Multiphysics Modeling of Pollutant Uptake by Mangroves

The lack of understanding the space-time dynamics of water and matter transport in the soil-plant continuum of estuarine ecosystems remains an impediment to accurate prediction to support the establishment of appropriate strategies for pollution control and environmental protection. In this paper, a three dimensional model of water and substance flow in the soil plant system is set up based on cohesion-tension theory. Water transport in soil and tree is conceived as a continuous hydraulic process, which is driven by canopy transpiration. State variables of the model are water potential and contaminant concentrations in the soil, roots, xylem, core and canopy. The model equations are obtained by application of Richards equations with van Genuchten-Mualem approaches for hydraulic conductivity and water retention curves. The water transport equations are coupled to the contaminant transport equations via the Darcy velocity and the dispersion tensor. Exchanges between compartments are mediated by a diffusion model on the boundary for transport across membranes. Water evaporation from leaf mesophyll cells is taken into account by a transpiration sub model, which is driven by environmental variables such as air water potential, wind speed, radiation and temperature. The governing equations consist of a system of coupled nonlinear partial differential equations with reaction terms, which were implemented into the finite element tool COMSOL MULTIPHYSICS based on the Petrov-Galerkin scheme. First results show that the model is capable of reproducing typical spatial concentration patterns of metals in young mangrove plants.