Field, lab and modelling studies will quantify the capacity for attenuation of nutrients (e.g. nitrate and phosphate) and organic contaminants (e.g. chlorinated aliphatic hydrocarbons, pharmaceuticals, personal care products and endocrine disrupting compounds) at the biogeochemically active groundwater-surface water interface (GSI) in lowland river catchments. Using existing test sites, field studies will determine the spatial and temporal variability in attenuation, linking these to hydrogeology, relevant geochemical properties, redox conditions and microbiological activity. The research will study the effect of heterogeneity in flow, transport and reactions at the GSI continuum on contaminant attenuation. The expected variations in degradation rate constants, microbiological data and geochemical data will be obtained using results of experimental lab-scale tests carried out in the framework of other EU projects. Additional field data (from multilevel samplers and flux samplers) will be collected to deduce the heterogeneity of flow and transport across the GSI. The results will be interpreted with advanced modelling tools, to validate conceptual models of contaminant attenuation, deduce transfer functions that allow up-scaling of attenuation and identify how variation in these due to heterogeneity affects the prediction of attenuation. The output will be a methodology allowing natural attenuation at the GSI to be included in catchment water quality assessments.