Effect of subcritical hydrophobicity in a sandy soil on water infiltration and mobile water content

Recent research shows that most soils are more or less water repellent. Already subcritical water repellency may cause incomplete soil wetting and preferential flow. Both processes potentially reduce the residence time of water and solutes in the vadose zone, resulting in an enhanced risk of groundwater contamination. The objective of the present paper is, therefore, to evaluate the impact of reduced soil wettability on the soil water infiltration rate and to investigate the tendency towards preferential flow with the analysis of the immobile water content in the infiltration zone. In november 2002, a field experiment was done in a coniferous forest, 30 km N of Hannover, Germany. Soil hydrophobicity was quantified by measuring the contact angles. The hydraulic conductivity of the podsolic sandy soil was measured depth-dependent with a double-ring tension infiltrometer in three soil horizons. To quantify possible preferential-flow effects, a LiBr-Tracer was added to the infiltrating water to evaluate the mobile water-content fraction after infiltration. Additionally, infiltration rates of water were compared with infiltration rates of ethanol which were determined after water infiltration at the same locations. Results show that the actual water repellency of field-moist soil was mainly subcritical (contact angle < 90 degrees). Water infiltration rates were reduced due to subcritical repellency by a factor of 3-170 compared with ethanol infiltration rates (exclusion of wetting effects). This spatially variable infiltration behavior was not clearly reflected neither by the small-scale contact-angle measurements nor by the analysis of the average immobile soil water content in the infiltration zone. We conclude that this specific infiltration behavior of water caused by small-scale wettability effects may temporarily reduce the local connectivity of water-flow pathways.