A classification scheme for pollutant natural attenuation potential at the groundwater–surface water interface is presented, and its predictive power for explaining baseflow river nitrate concentration investigated. Both the classification scheme and statistical analysis are undertaken at Water Framework Directive surface water body scale for England and Wales, in baseflow conditions when relative groundwater contribution to rivers is greatest. The results of multiple regression analyses demonstrate statistically significant relationships between the classification of natural attenuation potential, its component properties, and baseflow river nitrate concentration. Natural attenuation at the groundwater–surface water interface is shown to be a significant control on observed river nitrate concentrations, albeit less influential than land-use descriptors. The results indicate that natural attenuation processes have a measurable impact on baseflow river chemistry at surface water body scale, and that consideration of natural attenuation processes at the groundwater–surface water interface would improve regional and catchment-scale risk prediction, and could help in the design of more sustainable catchment management strategies.