Present limitations and future prospects of stable isotope methods for nitrate source identification in surface- and groundwater

Nitrate (NO3(-)) contamination of surface- and groundwater is an environmental problem in many regions of the world with intensive agriculture and high population densities. Knowledge of the sources of NO3(-) contamination in water is important for better management of water quality. Stable nitrogen (delta15N) and oxygen (delta18O) isotope data of NO3(-) have been frequently used to identify NO3(-) sources in water. This review summarizes typical delta15N- and delta18O-NO3(-) ranges of known NO3(-) sources, interprets constraints and future outlooks to quantify NO3(-) sources, and describes three analytical techniques ("ion-exchange method", "bacterial denitrification method", and "cadmium reduction method") for delta15N- and delta18)O-NO3(-) determination. Isotopic data can provide evidence for the presence of dominant NO3(-) sources. However, quantification, including uncertainty assessment, is lacking when multiple NO3(-) sources are present. Moreover, fractionation processes are often ignored, but may largely constrain the accuracy of NO3(-) source identification. These problems can be overcome if (1) NO3(-) isotopic data are combined with co-migrating discriminators of NO3(-) sources (e.g. (11)B), which are not affected by transformation processes, (2) contributions of different NO3(-) sources can be quantified via linear mixing models (e.g. SIAR), and (3) precise, accurate and high throughput isotope analytical techniques become available.