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SOIL PHYSICS

Upward Infiltration into Porous Media as Affected by Wettability and Anionic Surfactants

^a Graduate School of Environmental Science, Okayama Univ., 3-1-1 Tsushima-naka, Okayama 700-8530, Japan
^b Toray Industries, Inc., Chuo-ku, Tokyo 103-8666, Japan

^* Corresponding author (ishi{at}cc.okayama-u.ac.jp).

The influence of surfactants on water infiltration in soil is not fully understood. The objective of this study was to propose a model for evaluating effects of an anionic surfactant on upward infiltration under saturated conditions in porous materials with highly contrasting wettability. The simplified equation for upward infiltration based on Darcy's law is equivalent to the widely used Washburn equation. We experimentally determined upward infiltration of sodium dodecyl sulfate (SDS) solution (0–700 mol m^–3) into 60-cm-long, 2-cm-diameter columns filled with air-dry materials (glass beads, sand, leaf mold, peat moss, or polyethylene particles). In hydrophilic glass beads and sand, the infiltration rate decreased as the SDS concentration increased due to a decrease in solution surface tension (from 72 to 38 mN m^–1). The proposed model could describe the infiltration in all materials and at all concentrations when fitting to the initial parts of the curve of infiltration front vs. time. Contact angles were obtained by fitting the model to the measured height of the infiltration front in the saturated range as a function of time. In columns filled with hydrophobic materials, the infiltration rate increased with SDS concentration, corresponding with a decrease in contact angles from >125 to 69° for polyethylene particles and from 102 to 43° for peat moss. In leaf mold, the infiltration rate decreased as the SDS concentration increased, probably due to swelling. The proposed equation was found useful for calculating saturated hydraulic conductivity and contact angles but limited in the case of swelling porous material.

Abbreviations: SDS, sodium dodecyl sulfate