ exchange at relatively low pH prev iously reported for studies involving kaolinite soils. The results of this laboratory study tend to confirm previous field studies that indi cate diffusion-dominated transport at the low flow rates common in fin e-grained barrier materials.