The Expression of Dynamic Soil Water Repellency during Infiltration of Water and Reduced Surface Tension Fluids.

Many soils exhibit reduced wettabilities, fractional wettability, and long duration hydrophobicity. While these phenomena are generally understood to limit and impede infiltration at the small scale, their contribution is difficult to measure directly. Understanding this mechanistic contribution is important for a range of infiltration scenarios, and thusly, a wide scope of research has been generated in disciplines that include soil physics, hydrology, geomorphology, agriculture, and material science. In larger contexts of groundwater fluxes and overland flow generation, there is growing consensus that many soils exhibit complex wettabilities, with soil matrices being comprised of particle and regional fractions that exhibit a range of wettabilities that can change over time. This work seeks to develop mechanistic insight in systems with dynamic soil water repellency to better understand the nature of its influence in natural porous media and implications for longer/larger scale infiltration behaviours. Through a combination of (1D) laboratory infiltration experiments using tension infiltrometers (4.4cm and 8.5cm), materials collected from post-wildfire sites were studied through the combined lenses of fractional wettability and contact angle dynamics. Negative pressures were maintained throughout repellent layers to remove the influence internal ponding during infiltration. The experimental elimination of positive pore pressures facilitated the observation of infiltration driven by capillary pull and changes in wettability over time. Semi-quantitative infiltration relationships generated more insight in these complex systems than analytical approaches based on idealized wettable system principles. Early- and late- time infiltration behaviours of water provided insight into the nature of fractional wettability in mechanically layered materials and showed a systematic transition between primarily non-wettable and primarily wettable behaviours. The application of water, ethanol, and an aqueous ethanol solution showed varying accelerated changes in the wettability of homogenized materials based on the interaction of fluid properties, changing wettabilities, and applied pressures. There is great opportunity to develop new approaches for developing mechanistic insight into these systems, particularly over times scales of infiltration and groundwater recharge.