A novel method to assess the infiltration and filtration functions of urban soils

Urban areas are increasingly under pressure due to rapid expansion and extreme rainfall events driven by climate change. Rather than relying solely on pipes to regulate stormwater, new management systems promote direct infiltration of water and pollutants into the soil. Soils fulfill multiple functions, including water infiltration and pollutant filtration. Urban soils, however, are often highly heterogeneous; the presence of macropores can accelerate water flow while reducing pollutant retention. Yet, infiltration and filtration are typically assessed using surface measurements and soil‐destructive methods, which fail to capture the complexity of water and pollutant transport pathways in the subsurface. In this study, we present a prototype of a large‐ring infiltrometer coupled with a ground‐penetrating radar (GPR) to simultaneously monitor water and nanotracers movement in the subsurface. This provides a more comprehensive characterization of both infiltration and filtration functions. Superparamagnetic iron oxide nanoparticles (SPIONs) were chosen as nanotracers due to their nontoxicity, small size, and advantageous electromagnetic properties. Although distinguishing between subsurface zones imbibed with water and those containing SPIONs was challenging, GPR imaging revealed distinct spatial patterns and temporal dynamics even under identical infiltration conditions. Comparative analyses with water and solute transport models further elucidated the underlying subsurface processes. Typologies of infiltration and filtration patterns, linked to soil heterogeneity, are presented. Our results emphasize the need for a more comprehensive characterization of infiltration and filtration functions to improve flood risk assessment and evaluations of groundwater pollution threats.