GEODENS is a FORTRAN code for modeling partly or fully saturated density dependent flow and multispecies reactive solute transport in porous media under both local chemical equilibrium and kinetic conditions. It can handle geochemical reactions such as mineral dissolution-precipitation processes. Its main purpose is to represent the physicochemical processes in the subsurface system. The code considers flow, transport and geochemical reactions in porous media. GEODENS comprises two modules, (i) the density dependent flow and multispecies transport module and (ii) the geochemical module. The mathematical formulation of the first module leads to a nonlinear and strongly coupled equations. In order to solve such equations, a finite element method has been developed with a consistent numerical scheme of gravity terms to calculate Darcy velocities (Voss, C. I., 1984. SUTRA: a finite element simulation model for saturated–unsaturated fluid density dependent groundwater flow with energy transport of chemically-reactive simple-species solute transport. US Geological Survey Water-Resources Investigations 84-4369). The second module focuses on salts and brine geochemistry using the Pitzer model (Pitzer, K. S., Peiper, J. C., Busey, R. H., 1984. Thermodynamic properties of aqueous sodium chloride solutions. J. Phys. Chem. Ref. Data 13, 1-102).. This geochemical module allows the calculation of ions and solvent activities as well as the density of the solution. Reactions of salts including dissolution- precipitation processes are controlled by diffusion and are represented by a first order kinetic law.
The hydrogeochemical model presented in this paper integrates the two modules described above. The code iteratively calculates the quantities of the different salts that may precipitate or dissolve in a solution when the system is displaced from its equilibrium. This may occur due to evaporation or mixing with a different solution. The system’s hydrodynamic parameters may also change due to mineral dissolution-precipitation reactions. Cases of study presented here are related to seawater intrusion in carbonate coastal aquifers and to Sebkha genesis with salts deposit after seawater evaporation through geological time.