Disentangling nitrogen turnover in Nature Based Solutions: hydrodynamic properties and reactive behavior

Nature Based Solutions (NBS) are known to play a key role in urban water management by increasing the infiltration, retention, and evapotranspiration capacity of urban areas. However, their potential use for contaminant removal has only been partially investigated. To address this issue, this study presents an experimental analysis of the nitrogen turnover in selected typical NBS substrates. Soil column experiments were combined with laboratory methods to characterize the hydrodynamic properties of porous media and elucidate the nitrification process in NBSs. In a first experimental campaign, saturated soil columns were injected with a natural tracer (deuterium) to characterize non-reactive solute transport in different substrates. Breakthrough curves exhibit significant tailing, thus suggesting the existence of a complex interplay between a mobile and an immobile domain. A second experimental campaign was carried out in larger unsaturated soil columns periodically injected with wastewater. Nitrogen species were measured in the effluent to describe the nitrogen turnover in soils. Results are characterized by two distinct phases, in which nitrate is initially not detectable in the outflow but later becomes the dominant species. This behavior indicates the existence of an initial microbial adaptation phase, followed by an efficient nitrification process supported by the oxic conditions in the substrate. Altogether, observations highlight the complex hydraulic and reactive behavior of NBSs substrates, which should be properly combined with modeling to better understand and design NBS systems for pollutant treatment.