MODFLOW and MODPATH for hydrodynamic simulation of porous media in horizontal subsurface flow constructed wetlands: A tool for design criteria

Abstract Constructed wetlands are important alternatives for decentralized wastewater treatment. In recent years, a large number of numerical models have been developed and refined in order to assist in the development and understanding of these facilities. This study presents a three-dimensional numerical modeling of flow in HSSF-CWs. The software MODFLOW and MODPATH, based on the finite difference method, were used to numerically simulate flow patterns in HSSF-CWs according to the project design and changes in porous medium hydrodynamics and flow distribution. The model proved to be a powerful tool for 3D simulation, allowing the representation of flow distribution, flow velocities, hydraulic head and particles trajectories. An interdependence of the hydrodynamic parameters and hydraulic characteristics in the porous medium was observed. Changes to hydraulic conductivity and flow distribution mechanisms can improve the flow patterns inside the porous medium, mainly because these contribute to homogenizing flow and eliminating low velocity zones, presenting the best results when coarse sand and two distribution pipes (located at depths of 0.1 and 0.25 m) were used. Proper design is important to improve the treatment conditions in these facilities and to prevent the occurrence of damage, such as clogging.