Tracking the sources of nitrate in groundwater using coupled nitrogen and boron isotopes: a synthesis

Nitrate (NO3) is one of the world's major pollutants of drinking water resources. Although recent European Directives have reduced input from intensive agriculture, NO3 levels in groundwater are approaching the drinking water limit of 50 mg L(-1) almost everywhere. Determining the sources of groundwater contamination is an important first step toward improving its quality by emission control. It is with this aim that we review here the benefit of using a coupled isotopic approach (delta15N and delta11B), in addition to conventional hydrogeological analyses, to trace the origin of NO3 in water. The studied watersheds include both fractured bedrock and alluvial (subsurface and deep) hydrogeological contexts. The joint use of nitrogen and boron isotope systematics in each context deciphers the origin of NO3 in the groundwater and allows a semi-quantification of the contributions of the respective pollution sources (mineral fertilizers, wastewater, and animal manure).