Which hydraulic model should be used to model flow in vertical flow constructed Wetlands?

Modelling water flow in a Vertical Flow Constructed Wetland (VFWC) is a prerequisite to model wastewater treatment using process based filtering models. The material used in VFCW is very susceptible to generate preferential flow. If this occurs, water will bypass most of the soil porous matrix in a largely unpredictable way. Even if it is possible to simulate water content variations within a VFCW, we can not correctly model outflow with the standard van Genuchten-Mualem function if preferential flow occurs. A number of various model approaches have been proposed to overcome this problem. These models mostly try to separately describe flow and transport in preferred flow paths and slow or stagnant pore regions. The objective of this study was to study and simulate the hydrodynamic behaviour and the solute transport through a simplified representation of a French VFCW by using both a classical equilibrium model and a non-equilibrium model (dual-porosity model: mobile-immobile water model, with water content mass transfer) included in the HYDRUS-1D software package. Modelling results were compared to a solute breakthrough curve obtained from a tracer experiment, carried out on an existing VFCW. The comparison between measured and simulated tracer breakthrough curves indicates that the non-equilibrium approach seem to be the most appropriate for simulating preferential flow paths