Analytical and Numerical Modeling Assessment of Capillary Barrier Performance Degradation due to Contaminant-Induced Surface Tension Reduction

Analytical and numerical models of capillary barrier performance commonly use hydraulic characteristics measured using pure water. However, the potential exists for an infiltrating solution to have a surface tension lower than that of pure water due to the presence of surface-active contaminants (surfactants). A lower surface tension solution may impact capillary barrier performance due to the dependence of capillarity on surface tension. An existing analytical solution for capillary diversion length (L) was modified to include the effect of surface tension reduction on steady-state capillary barrier performance during uniform and constant infiltration. The L for a surfactant-contaminated system was found to be less than for a pure water system and equal to L for a pure water system multiplied by the relative surface tension. Numerical modeling using HYDRUS-2D also showed that diversion was less in the surfactant-contaminated system and that the difference in the performance of the two systems was due to the fact that the fine layer in the capillary barrier retains less liquid when wetted with surfactant solution.