This book addresses the application of the coupling interface OGS-Phreeqc (open-source scientific software) to model reactive mass transport processes in environmental subsurface systems. It provides general information regarding reactive transport modeling and step-by-step model set-ups with OGS-Phreeqc and related components such as GINA and ParaView. In addition, benchmark examples (1D to 2D/3D) are presented in detail. The book is primarily intended for graduate students and applied scientists whose work involves reactive transport modeling. It also offers valuable information for all professional geoscientists interested in deepening their grasp of numerical simulation, with a focus on nitrate fate and transport. It is the third volume in a series exploring the further applications of computational modeling in hydrological science.