Biogeochemical cycling at redox interfaces contributes to regulating ecology, biological productivity and environmental quality in the aquatic environment. Such dynamic systems are best studied using a range of tools including field measurements, laboratory experiments and mathematical modelling. I use such approaches to unravel the biogeochemical transformations of key elements at redox-interfaces throughout watersheds. In this presentation three case studies will feature soil-water, sediment-water and oxic-anoxic interfaces, focusing on potentially toxic metal(loid)s (As, Se, Cr), major elements (Fe, Mn, S) and essential elements (C, P, O). Information on elemental transformation pathways and rates are combined into a reaction network, drawing from the results of field campaigns (sediment cores, porewater concentration profiles, monitoring data) and laboratory experiments (redox-oscillating bioreactors, column and batch experiments). The reaction network is then coupled to physical transport models addressing relevant spatial and temporal dynamics in water-logged soils, sediments and the lake water-column to predict the magnitude and direction of biogeochemical fluxes. These coupled models are used under various future scenarios, such as water-table oscillation, increased carbon loads, climate and land-use changes. Finally, the results are discussed in light of the increasing need for accurate water quality forecasting under environmental changes.