Water–soil interactions: Unravelling the processes and stages involved in the wetting of water repellent soils

The water repellent behaviour of soils is a widely studied phenomenon given its implications for infiltration, runoff, erosion and preferential flow. However, the principles underlying the eventual penetration of water into affected soils remain poorly understood. Theoretical considerations of the energetics and kinetics involved as a water drop makes contact with a water repellent soil surface and eventually penetrates into the soil suggest three distinct stages in the overall process. These stages are 1) adhesional wetting as soil and water first make contact, followed by 2) a kinetic barrier transitional stage in which molecular reorganisation of organics on soil reduces the water-soil contact angle to allow the water drop to sit deeper over soil particles of initial contact such that there is contact with particles in directly underlying soil layers, and finally 3) branching interstitial wetting as water penetrates into the bulk soil. Studies presented here of optical microscopy, mass of soil initially wetted, penetration time through layers of soil of different thicknesses, and time-dependent measurements of contact angle, volume of water penetrated, and mass of soil wetted, all give results consistent with this model. However, only for highly water repellent soils can distinct stages in wetting be clearly resolved experimentally, presumably because only these soils have a high enough kinetic barrier in the transitional stage for good separation between stages. For less water repellent soils, while the general time dependent behaviour remains consistent with the model, the distinction between the three stages is not so easy to resolve experimentally. The roles of contact angle, particle size distribution and drop size in determining the rates of these stages is considered, and the implications of the model for understanding soil water repellency are discussed.