Incorporating Electrostatic Coupling Effects into Multispecies Solute Transport Simulations with <scp>MODFLOW</scp>

Solute transport simulators aiming to accurately describe the transport of charged chemical species in porous media need to account for electrostatic coupling effects. Each ion in pore water possesses a specific electric charge and molecular diffusion coefficient, properties that determine their mobility and the overall charge balance of aqueous solutions. Depending on the charge, concentration and aqueous diffusion coefficient, the displacement of an ion in solution influences, and is in turn influenced by, other ions in solution by means of electrostatic interactions. This phenomenon has been studied with experiments and numerical simulations in diffusion-dominated regimes, as well as in advection-dominated flow-through systems, showing that electrostatic coupling effects play a relevant role in the spatiotemporal prediction of ion concentrations. However, there is limited availability of solute transport codes incorporating electrostatic coupling, limiting applications of multispecies ionic transport at different scales. This article elaborates on the topic of electrostatic coupling and presents a methodology for incorporating the effect into multispecies solute transport simulations with MODFLOW. The integration is achieved through the Application Programming Interface of the program (MODFLOW-API). This interface enables the access to concentrations and dispersion coefficients of all species during the simulation, which are necessary to calculate a dispersive correction that effectively incorporates electrostatic coupling into the model. Numerical results demonstrate the effectiveness of the coupling strategy, benchmarking the implementation with previously validated numerical simulators and with experimental data.