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 (δ15N) and oxygen (δ18O) isotope data of NO3− have been frequently used to identify NO3− sources in water. This review summarizes typical δ15N- and δ18O-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 δ15N- and δ18O-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. 11B), 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.