This book explores the application of the open-source software OpenGeoSys (OGS) for hydrological numerical simulations concerning conservative and reactive transport modeling. It provides general information on the hydrological and groundwater flow modeling of a real case study and step-by-step model set-up with OGS, while also highlighting related components such as the OGS Data Explorer. The material is based on unpublished manuals and the results of a collaborative project between China and Germany (SUSTAIN H2O). Though the book is primarily intended for graduate students and applied scientists who deal with hydrological modeling, it also offers a valuable source of information for professional geoscientists ... Read morewishing to expand their knowledge of the numerical modeling of hydrological processes including nitrate reactive transport modeling. This book is the second in a series that showcases further applications of computational modeling in hydrological science.