Reactive Flow and Transport Simulator GeoSysBRNS. ... Focusing on the saturated zone, the reactive flow and transport simulator GeoSysBRNS (coupling the flow and transport model OGS with the biogeochemical reaction network simulator BRNS) allows for the integrated simulation of groundwater flow, transport of chemically reactive species, and (bio-) chemical processes transforming them in up to three spatial dimensions. Flexibility with respect to the description of reactive transformation processes has been a priority during the development. Reactive species in GeoSysBRNS can refer to actual chemical compounds, or to pseudo-species. These can, for example, refer to microbial biomass of species catalyzing particular transformations, or to the isotopologically heavy fraction of an organic compound. Chemical and biochemical processes can be kinetically controlled (e.g., by Monod-kinetics), or are assumed to be at equilibrium (e.g., for acid base dissociations). A major advantage of GeoSysBRNS is its way of defining reactive processes: instead of providing a pre-defined fixed set of kinetics, or requiring the user to apply changes at the source code level, chemical processes are written as ordinary differential equations of arbitrary form in the Maple environment. An easy Drag&Drop operation performs the required generation of Fortran source code and the subsequent compilation of the process network specification to a dynamic link library. This flexibility is obtained by employing an operator-splitting scheme, and allows for fast development times for the setup of new scenarios.