Assessment of Physical Properties of Water Repellent Soils

Abstract: This note presents a comprehensive characterization of physical and mechanical properties of water repellent (hydrophobic) soil collected from Cleveland National Forest in California immediately after the Holy Fire, 2018, and delineates comparisons with chemically induced hydrophobic sand in the laboratory. Hydrophobicity is a particle surface characteristic that governs different levels of attraction between water molecules and solid particles. Wildfires can cause different levels of hydrophobicity in shallow soil layers based on fire severity, vegetation, and chemical structure of the soil. Natural and chemically induced regular and hydrophobic sands are characterized by grain size distribution, water retention curve, water contact angle and electron microscopic imaging, including the relationship between water entry value and the drop contact angle in hydrophobic soil. Comparative knowledge of natural and chemically induced hydrophobic soil properties will help future research to better predict soil behavior and improve insights into post-wildfire soil erosion and mudflow mechanisms. This note contributes to a database of wildfire-induced hydrophobic soil with detailed properties and assesses the applicability of laboratory made hydrophobic soils for studying mudflows by comparison to the natural water repellent soil collected from the burned site.