The influence of surfactant-application method on the effectiveness of water-repellent soil remediation

Soil water repellency (SWR) has a substantial effect on soil water hydrology: it hinders infiltration, leading to enhanced surface runoff and soil erosion, and causes preferential flow in the soil profile beyond that from the soil's natural heterogeneity. SWR is associated with soil organic matter content, the latter added to the soil by vegetation exudates, litter and residues, forest fires, and replacement of fresh water by treated wastewater for irrigation. Surfactants are surface-active substances composed of organic molecules with hydrophobic tails and hydrophilic heads that can reduce the surface tension (gamma) of the aqueous solution, thereby reducing SWR, via adsorption to soil particles. Surfactants are commonly used to remediate water-repellent soils. We investigated the role of two surfactant-application methods on the efficacy of SWR remediation. Aqueous solutions of two commercial surfactants had a substantial effect on parameters used to characterize the persistence and severity of SWR. However, the efficacy of these surfactants in remediating sandy soils rendered water-repellent by irrigation with treated effluent was substantially affected by their application method. Whereas application of aqueous surfactant solution to the surface of water-repellent soil, the commonly used remediation method, formed finger-like plumes similar to those obtained for water application, bulbous-like plumes were formed when the soil was premixed with the aqueous surfactant solution prior to water application. These differences were attributed to the significant role of the rate-limited surfactant adsorption to the soil particles.