Background Nitrate pollution is a serious and well recognised problem for the sustainable management of surface and groundwater in many European river basins. There is a clear impact of agricultural practices, such as the use of fertilisers, manure or sewage sludge for crop fertilisation on quality of surface and ground-water with regard to nitrate concentrations. This has important consequences on the management of river basins (surface and groundwater) as well as on marine coastal environment, in relation with nutrient pollution, eutrophication, nitrogen balances in grassland and arable land. Similarly, wastewater effluents release from water treatment plants will also impact nitrogen mass balance in the surrounding environment. As a result of nitrate pollution, there are territories (for instance in French Brittany) where local authorities periodically issue warnings to the population to not use tap water for drinking and cooking purposes, and recommends (or provides) the use of bottled mineral water at great cost. The current approach to environmental management and control of water quality regarding nitrate is based on monitoring the concentrations. Chemical data alone, however, do not establish unambiguously the type, location and contribution of its different sources in a river basin. In particular, it is not possible to differentiate urban and agricultural origin (even by increasing the number of monitoring stations or samples). Thus, design and application of specifically targeted management plans for nitrate control is improbable. The demand for appropriate and reliable tools for improved management of nitrate pollution is very high at local, national and European levels. Expectations with respect to this issue in terms of public opinion, hence legislation, are also of critical importance in many Member States. Objectives Research shows that there is significant added value in using isotopic measurements (nitrogen and oxygen isotopes of nitrate, and boron isotopes) to precisely identify nitrate pollution sources (urban or agricultural), trace them in water and quantify their respective contributions. This is because the N and O isotopic ratios of nitrate from different sources (atmospheric, mineral fertilizer, urban or industrial wastewater, animal manure, soil organic matter) are most often significantly distinct. The Isonitrate project’s main aim was to demonstrate to policy makers the technical and economical feasibility of integrating the isotope approach as part of characterising water bodies and analysing pressure and impact of nitrate pollution, for a more effective implementation of environmental management measures on river basins. Results Four sampling sites were studied in the aquifer of Alsace in north-eastern France. Different cases of nitrate concentration and nitrate sources (animal manure, mineral fertilisers and wastewater) were analysed. These four scenarios confirmed that the multi-isotope approach usually brings non-equivocal constraints on the main NO3 contamination sources in water. This results in the contamination-source-specific isotope signatures found in the different types of NO3 sources. One case study estimated the cost-benefit of the isotope approach: it showed that the benefit of investing in isotope monitoring reduced the environmental return (expressed as monetary value of additional information via avoided environmental damage) of reducing nitrate management pollution by over €7 000 per monitoring campaign - suggesting a marginal rate of return of 62%. These promising results were presented at an International conference held at the end of the project. Two technical publications were also presented at the conference: the guidelines for water quality managers/ technicians, "Can measurement of Nitrate, Oxygen and Noron isotopes be useful for your nitrate problem?” and a practical manual for engineers and technicians responsible for nitrate monitoring entitled: "N, O and B isotopes to evaluate nitrate pollution in water”. The project’s goals, however, were not fully reached as concrete mitigation measures are still to be undertaken, after obtaining the sampling results of the project. Some contacts were established with policy makers and managers of water quality, and these will be further expanded in the after-LIFE period, expecting to reach a wider knowledge transfer of the project's results.