Pore-scale observation of deposit within the gravel matrix of a vertical flow constructed wetland

Lifespan and well-operation of French vertical flow constructed wetlands (VFCW) depend on how the organic deposit forms and evolves within filter media. This study aimed to demonstrate the feasibility of thin section methods application to VFCW. Unfortunately, constructed wetland scientists are currently missing tools to observe how deposit physically occupies pore space. Thin sections allow a direct and undisturbed observation of filter media and deposit. Undisturbed samples were taken from the surface of an experimental VFCW. Water was exchanged with a solvent before resin impregnation to preserve the sample structure. Several thin sections were successfully produced. Results highlight that deposit significantly reduces pore space. It forms a structured media crossed by large channels which can participate to fast gravity-driven flow and media oxygenation. The deposit structure seemed also made of a large bundle of small pores less than 100 mu m in radius. They can effectively store water by capillarity and provide a large surface for potential pollutant adsorption. An image analysis of thin sections provided hints at understanding the structuration of the porous media linked to organic matter deposition.