Simulation tool for variably saturated flow with comprehensive geochemical reactions in two- and three-dimensional domains

1 We present a software tool for simulations of flow and multi‐component solute transport in 2 two and three‐dimensional domains in combination with comprehensive intra‐phase and 3 inter‐phase geochemistry. The software uses IPhreeqc as a reaction engine to the multi‐ 4 purpose, multidimensional finite element solver COMSOL Multiphysics® for flow and 5 transport simulations. Here we used COMSOL to solve Richards' equation for aqueous phase 6 flow in variably saturated porous media. The coupling procedure presented is in principle 7 applicable to any simulation of aqueous phase flow and solute transport in COMSOL. The 8 coupling with IPhreeqc gives major advantages over COMSOL's built‐in reaction capabilities, 9 i.e., the soil solution is speciated from its element composition according to thermodynamic 10 mass action equations with ion activity corrections. State‐of‐the‐art adsorption models such 11 as surface complexation with diffuse double layer calculations are accessible. In addition, 12 IPhreeqc provides a framework to integrate user‐defined kinetic reactions with possible 13 dependencies on solution speciation (i.e., pH, saturation indices, and ion activities), allowing 14 for modelling of microbially mediated reactions. Extensive compilations of geochemical 15 reactions and their parameterization are accessible through associated databases. Research highlights 20  Coupling of COMSOL and PHREEQC facilitates simulation of variably saturated flow 21 with comprehensive geochemical reactions. 22  The use of finite elements allows for the simulation of flow and solute transport in 23 complex 2 and 3D domains. 24  Geochemical reactions are coupled via sequential non‐iterative operator splitting. 25  The software tool provides novel capabilities for investigations of contaminant 26 behaviour in variably saturated porous media and agricultural management. 27 3 Software requirements 28 COMSOL Multiphysics® including Earth Science Module (tested version: 3.5a; due to a 29 memory leak in versions 4.0 and 4.0a, these are not suitable for the presented coupling) 30 Price for single user academic license including Earth Science Module ca. 2000 € 31 Matlab® (tested versions: 7.9, 7.10) 32 Price for single user academic license including Parallel Computing Toolbox ca. 650 € 33 IPhreeqc (COM‐version, available free of charge at 34 The coupling files together with animations of the presented simulations are available at 36