285527 Hydrogeochemical Modelling in Coastal Carbonate Aquifer: Porosity and Permeability Evolution

Thursday, November 1, 2012: 10:35 AM
301 (Convention Center )
Rachida Bouhlila and Ezzeddine Laabidi, LMHE, National Engineering School of Tunis-Tunis EL Manar University, Tunis, Tunisia

Hydrogeological properties that govern subsurface flow are porosity, permeability, water-water and water-solid interaction, pore and particle size distributions. Porosity and permeabilities can be modified due to dissolution and precipitation of minerals. In several hydrogeochemical situations, there is an interaction between the solid matrix and the carbonate species according to salts dissolution and precipitation reactions (evaporate deposition in Sebkha anc Chotts, calcite dissolution in seawater-freshwater mixing zone). The dissolution of such rocks can easily induce a development of the porosity and permeability as a result of the mixing processes in the coastal carbonate aquifers (Romanov and Dreybrodt, 2006). The modeling of such problem requires a set of highly nonlinearly coupled equations. GEODENS model, presented in this work, can solve these equations by a finite element procedure. GEODENS is a FORTRAN code (Bouhlila and Laabidi, 2008) 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 called has been developed with a consistent numerical scheme of gravity terms to calculate Darcy velocities (Voss, C. I., 1984). The second module focuses on salts and brine geochemistry, using the Pitzer model (Pitzer, K. S. et al., 1984). 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 by mixing with a different solution.  The system’s hydrodynamic parameters may also change due to mineral dissolution-precipitation reactions. It is hoped that the code will be a useful tool for understanding hydrological and geochemical processes in arid regions. Through the examples simulated in this work we have shown firstly that our model is in good agreement with the previous experiment work (Singurindy and Berkowitz, 2004) and numerical one (Romanov and Dreybrodt, 2006). The model tested and validated is used to simulate the effect of the calcite dissolution on the penetration length of the seawater intrusion, and in order to provide general results we simulate these processes using Henry problem geometry with the same hydrodynamic parameters and boundary conditions.

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