Calcite dissolution, porosity development are strongly influenced by mixing phenomenon, fluid density and heterogeneity (variability of the hydraulic proprieties). This study presents an approach for calculating porosity change in a heterogeneous porous media during mixing of seawater and freshwater. The proposed approach is based on the Phillip’s analytical solution and is determined by five main steps: (1) Generate a random K-field, (2) Calculate the flow and transport of the heterogeneous porous media, (3) Evaluate the dispersivity coefficient results of the heterogeneity aspect, (4) Evaluate the dissolution capacity of calcite during mixing of two solutions and (5) calculate the porosity change. This approach takes into account the effect of the aquifer heterogeneity on the dispersivity and is applied to simulate the effect of calcite dissolution on the porosity development in a heterogeneous idealized aquifer during seawater intrusion. Results presented in this work are preliminary to assess the effect of heterogeneity on the geochemical reaction during mixing.