The increasing production of food and energy worldwide has multiplied today's anthropogenic emissions of nitrogen compounds in the atmosphere, which are transported and deposited through precipitation on the terrestrial and aquatic systems. Disruption of N-balance, eutrophication in rivers and lakes, and stream acidification are a few of the negative consequences, when N-deposition exceeds the remediation capacity or critical load of the aquatic system. Environmental isotopes have been widely used not only to assess hydrological processes, but also to address water quality issues, such as nitrogen pollution. This CRP aims to determine the spatio-temporal distribution of nitrogen isotopes in precipitation as a potential source of nitrogen pollution of water resources, especially of aquatic systems sensitive to eutrophication stressors. This CRP aims to combine N-isotopes with atmospheric chemistry and hydrometeorological techniques to better understand wet deposition processes and to characterize the origin of inorganic nitrogen loadings. Overall, the CRP will aim at improving the capability and expertise among participating Member States in the use of environmental isotopes to assess impacts of nitrogen wet deposition on water quality and ecosystems.