This work package will develop an improved conceptual model for contaminant attenuation in the vadose zone (VZ) and fluxes to the saturated (SZ), and evidence-based predictive tools for groundwater impact assessment and optimization of remediation design. This will involve lab, field and modelling studies that will determine how VZ hydraulic and physico-chemical properties control the attenuation of common contaminants. Molecular microbiology, stable isotopes, high-resolution multilevel samplers (MLS), tracer tests and hydrogeophysical techniques (e.g. electrical resistivity tomography, ground penetrating radar, electromagnetic induction tomography) will be used to quantify contaminant transport, biogeochemical processes and degradation at the field scale, supported by lab-scale model systems to quantify attenuation according to saturation, contaminant flux, oxidant availability, organic matter content and microbiology. Control plane approaches at the VZ–SZ interface and in groundwater will be used to quantify contaminant attenuation and mixing (dilution) efficiency. This will establish how heterogeneity and up-scaling affects attenuation. The analysis will identify key processes for contaminant attenuation in the VZ, the measurements needed to predict attenuation and provide data for modelling. 2-D and 3-D finite-element modelling tools will be used to interpret the results.