Applicability of Ethanol for Measuring Intrinsic Hydraulic Properties of Sand with Various Water Repellency Levels

Many soils worldwide show water repellency to some degree. Soil water repellency (SWR) is known to alter hydraulic processes. Particularly in water-repellent soils a decreased water infiltration rate can be observed. In this case soil hydraulic properties, like the hydraulic conductivity, not only are a result of the soil's pore system but also depend on the physicochemical properties of the pore surfaces (water repellency). Ethanol as a completely wetting liquid is not influenced by the soil's water repellency. In this study we introduce the concept of intrinsic soil hydraulic properties, that is, the hydraulic properties that are only dependent on the porous system and independent of its surface properties. We used the concept of intrinsic permeability, originally developed for saturated conditions. The effect of different liquid surface tensions of water and ethanol, important under unsaturated conditions, was incorporated using a correction factor for the matric potential. Retention and saturated and unsaturated liquid conductivity of water and ethanol were systematically measured in sand and glass-bead porous media with different wettabilites. Results showed no difference between the intrinsic hydraulic properties (measured with ethanol) and the hydraulic properties (measured with water) in fully wettable porous media. In water-repellent porous media, intrinsic hydraulic properties deviate from measured hydraulic properties. Measurements further showed that the influence of soil water repellency on hydraulic conductivity and retention of water can be predicted as a function of the macroscopic contact angle (CA) in these model substrates. In summary, at least for well-defined substrates such as sands, we suggest measuring the hydraulic conductivity and intrinsic liquid retention of the pore system with ethanol as a standard procedure.