Simulating electrochemical migration and anion exclusion in porous and fractured media using PFLOTRANNP

Multicomponent transport of dissolved charged species often involves electrostatic interactions among the different ions. These interactions are the result of the different diffusion rates of the chemical species, which create a diffusion potential and thus an electromigration term in the transport equation that is additive to the Fickian diffusion term due to the concentration gradient. The explicit consideration of the electromigration term involves the use of the Nernst-Planck equation, which tightly couples the transport of the charged species through the electro-diffusive term. Here, we have implemented the Nernst-Planck equation in the open source computer code PFLOTRAN. The advantages of the customised code, denoted as PFLOTRAN(NP), in comparison with other recent similar developments, are that (i) the implementation is extended also to the multiple continuum module of PFLOTRAN, which makes the code suitable for the modelling of electromigration processes in fractured rock, (ii) PFLOTRAN(NP) leverages the parallelisation capabilities of one of the latest stable PFLOTRAN versions and thus it is suited for large-size simulations in supercomputers and (iii) the developed code is made available through a public repository. PFLOTRAN(NP )was successfully validated against a standard benchmark involving 1D transport of a multicomponent electrolyte solution and a verification case, involving a fracture-matrix system, was also presented. Finally, PFLOTRAN(NP) was used to interpret field and experimental data at Olkiluoto (Finland), which show that an imbalance exists between chloride concentration in the matrix pore water and in the fracture filling groundwater. The results of the simulations show that this imbalance is indeed caused by anion exclusion processes, which are more significant in gneiss rock types than in pegmatitic granitic rock.