The effect of ionic strength on surfactant-induced unsaturated flow

The effect of ionic strength of soil water on surfactant-induced unsaturated flow is investigated for the anionic surfactant DOWFAX 8390. Instrumented infiltration experiments were performed to study the impact of surfactant and ionic strength of the soil-water solution on unsaturated flow. A solution having a measured surface tension of 68 mN/m was prepared with 0.014% w/w DOWFAX using distilled water and applied to the soil column instrumented with time domain reflectometry probes and pressure transducer equipped tensiometers. Results of the experiment were compared with those of pure water to investigate surfactant-induced flow and the corresponding influence of the ionic strength of soil water. The DOWFAX solution behaved like a solution having a surface tension of 49 mN/m, which was directly attributed to the interaction between the surfactant and soil water. Soil-moisture characteristic curves for soil wetted with water and DOWFAX were also measured, and a scaling relationship as a function of reduced surface tension is proposed.