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.