In recent decades, field and laboratory studies have demonstrated that water flow in the unsaturated zone often occurs through a small fraction of the soil along preferential flow paths. Preferential flows strongly impact mass transfer in soil. Preferential flow can reduce the access of pollutants to the soil sorption sites, thus reducing the capability of soil to filtrate water and protect the quality of groundwater body. Conversely, preferential flows may reduce the contact of soil particle with fresh water, thus reducing the potential for the desorption of pollutants previously sorbed onto the solid, and permitting a long term storage of sorbed pollutants. Significant effects are also expected for geochemical processes that govern soil evolution, including mineral precipitation and dissolution, and weathering processes. The impacts of preferential and non-equilibrium flows on the contact between soil matrices and infiltrating water and related consequences on geochemical processes, soil evolution (precipitation, soil erosion, etc.) and mass transfer including pollutant transfer still need to be investigated. There is an urgent need for scientific advances in developing new measurement techniques and theoretical approaches on this topic. The proposed session will welcome studies on the following topics, but not limited to: • Tracking preferential flows and mass transfers in soils using high-tech tracer techniques including MRI, tomography CAT, etc. • Linking preferential flow pattern with soil heterogeneity including soil structure (pore size distribution, pore connectivity, type of macroporosity) and lithological heterogeneity (heterogeneous soils made of lithofacies with contrasting transfer properties) • Linking preferential flow pattern with soil geochemical properties (e.g. organic matter and hydrophobicity) • Coupling physical processes of preferential flows and geochemical processes for understanding solute sorption and solute desorption • Coupling physical processes of preferential flow and geochemical processes for understanding mineral precipitation and dissolution (weathering processes) and impacts on soil evolution (soil erosion, crusting, etc.).