Improving the management of nitrate pollution in water by the use of isotope monitoring: the δ15N, δ18O and δ11B triptych

In spite of increasing efforts to reduce nitrogen inputs into ground water from intensive agriculture, nitrate (NO3) remains one of the major pollutants of drinking-water resources worldwide, with NO3 levels approaching the defined limit of 50mgl1 in an increasing number of water bodies. Determining the source(s) of contamination in water is an important first step for improving its quality by emission control. The Life ISONITRATE project aimed at showing the benefit of a multi-isotope approach (15N and 18O of NO3, and 11B), in addition to conventional hydrogeological analysis, to track the origin of NO3 contamination in water. Based on land use and local knowledge, four distinct cases were studied: (1) natural soil NO3, (2) natural denitrification, (3) single source of NO3 pollution and (4) multiple sources of NO3 pollution. Our results show the added value of combining isotope information, compared to knowledge based on local authorities' experience, land use and the classical' chemical approach, by efficiently identifying the number and type of NO3 source(s) for each watershed studied.