Significant advance in the understanding of water transit times, subsurface structure controls on biogeochemical reactions, and the quantification of catchment scale weathering rates have resulted in the convergence of biogeochemical and hydrological frameworks. Although such convergence is necessary for a mechanistic understanding of the Critical Zone, many challenges still exist. Perhaps the most difficult of all is to reach a unifying hydro-biogeochemical theory that can compare catchments across gradients of climate, geology, and vegetation, while also considering the compositional nature of geochemical data. Understanding the processes driving the evolution of chemical tracers as they move through space and time is of cardinal importance to validating hypotheses regarding Critical Zone features such as the residence time of water in catchments, concentration-discharge relationships, or nutrient cycling through ecosystems. This session aims to initiate and/or intensify exchanges between two of the main scientific fields in Critical Zone science: hydrology and biogeochemistry. We are expecting novel approaches that allow merging hydrological modelling with studies of biogeochemical processes.